UBC Theses and Dissertations

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UBC Theses and Dissertations

Studies of maternal age as a source of variation in two insect species Mackay, Patricia Ann 1974

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STUDIES OF MATERNAL AGE AS A SOURCE OF VARIATION IN TWO INSECT S P E C I E S b y P A T R I C I A ANS MACKAY B . S c , U n i v e r s i t y o f T o r o n t o , 1967 M . S c , U n i v e r s i t y o f T o r o n t o , 1971 A THESIS SUBMITTED IN P A R T I A L FULF ILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n t h e D e p a r t m e n t o f P l a n t S c i e n c e we a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF B R I T I S H COLUMBIA A u g u s t , 1974 In presenting this thesis in partial fulfilment of the requirements f o r an advanced degree at the University of British Columbia, I agree t h a t the Library shall make it freely available for reference and s t u d y . I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department: o r by his representatives. It is understood that copying or p u b l i c a t i o n of this thesis for financial gain shall not be allowed without my written permission. Depa rtment The University of British Columbia Vancouver 8, Canada Date AUGUST 7 /??/ i i A b s t r a c t D i f f e r e n c e s i n p r o g e n y c h a r a c t e r i s t i c s a r i s i n g f r o m d i f f e r e n c e s i n m a t e r n a l age a t t i m e o f b i r t h a r e d i s c u s s e d a s a p o s s i b l e s o u r c e o f e c o l o g i c a l l y i m p o r t a n t v a r i a t i o n . The l i t e r a t u r e on e f f e c t s o f m a t e r n a l age on p r o g e n y g u a l i t y i s r e v i e w e d . E x p e r i m e n t s d e s i g n e d t o document any e f f e c t s o f m a t e r n a l age on two b i o l o g i c a l p r o c e s s e s a r e d e s c r i b e d . The p r o c e s s e s s e l e c t e d were p r o d u c t i o n o f a l a t a e by an a p h i d , ASiSiioSiEiiOS. £ i s u m , and i n c i d e n c e o f d i a p a u s e i n a s a w f l y , E r i o c a m o a o v a t a . M a t e r n a l - a g e e f f e c t s i n f l u e n c e d t h e p r o d u c t i o n o f a l a t a e by t h e a p h i d . E a r l y - b o r n p r o g e n y o f w i n g l e s s a p h i d s were more l i k e l y t o r e s p o n d t o a s t a n d a r d c r o w d i n g s t i m u l u s by p r o d u c i n g a l a t a e t h a n were l a t e - b o r n p r o g e n y . E a r l y - b o r n a l a t a - p r o d u c e r s a l s o p r o d u c e d h i g h e r numbers o f a l a t a e t h a n l a t e - b o r n o n e s . M a t e r n a l age a f f e c t e d t h e p r o g e n y o f w i n g e d a p h i d s , b u t i n t h e o p p o s i t e m a n n e r . E a r l y - b o r n p r o g e n y o f t h i s morph u s u a l l y d i d n o t p r o d u c e a l a t a e a t a l l , w h e r e a s l a t e - b o r n p r o g e n y d i d . The i n h i b i t i o n o f a l a t a - p r c d u c t i o n i n t h e l i n e a g e o f an a l a t e a p h i d was s a t i s f a c t o r i l y e x p l a i n e d by m a t e r n a l age e f f e c t s a l o n e . A t i m i n g m e c h a n i s m s u c h a s t h e " i n t e r v a l t i m e r " p r o p o s e d by L e e s (1966) d i d n o t a p p l y . Ho s i g n i f i c a n t e f f e c t s o f m a t e r n a l age on t h e i n c i d e n c e o f d i a p a u s e i n t h e s a w f l y c o u l d be f o u n d . H o w e v e r , d i a p a u s e was i i i shown t o be i n f l u e n c e d by a t l e a s t two f a c t o r s . More a n i m a l s d i a p a u s e d a t l o w h u m i d i t i e s . S e v e n - i n s t a r l a r v a e d i a p a u s e d more f r e q u e n t l y t h a n d i d s i x - i n s t a r l a r v a e . L a r v a l d e v e l o p m e n t a l t i m e s v a r i e d w i t h t h e number o f i n s t a r s , t h e o c c u r r e n c e o f d i a p a u s e , and t h e d a t e o f e g g - l a y i n g . L e n g t h o f t h e c c c o c n s t a g e o f n c n - d i a p a u s i n g i n d i v i d u a l s v a r i e d w i t h t h e d a t e o f e g g -l a y i n g . I t i s c o n c l u d e d t h a t m a t e r n a l age e f f e c t s , a l t h o u g h not u n i v e r s a l , a r e a common b i o l o g i c a l p h e n o m e n o n , and p r o b a b l y o f e c o l o g i c a l s i g n i f i c a n c e a s a s o u r c e o f v a r i a t i o n . i v T a b l e o f C o n t e n t s C h a p t e r P a g e A b s t r a c t i i L i s t o f T a b l e s v i i L i s t o f F i g u r e s i x A c k n o w l e d g m e n t s x i i 1 I n t r o d u c t i o n 1 2 M a t e r n a l a g e a s a s o u r c e o f v a r i a t i o n i n i n s e c t s 3 2.1 V a r i a t i o n b e t w e e n i n d i v i d u a l s i n i n s e c t e c o l o g y 3 2 . 2 M a t e r n a l age a s a s o u r c e o f v a r i a t i o n : some t h e o r e t i c a l i d e a s 6 2 . 3 M a t e r n a l a g e a s a s o u r c e o f v a r i a t i o n : l i t e r a t u r e r e v i e w 14 3 V a r i a t i o n i n t h e p r o d u c t i o n o f a l a t a e i n l £ l £ t h o s i o h o n p i s u m ( H a r r i s ) , t h e pea a p h i d ( H o m o p t e r a : A p h i d i d a e ) 25 3.1 I n t r o d u c t i o n 25 3 .2 M a t e r i a l s and M e t h o d s 27 3.2.1 L i f e h i s t o r y 27 3 . 2 . 2 R e a r i n g c o n d i t i o n s 28 3 . 2 . 3 R e a r i n g t e c h n i q u e s 30 3 . 2 . 4 E x p e r i m e n t a l d e s i g n 34 3 . 3 R e s u l t s 39 V 3 . 3 . 1 R e p r o d u c t i v e p a t t e r n s o f a p t e r a e and a l a t a e 39 3 . 3 . 2 E f f e c t o f g r a n d p a r e n t a l age on p a r e n t a l a b i l i t y t o p r o d u c e a l a t e o f f s p r i n g 4 9 3 . 3 . 2 . 1 A p t e r o u s g r a n d p a r e n t s 49 3 . 3 . 2 . 2 a l a t e g r a n d p a r e n t s 57 3 . 3 . 2 . 3 A l a t e g r e a t - g r a n d p a r e n t s / a p t e r o u s g r a n d p a r e n t s 69 3 . 3 . 3 E f f e c t o f a g i n g o f a l a t a e on t h e i r a b i l i t y t o p r o d u c e a l a t e p r o g e n y 75 3 . 3 . 4 E f f e c t o f g r a n d p a r e n t a l a g e on p a r e n t a l a b i l i t y t o p r o d u c e a l a t e o f f s p r i n g : a p t e r o u s g r a n d p a r e n t , c a g e d p o p u l a t i o n s 82 3 . 4 D i s c u s s i o n 89 3 . 5 Summary 101 V a r i a t i o n i n i n c i d e n c e o f d i a p a u s e a n d g r o w t h c h a r a c t e r i s t i c s i n E r i o c a m g a o v a t a L , t h e r e d -b a c k e d s a w f l y , ( H y m e n o p t e r a : T e n t h r e d i n i d a e ) 103 4.1 I n t r o d u c t i o n 103 4 . 2 M a t e r i a l s ana M e t h o d s 104 4 . 2 . 1 A d u l t s and e g g s 104 4 . 2 . 2 L a r v a e 106 4 . 2 . 3 P r e p u p a e and c o c o o n s 107 v i 4 . 3 R e s u l t s 108 4 . 3 . 1 L i f e h i s t o r y and h a b i t s 108 4 . 3 . 1 . 1 A d u l t s 108 4 . 3 . 1 . 2 E g g s 113 4 . 3 . 1 . 3 L a r v a e 113 4 . 3 . 1 . 4 P r e p u p a e and c o c o o n s 119 4 . 3 . 2 V a r i a t i o n i n some d e v e l o p m e n t a l c h a r a c t e r i s t i c s 123 4 . 3 . 2 . 1 V a r i a t i o n w i t h t i m e 123 4 . 3 . 2 . 2 V a r i a t i o n i n i n c i d e n c e o f d i a p a u s e 131 4 . 3 . 2 . 3 V a r i a t i o n i n d u r a t i o n o f t h e l a r v a l s t a g e 137 4 . 4 D i s c u s s i o n 143 4 . 5 Summary 144 5 C o n c l u s i o n 145 L i t e r a t u r e c i t e d 146 A p p e n d i x I 159 A p p e n d i x I I 162 V l l / l i s t o f T a b l e s D e m o g r a p h i c d a t a f o r a p t e r o u s and a l a t e p a r t h e n o g e n e t i c A_. £ i s u m f e m a l e s , w i t h c o m p a r a t i v e d a t a f r o m F r a z e r (1972a) f o r a p t e r a e , C h a r a c t e r i s t i c s o f r e p r o d u c t i o n o f p a r e n t a p h i d s a f t e r a s t a n d a r d c r o w d i n g s t i m u l u s : a p t e r o u s g r a n d p a r e n t s . E f f e c t o f g r a n d p a r e n t a l age a t t i m e o f p a r e n t a l b i r t h on t h e number o f a l a t e o f f s p r i n g p r o d u c e d i n r e s p o n s e t o c r o w d i n g : a p t e r o u s g r a n d p a r e n t s . E f f e c t o f g r a n d p a r e n t a l age a t t i m e o f p a r e n t a l b i r t h on p a r e n t a l r e s p o n s e t o c r o w d i n g : a p t e r o u s g r a n d p a r e n t s . C h a r a c t e r i s t i c s o f r e p r o d u c t i o n o f p a r e n t a p h i d s a f t e r a s t a n d a r d c r o w d i n g s t i m u l u s : a l a t e g r a n d p a r e n t s . E f f e c t o f g r a n d p a r e n t a l age a t t i m e o f p a r e n t a l b i r t h on t h e number o f a l a t e o f f s p r i n g p r o d u c e d i n r e s p o n s e t o c r o w d i n g : a l a t e g r a n d p a r e n t s . E f f e c t o f g r a n d p a r e n t a l age a t t i m e o f p a r e n t a l b i r t h on p a r e n t a l r e s p o n s e t o c r o w d i n g : a l a t e g r a n d p a r e n t s . C h a r a c t e r i s t i c s o f r e p r o d u c t i o n o f p a r e n t a p h i d s a f t e r a s t a n d a r d c r o w d i n g s t i m u l u s : a l a t e g r e a t - g r a n d p a r e n t s / a p t e r o u s g r a n d -p a r e n t s . E f f e c t o f g r a n d p a r e n t a l age a t t i m e o f p a r e n t a l b i r t h on t h e number o f a l a t e o f f s p r i n g p r o d u c e d i n r e s p o n s e t o c r o w d i n g : a l a t e g r e a t - g r a n d p a r e n t s / a p t e r o u s g r a n p a r e n t s . E f f e c t o f g r a n d p a r e n t a l age a t t i m e o f p a r e n t a l b i r t h on p a r e n t a l r e s p o n s e t o c r o w d i n g : a l a t e g r e a t - g r a n d p a r e n t s / a p t e r o u s g r a n d p a r e n t s . v i i i T a b l e 11 . T a b l e 1 2 . T a b l e 1 3 . T a b l e 14 . T a b l e 1 5 . T a b l e 16 . T a b l e 1 7 . T a b l e 18 . T a b l e 1 9 . T a b l e 2 0 . T a b l e 2 1 . E f f e c t o f a g e o f a p t e r o u s g r a n d p a r e n t s on t h e number o f a l a t e o f f s p r i n g p r o d u c e d i n r e s p o n s e t o a more n a t u r a l c r o w d i n g s t i m u l u s . R e p r o d u c t i v e c h a r a c t e r i s t i c s f e m a l e s and t h e d e v e l o p m e n t a l o f t h e i r o f f s p r i n g . o f 32 | i o v a t a c h a r a c t e r i s t i c s E f f e c t o f h u m i d i t y ( r e a r i n g c o n t a i n e r ) m o r t a l i t y a n d i n c i d e n c e o f d i a p a u s e . on R e l a t i o n s h i p b e t w e e n i n c i d e n c e o f d i a p a u s e and m a t e r n a l a g e . R e l a t i o n s h i p b e t w e e n number o f l a r v a l and m a t e r n a l a g e . i n s t a r s R e l a t i o n s h i p b e t w e e n number o f l a r v a l i n s t a r s and m a t e r n a l age f o r n o n - d i a p a u s i n g and d i a p a u s i n g l a r v a e s e p a r a t e l y . R e l a t i o n s h i p be tween i n c i d e n c e o f d i a p a u s e a n d m a t e r n a l a g e f o r 6 - i n s t a r a n d 7 - i n s t a r l a r v a e s e p a r a t e l y . R e l a t i o n s h i p b e t w e e n number o f l a r v a l and i n c i d e n c e o f d i a p a u s e . i n s t a r s V a r i a t i o n i n l a r v a l d e v e l o p m e n t a l t i m e s (days ) f o r 6 - i n s t a r and 7 - i n s t a r , n o n - d i a p a u s i n g a n d d i a p a u s i n g l a r v a e . V a r i a t i o n i n l a r v a l d e v e l o p m e n t a l t i m e s i n n o n - d i a p a u s i n g and d i a p a u s i n g , 6 - i n s t a r and 7 -i n s t a r l a r v a e i n r e l a t i o n t o m a t e r n a l a g e . C o m p a r i s o n o f r e p r o d u c t i v e c h a r a c t e r i s t i c s and o f f s p r i n g c h a r a c t e r i s t i c s o f p a r e n t s w i t h 6-i n s t a r and 7 - i n s t a r l a r v a l h i s t o r i e s . 87 112 122 132 133 134 135 136 139 141 142 IX L i s t o f F i g u r e s F i g u r e 1. F i g u r e 2 . F i g u r e 3 . F i g u r e 4 . F i g u r e 5 . F i g u r e 6 . F i g u r e 7 . F i g u r e 8 . F i g u r e 9 . Me thod f o r o b t a i n i n g a s t o c k c u l t u r e o f hs. £ i § M 5 ! i f r o m o n e i n d i v i d u a l , w h i c h w o u l d y i e l d a d a i l y s u p p l y o f n e w l y - m a t u r e d e a r l y -b o r n a p h i d s . S e q u e n c e o f e v e n t s f o r one e x p e r i m e n t a l u n i t f o r t e s t i n g t h e e f f e c t s o f g r a n d p a r e n t a l age on p a r e n t a l a b i l i t y t o p r o d u c e a l a t e o f f s p r i n g . A g e - s p e c i f i c f e c u n d i t y o f a p t e r o u s p a r t h e n o g e n e t i c a p h i d s . and a l a t e A g e - s p e c i f i c s u r v i v a l o f a p t e r o u s and a l a t e p a r t h e n o g e n e t i c a p h i d s . E x p e r i m e n t a l d e s i g n f o r i n v e s t i g a t i n g t h e e f f e c t s o f g r a n d p a r e n t a l age a t t i m e o f p a r e n t a l b i r t h on p a r e n t a l a b i l i t y t o p r o d u c e a l a t e o f f s p r i n g : a p t e r o u s g r a n d p a r e n t s . T h e number o f p a r e n t s p r o d u c i n g d i f f e r e n t n u m b e r s o f a l a t e o f f s p r i n g i n e a c h b a t c h , e x p r e s s e d a s a p e r c e n t o f t h e p a r e n t s f r o m e a c h b a t c h t h a t r e s p o n d e d : a p t e r o u s g r a n d p a r e n t s . E x p e r i m e n t a l d e s i g n f o r i n v e s t i g a t i n g t h e e f f e c t o f g r a n d p a r e n t a l age a t t i m e o f p a r e n t a l b i r t h on p a r e n t a l a b i l i t y t o p r o d u c e a l a t e o f f s p r i n g : a l a t e g r a n d p a r e n t s ; a l a t e g r e a t - g r a n d p a r e n t s / a p t e r o u s g r a n d p a r e n t s . The number o f p a r e n t s p r o d u c i n g d i f f e r e n t n u m b e r s o f a l a t e o f f s p r i n g i n e a c h b a t c h , e x p r e s s e d a s a p e r c e n t o f t h e p a r e n t s f r o m e a c h b a t c h t h a t r e s p o n d e d : a l a t e g r a n d p a r e n t s . The number o f p a r e n t s p r o d u c i n g d i f f e r e n t n u m b e r s o f a l a t e o f f s p r i n g i n e a c h b a t c h , e x p r e s s e d a s a p e r c e n t o f t h e p a r e n t s f r o m e a c h b a t c h t h a t r e s p o n d e d : a l a t e g r e a t -g r a n d p a r e n t s / a p t e r o u s g r a n d p a r e n t s . P a g e 31 35 40 41 51 56 61 68 74 X F i g u r e 10 . F i g u r e 1 1 . F i g u r e 12 . F i g u r e 1 3 . F i g u r e 14, F i g u r e 15 . F i g u r e 1 6 . F i g u r e 17. F i g u r e 1 8 . F i g u r e 19 . F i g u r e 2 0 . E x p e r i m e n t a l d e s i g n f o r i n v e s t i g a t i n g t h e e f f e c t o f a g i n g i n a l a t e a p h i d s on t h e i r a b i l i t y t o p r o d u c e a l a t a e . 77 R e s p o n s e t o a s t a n d a r d c r o w d i n g s t i m u l u s o f s u c c e s s i v e g e n e r a t i o n s o f a p h i d s o f t h e g r e e n s t r a i n o f A_. _gisum . 79 E x p e r i m e n t a l d e s i g n f o r i n v e s t i g a t i n g t h e e f f e c t o f g r a n d p a r e n t a l age a t t i m e o f p a r e n t a l b i r t h o n p a r e n t a l a b i l i t y t o p r o d u c e a l a t e o f f s p r i n g : a p t e r o u s g r a n d p a r e n t s , c a g e d p o p u l a t i o n s . 83 E f f e c t o f g r a n d p a r e n t a l age a t t h e t i m e o f p a r e n t a l b i r t h on t h e p r o p o r t i o n o f p a r e n t s p r o d u c i n g a l a t e o f f s p r i n g : a) and b) a p t e r o u s g r a n d p a r e n t s ; c ) a l a t e g r a n d p a r e n t s . 90 E f f e c t o f g r a n d p a r e n t a l a ge a t t h e t i m e o f p a r e n t a l b i r t h on t h e mean number o f a l a t e o f f s p r i n g p r o d u c e d p e r p a r e n t r e s p o n d i n g : a ) and b) a p t e r o u s g r a n d p a r e n t s ; c ) a l a t e g r a n d p a r e n t s . 91 E f f e c t o f g r a n d p a r e n t a l age a t t h e t i m e o f p a r e n t a l b i r t h on t h e t o t a l p r o p o r t i o n o f a l a t e o f f s p r i n g p r o d u c e d : a) a n d b) a p t e r o u s g r a n d p a r e n t s ; c ) a l a t e g r a n d p a r e n t s . 92 A v e r a g e d a i l y f o o d c o n s u m p t i o n o f a n E_. o v a t a l a r v a (n=12) . 115 T o t a l f o o d c o n s u m p t i o n ( w i t h s t a n d a r d d e v i a t i o n s ) o f l a r v a e r e a r e d a t d i f f e r e n t d e n s i t i e s . 117 Number o f l a r v a e w h i c h h a t c h e d a t d i f f e r e n t t i m e s d u r i n g t h e e x p e r i m e n t a l p e r i o d and were m a i n t a i n e d a n d r e a r e d s u c c e s s f u l l y ( n =967 ) . 124 P r o p o r t i o n o f l a r v a e h a t c h i n g a t d i f f e r e n t t i m e s d u r i n g t h e e x p e r i m e n t a l p e r i o d w h i c h e v e n t u a l l y e n t e r e d d i a p a u s e . 125 P r o p o r t i o n o f l a r v a e h a t c h i n g a t d i f f e r e n t t i m e s d u r i n g t h e e x p e r i m e n t a l p e r i o d w h i c h e v e n t u a l l y went t h r o u g h 7 i n s t a r s . 126 F i g u r e 2 1 . R e l a t i o n s h i p o f d u r a t i o n o f t h e l a r v a l s t a g e o f a) n o n - d i a p a u s i n g , a n d b) d i a p a u s i n g l a r v a e w i t h 6 i n s t a r s , and 7 i n s t a r s , t o l a r v a l h a t c h i n g d a t e . 128 F i g u r e 2 2 . R e l a t i o n s h i p o f d u r a t i o n o f t h e c o c o o n s t a g e o f 6 - i n s t a r and 7 - i n s t a r n o n - d i a p a u s i n g l a r v a e t o l a r v a l h a t c h i n g d a t e . 129 F i g u r e 2 3 . R e l a t i o n s h i p o f p r o p o r t i o n o f o f f s p r i n g d i a p a u s i n g t o p r o p o r t i o n o f o f f s p r i n g w i t h 7 l a r v a l i n s t a r s f o r 32 f e m a l e s . 138 x i i A c k n o w l e d g m e n t s I w i s h t o t h a n k a l l t h e members o f my c o m m i t t e e , D r s . J . P . K i m m i n s , H. R. M a c C a r t h y , A . J . R e n n y , V . C , R u n e c k l e s , and W. G . W e l l i n g t o n , f o r t h e i r i n t e r e s t a n d s u g g e s t i o n s t h r o u g h o u t t h e p e r i o d o f t h i s s t u d y . B u t p a r t i c u l a r l y , I am g r a t e f u l t o D r . W e l l i n g t o n , n o t o n l y f o r a l l h i s t i m e and e f f o r t , b u t a l s o h i s f i n a n c i a l and m o r a l s u p p o r t d u r i n g t h e p r o j e c t . J o h n K i n g , T e r r y K o p e , and E v e Y i a n i t s o p o u l o u , summer a s s i s t a n t s d u r i n g t h e e x p e r i m e n t a l p h a s e , a l l c o n t r i b u t e d i d e a s as w e l l a s h a r d work . N e i l G i l b e r t and h i s s t a t i s t i c a l p r o g r a m s were i n d i s p e n s i b l e f o r t h e d a t a a n a l y s i s . D r . M a c C a r t h y e d i t e d m a n u s c r i p t s f o r me a t d i f f e r e n t s t a g e s o f t h e p r o j e c t and i m p r o v e d them i m m e a s u r a b l y . F i n a l l y I w o u l d l i k e t o t h a n k a l l t h o s e who r e a d and commented o n t h e t h e s i s . 1 C h a p t e r 1 I n t r o d u c t i o n In e c o l o g y , a p r i m a r y g o a l h a s b e e n t o u n d e r s t a n d how a p o p u l a t i o n o f o r g a n i s m s , o r an i n t e r a c t i n g s e t o f p o p u l a t i o n s i s g o v e r n e d . T h a t i s , what a r e t h e f a c t o r s t h a t c o n t r o l t h e l e v e l s o f t h e d i f f e r e n t p o p u l a t i o n s a s t h e y f l u c t u a t e o r s t a b i l i z e . E v e r y p o p u l a t i o n w i l l be a f f e c t e d by a g r e a t many f a c t o r s i n i t s e n v i r o n m e n t . H o w e v e r , i t i s p r o b a b l e t h a t f o r any one p o p u l a t i o n , r e l a t i v e l y few f a c t o r s w i l l be o f c r u c i a l i m p o r t a n c e , a n d t h a t a t any one t i m e , e v e n f e w e r w i l l h a v e an i m p o r t a n t i n f l u e n c e . I f t h i s were n o t s o , t h e s t u d y o f e c o l o g y m i g h t p r o v e t o be a h o p e l e s s t a s k . T h e r e a r e two ways t o a p p r o a c h an e c o l o g i c a l s t u d y . One c a n c h o o s e a s p e c i e s and s t u d y t h e manner i n w h i c h i t s p o p u l a t i o n s b e h a v e , and t h e f a c t o r s w h i c h a f f e c t t h e m . Or o n e c a n c h o o s e a f a c t o r and l o o k a t how i t a f f e c t s d i f f e r e n t s p e c i e s . I h a v e c h o s e n t h e l a t t e r a p p r o a c h i n c a r r y i n g o u t t h i s s t u d y . T h e f a c t o r I h a v e c h o s e n t o l o o k a t i s m a t e r n a l age and t h e v a r i a t i o n i t may c a u s e . The p r o b l e m o f why s o few p o p u l a t i o n s e x p a n d t o t h e p o i n t o f d e s t r o y i n g t h e i r own h a b i t a t and c a u s i n g t h e i r own e x t i n c t i o n h a s l e d many e c o l o g i s t s t o b e l i e v e t h a t i n t e r n a l s e l f - r e g u l a t o r y m e c h a n i s m s must e x i s t . Many p o p u l a t i o n s may be r e g u l a t e d by g e n e t i c c h a n g e s r e s u l t i n g f r o m v a r i a t i o n i n s e l e c t i o n p r e s s u r e a r i s i n g f r o m e n v i r o n m e n t a l c h a n g e s . O t h e r s may be r e g u l a t e d 2 t h r o u g h p h y s i o l o g i c a l c h a n g e s i n i n d i v i d u a l s i n r e s p o n s e t o e n v i r o n m e n t a l c h a n g e s . The i m p o r t a n t r e q u i r e m e n t i s t h a t p o p u l a t i o n s a t d i f f e r e n t t i m e s be made up o f d i f f e r e n t t y p e s o f i n d i v i d u a l s . T h o s e d i f f e r e n c e s may be a c h i e v e d e i t h e r by a l t e r i n g t h e b r e e d i n g s u c c e s s a n d s u r v i v a l r a t e s o f c e r t a i n t y p e s o f i n d i v i d u a l s o r by d i r e c t l y a l t e r i n g t h e c h a r a c t e r i s t i c s o f i n d i v i d u a l s . The e s s e n t i a l a s p e c t o f a c o n c e p t o f s e l f -r e g u l a t i o n i s t h a t t h e r e be a c e r t a i n amount o f v a r i a b i l i t y a v a i l a b l e w i t h i n t h e p o p u l a t i o n . M a t e r n a l age a t t h e t i m e o f an i n d i v i d u a l ' s b i r t h i s c e r t a i n l y a s o u r c e o f v a r i a b i l i t y b e t w e e n i n d i v i d u a l s , p a r t i c u l a r i l y i n i n s e c t s . T h e r e a r e a number o f t h e o r e t i c a l a r g u m e n t s t h a t c a n be made t h a t l e d me t o b e l i e v e t h a t t h e s e d i f f e r e n c e s o f f e r a s o u r c e o f v a r i a b i l i t y w h i c h m i g h t be a d v a n t a g e o u s t o a s p e c i e s i n a number o f w a y s , i n c l u d i n g a l l o w i n g t h e e v o l u t i o n o f s e l f - r e g u l a t i o n . F o r t h i s r e a s o n , I b e g a n s t u d i e s on two d i f f e r e n t o r g a n i s m s f o r w h i c h m a t e r n a l - a g e e f f e c t s had n o t b e e n r e p o r t e d . My i n t e n t i o n was t o t r y t o d o c u m e n t m a t e r n a l - a g e e f f e c t s on e c o l o g i c a l l y i m p o r t a n t c h a r a c t e r i s t i c s i n t h e s e s p e c i e s . T h e r e was one e s s e n t i a l c r i t e r i o n f o r c h o o s i n g an o r g a n i s m . I t had t o e x h i b i t a p r o c e s s t h a t was e c o l o g i c a l l y s i g n i f i c a n t t o i t s s u r v i v a l b u t w h i c h c o n t a i n e d t h e p o t e n t i a l f o r v a r i a t i o n i n i t s e x p r e s s i o n f r o m i n d i v i d u a l t o i n d i v i d u a l , and m i g h t t h e r e f o r e be s u b j e c t t o e x t e r n a l i n f l u e n c e . T h e two p r o c e s s e s t h a t I c h o s e t o s t u d y were p r o d u c t i o n o f w inged f o r m s by an a p h i d , a n d i n c i d e n c e o f d i a p a u s e i n a s a w f l y . 3 C h a p t e r 2 M a t e r n a l age a s a s o u r c e o f v a r i a t i o n i n i n s e c t s 2 . 1 V a r i a t i o n b e t w e e n i n d i v i d u a l s i n i n s e c t e c o l o g y F o r a s l o n g a s e v o l u t i o n h a s b e e n r e c o g n i z e d a s a b i o l o g i c a l p r o c e s s , v a r i a t i o n h a s been r e c o g n i z e d a s i t s t o o l . N a t u r a l s e l e c t i o n c a n o c c u r o n l y b e c a u s e t h e i n d i v i d u a l o r g a n i s m s c o m p r i s i n g a s p e c i e s a r e a l l s l i g h t l y d i f f e r e n t f r o m one a n o t h e r . In t h e p a s t , t h e i m p o r t a n c e o f t h e s e d i f f e r e n c e s h a s b e e n s e e n o n l y i n r e l a t i o n t o l o n g - t e r m c h a n g e s . T h e y were u s u a l l y c o n s i d e r e d t o a f f e c t o n l y a s p e c i e s ' p a t h t o e x t i n c t i o n o r i t s e v o l u t i o n i n t o e n t i r e l y new s p e c i e s . More r e c e n t l y , h o w e v e r , a number o f w o r k e r s h a v e b e g u n t o s t r e s s t h e i m p o r t a n c e o f v a r i a t i o n among i n d i v i d u a l s a s i t a f f e c t s s h o r t t e r m p o p u l a t i o n p r o c e s s e s . F r a n z (1949) s u g g e s t e d t h a t d i f f e r e n c e s i n t h e q u a l i t y o f i n d i v i d u a l s c o m p r i s i n g a p o p u l a t i o n were r e s p o n s i b l e f o r o u t b r e a k s a n d c o l l a p s e s among some E u r o p e a n i n s e c t s . H o w e v e r , few s c i e n t i s t s t o o k up work on t h i s a s p e c t o f p o p u l a t i o n b i o l o g y . T h e n W e l l i n g t o n ( 1 9 5 7 ) , w o r k i n g on w e s t e r n t e n t c a t e r p i l l a r s , and C h i t t y ( 1 9 6 0 ) , w o r k i n g on v o l e s , a l s o pu t f o r w a r d t h e t h e o r y t h a t d i f f e r e n c e s i n t h e q u a l i t y o f i n d i v i d u a l s were r e s p o n s i b l e f o r d i f f e r e n c e s i n p o p u l a t i o n l e v e l s f r o m p l a c e t o p l a c e a t t h e same t i m e , o r i n t h e same p l a c e a t d i f f e r e n t t i m e s . U v a r o v (1961) s u g g e s t e d t h a t t h e k i n d s 4 o f q u a l i t a t i v e d i f f e r e n c e s e x h i b i t e d by l o c u s t s m i g h t be much more common i n o t h e r o r g a n i s m s t h a n was u s u a l l y s u p p o s e d . J u s t a s p h a s e p o l y m o r p h i s m h a s a t r e m e n d o u s i m p a c t on l o c u s t p o p u l a t i o n b i o l o g y , s o m i g h t s u b t l e r d i f f e r e n c e s f o u n d i n o t h e r g r o u p s a f f e c t t h e i r b i o l o g y . S i n c e t h e e a r l y 1 9 6 0 ' s , i n t e r e s t i n t h e e f f e c t s o f q u a l i t a t i v e c h a n g e s i n p o p u l a t i o n s h a s been s l o w l y i n c r e a s i n g among b i o l o g i s t s s t u d y i n g i n s e c t e c o l o g y . I n s e c t s o f f e r n u m e r o u s e x a m p l e s o f s p e c i e s f o r w h i c h i n d i v i d u a l d i f f e r e n c e s o f one k i n d o r a n o t h e r a r e e s s e n t i a l i n some way t o t h e m a i n t e n a n c e o f a p o p u l a t i o n . T h e H y m e n o p t e r a and I s o p t e r a w i t h t h e i r c o m p l e x s y s t e m s o f s o c i a l p o l y m o r p h i s m a r e two g r o u p s f o r w h i c h m a i n t e n a n c e o f i n d i v i d u a l d i f f e r e n c e s i s n e c e s s a r y ( e . g . , s e e W i l s o n , 1 9 7 1 ) . The A p h i d i d a e i s a n o t h e r l a r g e g r o u p i n w h i c h i n d i v i d u a l d i f f e r e n c e s a r e e s s e n t i a l t o t h e s u r v i v a l o f p o p u l a t i o n s ( e . g . , s e e Kennedy and S t r o y a n , 1 9 5 9 ) . The p r o d u c t i o n o f d i s p e r s e r s and s e x u a l e s a t p a r t i c u l a r t i m e s o b v i o u s l y h a s t r e m e n d o u s i m p a c t o n p o p u l a t i o n n u m b e r s . T h e l o c u s t s ( O r t h o p t e r a ) a r e p r o b a b l y t h e g r o u p f o r w h i c h most i s known a b o u t t h e e f f e c t s o f i n d i v i d u a l d i f f e r e n c e s on p o p u l a t i o n p r o c e s s e s ( e . g . , s e e K e y , 1950; K e n n e d y , 1 9 5 6 ; D e m p s t e r , 1 9 6 3 ) . A l t h o u g h i t i s by no means w e l l u n d e r s t o o d y e t , p h a s e p o l y m o r p h i s m h a s b e e n a n e x t e n s i v e l y s t u d i e d p h e n o m e n o n . A t l e a s t two d i s t i n c t l y d i f f e r e n t k i n d s o f i n d i v i d u a l v a r i a t i o n h a v e been d e s c r i b e d i n L e p i d o p t e r a . One i s s i m i l a r i n many ways t o p h a s e i n l o c u s t s ( e . g . , s e e I w a o , 1 9 6 8 ) . T h e o t h e r i n c l u d e s t h e g e n e t i c p o l y m o r p h i s m s s u c h a s i n d u s t r i a l m e l a n i s m ( e . g . , s e e K e t t l e w e l l , 1 9 6 1 ) , and t h e s y s t e m o f g e n e t i c p o l y m o r p h i s m s f o u n d 5 i n some P a p i l l i o b u t t e r f l i e s ( e . g . s e e S h e p p a r d , 1 9 6 1 ) . T h e r e a r e o f c o u r s e , many o t h e r e x a m p l e s l e s s s t a r t l i n g t h a n t h e s e . A l m o s t e v e r y i n s e c t o r d e r h a s a t l e a s t one i n s t a n c e o f a p o l y m o r p h i s m . B r i n k h u r s t (1959) d o c u m e n t e d a wing p o l y m o r p h i s m i n G e r r o i d e a w h i c h a f f e c t e d d i s p e r s a l a b i l i t y . G h e n t (1960) f o u n d d i f f e r e n c e s i n a c o l o n i a l s a w f l y i n i n d i v i d u a l a b i l i t y t o i n i t i a t e f e e d i n g a t a new s i t e . U n f o r t u n a t e l y h o w e v e r , v e r y few f i e l d s t u d i e s h a v e been c a r r i e d o u t . W e l l i n g t o n (1960 , 1961) s t u d i e d t h e q u a n t i t a t i v e c o n s e q u e n c e s o f p o p u l a t i o n q u a l i t y i n w e s t e r n t e n t c a t e r p i l l a r s . L e o n a r d (1970) h a s d o n e s i m i l a r work w i t h t h e g y p s y m o t h . B a l t e n s w e i l e r (1972) h a s s t u d i e d t h e r e l a t i o n s h i p o f l a r v a l c h a r a c t e r i s t i c s o f t h e l a r c h bud moth t o p o p u l a t i o n f l u c t u a t i o n s . T h e r e a p p e a r t o be o n l y two ways t h a t a n i m a l s c a n d i f f e r f r o m one a n o t h e r . T h e y may d i f f e r b e c a u s e t h e y a r e o f d i f f e r e n t g e n e t i c m a k e - u p . O r t h e y may h a v e r o u g h l y t h e same g e n e t i c make-up b u t e x h i b i t d i f f e r e n t p h e n o t y p e s a s a r e s u l t o f e x t r i n s i c f a c t o r s a f f e c t i n g t h e e x p r e s s i o n o f t h e i r g e n o t y p e s . T h u s i n a s e n s e , t h e f o r m e r a r e g e n e t i c v a r i a t i o n s , w h i l e t h e l a t t e r a r e n o n - g e n e t i c , a l t h o u g h e v e n t h e y must u l t i m a t e l y be g e n e t i c a l l y c o n t r o l l e d , a s i t i s g e n e s w h i c h d i c t a t e t h e p o s s i b l e r a n g e o f p h e n o t y p e s , e v e n t h o u g h t h e y may n o t d i c t a t e t h e p h e n o t y p e o f a p a r t i c u l a r i n d i v i d u a l . T h e f i e l d o f E c o l o g i c a l G e n e t i c s , o r G e n e t i c a l E c o l o g y i n c l u d e s t h e s t u d y o f t h e g e n e t i c v a r i a t i o n s w i t h i n and b e t w e e n p o p u l a t i o n s . I t i s d i f f e r e n c e s i n t h e n o n - g e n e t i c c a t e g o r y t h a t a r e o f 6 p a r t i c u l a r i n t e r e s t here. Most of the well-known examples of i n s e c t v a r i a t i o n or polymorphism f a l l i n t o t h i s category. The vast m a j o r i t y of the cases of s o c i a l polymorphism i n Hymenoptera and I s o p t e r a are c o n t r o l l e d by pheromonal or hormonal mechanisms, aphid polymorphisms are c o n t r o l l e d by such f a c t o r s as photoperiod, temperature, or p o p u l a t i o n d e n s i t i e s . Phase polymorphism i n l o c u s t s and i n l e p i d o p t e r a n s seems to be i n f l u e n c e d predominantly by p o p u l a t i o n d e n s i t y and r e l a t e d phenomena. In other words, i n each of these cases, every i n d i v i d u a l possesses the g e n e t i c p o t e n t i a l of developing i n t o any of the types or morphs. But e x t e r n a l f a c t o r s somehow c o n t r o l gene e x p r e s s i o n , and d i r e c t d i f f e r e n t i n d i v i d u a l s along d i f f e r e n t developmental pathways. 2.2 Maternal age as a source of v a r i a t i o n : some t h e o r e t i c a l i d e a s An e x t e r n a l f a c t o r which may be very important, but has not been s t u d i e d with any r e g u l a r i t y i s maternal age. I t has been shown f o r a number of s p e c i e s that c e r t a i n c h a r a c t e r i s t i c s of the o f f s p r i n g change as the mother ages. In other words, an i n s e c t produces one type of animal at one time i n her l i f e and another type at other times. These d i f f e r e n c e s are u s u a l l y not d i s c r e t e , but part of a continuum. They are not simply a r e s u l t of senescence as the mother approaches death; d i f f e r e n c e s have been documented throughout the whole r e p r o d u c t i v e p e r i o d . In f a c t , as w i l l be mentioned l a t e r , the d i f f e r e n c e s sometimes even 7 t a k e on a p a r a b o l i c s h a p e o v e r t h e l i f e o f t h e m o t h e r . F o r p a r t i c u l a r p a r a m e t e r s , o f f s p r i n g b o r n a t t h e b e g i n n i n g o r end o f t h e r e p r o d u c t i v e p e r i o d may be most s i m i l a r , and t h e r e i s o f t e n a maximum o r minimum n e a r t h e m i d - p o i n t o f t h e r e p r o d u c t i v e p e r i o d . A number o f a r g u m e n t s c a n be d e v e l o p e d w h i c h make d i f f e r e n c e s due t o m a t e r n a l age r a t h e r a p p e a l i n g f r o m a t h e o r e t i c a l p o i n t o f v i e w . The c o n c l u s i o n s t h a t c a n be drawn f r o m t h e s e l i n e s o f r e a s o n i n g s u g g e s t r a t h e r s t r o n g l y t h a t m a t e r n a l age e f f e c t s a r e p o t e n t i a l l y v e r y i m p o r t a n t i n t h e p o p u l a t i o n p r o c e s s e s o f a v a r i e t y o f s p e c i e s . I t w i l l p e r h a p s be u s e f u l t o d i s c u s s t h e s e a s p e c t s o f m a t e r n a l - a g e e f f e c t s p r i o r t o r e v i e w i n g what i n f o r m a t i o n i s a l r e a d y a v a i l a b l e . Den B o e r (1968) d i s c u s s e d i n some d e t a i l t h e i d e a t h a t p o p u l a t i o n n u m b e r s a r e s t a b i l i z e d t o some e x t e n t by a p r o c e s s he c a l l e d " s p r e a d i n g o f r i s k " . He s u g g e s t e d a number o f ways i n w h i c h t h e r i s k o f e x t i n c t i o n i s s p r e a d t h r o u g h o u t a p o p u l a t i o n , and p o p u l a t i o n f l u c t u a t i o n s t h e r e b y damped . One o f t h e s e was t h r o u g h p h e n o t y p i c v a r i a t i o n . F o r e x a m p l e , a t a n y one t i m e , a p o p u l a t i o n i s c o m p o s e d o f i n d i v i d u a l s w i t h v a r y i n g l e v e l s o f t e m p e r a t u r e t o l e r a n c e . T h e r e f o r e a l o c a l p o p u l a t i o n as a w h o l e h a s a much w i d e r t e m p e r a t u r e t o l e r a n c e t h a n any s i n g l e i n d i v i d u a l w i t h i n t h a t p o p u l a t i o n . As a r e s u l t , u n d e r t e m p e r a t u r e e x t r e m e s , a l t h o u g h l a r g e s e g m e n t s o f t h e p o p u l a t i o n may d i e , t h e p o p u l a t i o n a s a w h o l e a v o i d s e x t i n c t i o n . The d e a t h s due t o e x t r e m e t e m p e r a t u r e s c o u l d be f u r t h e r l e s s e n e d b y , f o r e x a m p l e , e n v i r o n m e n t a l h e t e r o g e n e i t y w h i c h w o u l d a l l o w a c e r t a i n 8 p r o p o r t i o n o f t h e p o p u l a t i o n t o a v o i d t h e e x t r e m e s . Den B o e r ' s b e l i e f i s t h a t r a t h e r t h a n t r y i n g t o d e v e l o p p e r f e c t a d a p t a t i o n s t o c u r r e n t c o n d i t i o n s , s p e c i e s a c t u a l l y u s e v a r i a t i o n a s a s t r a t e g y t o e n s u r e l o c a l p e r s i s t e n c e . The c o n c e p t o f s p r e a d i n g o f r i s k c a n be a p p l i e d n o t o n l y t o a p o p u l a t i o n , a s d e n B o e r d i d , b u t t o t h e o f f s p r i n g o f a s i n g l e i n d i v i d u a l a s w e l l . S i n c e t h e f u t u r e c a n n o t be p r e d i c t e d w i t h a n y c o n f i d e n c e , i t may be o f a d v a n t a g e t o an i n d i v i d u a l t o p r o d u c e o f f s p r i n g e x h i b i t i n g a s wide a v a r i e t y o f c h a r a c t e r i s t i c s a s p o s s i b l e i n o r d e r t o be p r e p a r e d f o r any e v e n t u a l i t y , t h e r e b y s p r e a d i n g t h e r i s k o f e x t i n c t i o n o f i t s p a r t i c u l a r g e n e t i c c o m p l e m e n t a s w i d e l y a s p o s s i b l e . Of c o u r s e , g e n e t i c a l l y - c o n t r o l l e d c h a r a c t e r i s t i c s w o u l d h e l p t o s p r e a d t h e r i s k i n t h i s m a n n e r . B u t d i f f e r e n c e s due t o m a t e r n a l age m i g h t b e e s p e c i a l l y a d v a n t a g e o u s b e c a u s e t h e y w o u l d , t o a c e r t a i n e x t e n t , be i n d e p e n d e n t o f t h e g e n e t i c m a k e - u p o f t h e p a r e n t s . F o r e x a m p l e , s u p p o s e f o r t h e moment t h a t m a t e r n a l age a f f e c t e d h e a t s e n s i t i v i t y i n o f f s p r i n g . I t t h e r e f o r e m i g h t a l l o w a g e n e t i c a l l y h e a t - s e n s i t i v e f e m a l e who h a p p e n e d t o mate w i t h a g e n e t i c a l l y h e a t - s e n s i t i v e ma le t o p r o d u c e a t l e a s t a few y o u n g t h a t were more r e s i s t a n t t o h e a t . S p r e a d i n g o f r i s k i s a k i n d o f d i v i s i o n o f l a b o u r t h a t i s i n some r e s p e c t s s i m i l a r t o t h e c o n c e p t o f " a l t r u i s m " . D i f f e r e n t members o f a p o p u l a t i o n o f t e n d e v e l o p c h a r a c t e r i s t i c s s u i t a b l e f o r v e r y d i f f e r e n t r o l e s . B u t i n many c a s e s , one g r o u p a p p e a r s t o s a c r i f i c e a c o n s i d e r a b l e amount i n t e r m s o f r e p r o d u c t i v e s u c c e s s i n c a r r y i n g o u t i t s r o l e . H e n c e i t i s t e r m e d a l t r u i s t i c . 9 F o r e x a m p l e , among t h e b e e s a n d a n t s , t h e r e a r e r e p r o d u c t i v e s a n d w o r k e r s . T h e s o l e t a s k o f t h e r e p r o d u c t i v e s i s e g g - l a y i n g . The o t h e r members o f t h e p o p u l a t i o n , a c o l o n y i n t h i s i n s t a n c e , c a r r y o u t a l l o t h e r n e c e s s a r y t a s k s and n o r m a l l y s a c r i f i c e t h e i r r e p r o d u c t i v e p o t e n t i a l e n t i r e l y . Among t h e a p h i d s , t h e w i n g l e s s morph b u i l d s up l a r g e p o p u l a t i o n s . T h e w i n g e d morph e s t a b l i s h e s new o n e s i n s u i t a b l e l o c a t i o n s , a p r o c e s s w h i c h s u b j e c t s i t t o v e r y h i g h p r e - r e p r o d u c t i v e m o r t a l i t y . T h e t h e o r e t i c a l d i f f i c u l t y i n e x p l a i n i n g s u c h b e h a v i o u r i n s t r i c t l y g e n e t i c t e r m s i s t h a t t h e r e i s a c o n s t a n t s e l e c t i o n p r e s s u r e a t t h e l e v e l o f t h e i n d i v i d u a l a g a i n s t t h e a l t r u i s t i c t r a i t . A d i s c u s s i o n o f d i s p e r s a l w i l l i l l u s t r a t e t h e p r o b l e m . D i s p e r s e r s p e r f o r m two f u n c t i o n s . T h e y may l o w e r t h e p o p u l a t i o n l e v e l o f t h e a r e a i n w h i c h t h e y were b o r n , by l e a v i n g i t . T h e y may a l s o e s t a b l i s h new p o p u l a t i o n s i n new a r e a s . H o w e v e r , i f t h e y l e a v e a p o p u l a t i o n p r i o r t o r e p r o d u c i n g , a s t h e y n o r m a l l y d o , t h e y remove t h e i r g e n e s f r o m t h a t p o p u l a t i o n . I n a d d i t i o n , t h e p r o b a b i l i t y o f s u r v i v i n g and f i n d i n g a s u i t a b l e h a b i t a t i n w h i c h t o b e g i n r e p r o d u c i n g i s f a i r l y low f o r a n y i n d i v i d u a l . T h i s i m p l i e s a c o n s i d e r a b l e s e l f - s a c r i f i c e on t h e p a r t o f t h e d i s p e r s e r . T h u s , t h e r e a r i s e s t h e p r o b l e m o f e x p l a i n i n g how any s u c h a l t r u i s t i c t e n d e n c y i s m a i n t a i n e d n o t o n l y w i t h i n t h e l o c a l p o p u l a t i o n , b u t w i t h i n t h e s p e c i e s a s a w h o l e . T h e r e h a s b e e n much t h e o r i z i n g a b o u t p o s s i b l e m e c h a n i s m s w h e r e b y t r a i t s c a n be m a i n t a i n e d w h i c h a p p e a r t o be d i s a d v a n t a g e o u s t o t h e i n d i v i d u a l e x h i b i t i n g t h e m . G r o u p s e l e c t i o n ( W y n n e - E d w a r d s , 1962) and k i n s e l e c t i o n ( H a m i l t o n , 10 1963 ; M a y n a r d S m i t h , 1964) a r e two t h e o r i e s t h a t h a v e b e e n p r o p o s e d t o e x p l a i n t h e e v o l u t i o n a n d m a i n t e n a n c e o f a p p a r e n t l y a l t r u i s t i c t r a i t s . A t h i r d p o s s i b l e e x p l a n a t i o n a r i s e s o u t o f m a t e r n a l age e f f e c t s . The d i f f i c u l t y w i t h a g e n e t i c e x p l a n a t i o n f o r a l t r u i s t i c t r a i t s l i e s i n t h e c o n s t a n t s e l e c t i o n p r e s s u r e a t t h e l e v e l o f t h e i n d i v i d u a l a g a i n s t t h e d e s i r e d c h a r a c t e r i s t i c . H y p o t h e s i z i n g m a t e r n a l - a g e e f f e c t s e l i m i n a t e s t h i s d i f f i c u l t y . T h e s e l e c t e d c h a r a c t e r i s t i c t h e n b e c o m e s n o t t h e a l t r u i s t i c t r a i t i n t h e o f f s p r i n g , b u t t h e p h y s i o l o g i c a l g r a d i e n t i n t h e m o t h e r . I t i s a d v a n t a g e o u s t o p r o d u c e some o f f s p r i n g t h a t a r e a l t r u i s t i c i f t h e i r p r e s e n c e e n s u r e s t h a t o t h e r o f f s p r i n g c a n r e a c h a d u l t h o o d and s u c c e s s f u l l y r e p r o d u c e , b u t t h e a l t r u i s t i c o f f s p r i n g may be a t a d i s a d v a n t a g e . I f t h e y a r e , and i f t h e y a r e a l s o g e n e t i c a l l y d i f f e r e n t f r o m t h e n o n - a l t r u i s t i c o f f s p r i n g , t h e r e w i l l be s e l e c t i v e p r e s s u r e a g a i n s t them w h i c h may o r may n o t be s u f f i c i e n t t o c o u n t e r t h e m a i n t e n a n c e ^ o f t h e t r a i t . On t h e o t h e r h a n d , i f t h e a l t r u i s t i c and n o n - a l r u i s t i c o f f s p r i n g a r e n o t g e n e t i c a l l y , b u t m e r e l y p h y s i o l o g i c a l l y d i f f e r e n t , t h e l o s s o f a l t r u i s t i c i n d i v i d u a l s need n o t c h a n g e t h e gene f r e q u e n c i e s w i t h i n t h e t o t a l o u t p u t o f t h e m o t h e r . M o r e o v e r , i f t h e p h y s i o l o g i c a l l y d e t e r m i n e d a l t r u i s m i n c r e a s e s t h e e v e n t u a l s u c c e s s o f a f e m a l e ' s o f f s p r i n g , t h e n t h e g e n e s w h i c h c a u s e d h e r t o p r o d u c e a r a n g e o f p h y s i o l o g i c a l l y - d e t e r m i n e d t y p e s w i l l be s e l e c t e d f o r , and t h e a l t r u i s t i c t r a i t w i l l become e s t a b l i s h e d i n t h e p o p u l a t i o n . T h i s p r o c e s s c a n be i l l u s t r a t e d w i t h a h y p o t h e t i c a l e x a m p l e 11 u s i n g d i s p e r s a l t e n d e n c y a s an a l t r u i s t i c t r a i t . S u c c e s s f u l d i s p e r s a l i s o f g r e a t b e n e f i t , b u t t h e p r o b a b i l i t y o f s u c c e s s f o r a n y i n d i v i d u a l may be q u i t e l o w . The s e l e c t i o n p r e s s u r e a t t h e l e v e l o f t h e i n d i v i d u a l a g a i n s t d i s p e r s a l o u t o f a l o c a l p o p u l a t i o n t h e r e f o r e may be h i g h . I f , t h r o u g h c h a n g e s i n h e r own p h y s i o l o g y , a f e m a l e a f f e c t s h e r o f f s p r i n g s o t h a t a t one t i m e i n h e r r e p r o d u c t i v e p e r i o d s h e p r o d u c e s y o u n g t h a t w i l l become d i s p e r s e r s , o r p r o d u c e r s o f d i s p e r s e r s , and a t a n o t h e r t i m e y o u n g t h a t w i l l become n o n - d i s p e r s e r s , o r p r o d u c e r s o f n o n -d i s p e r s e r s , t h e r e w i l l be no t e n d e n c y f o r d i s p e r s a l a b i l i t y t o d e c l i n e w i t h i n t h e l o c a l p o p u l a t i o n . A n o t h e r p r o p e r t y t h a t m a t e r n a l - a g e e f f e c t s may h a v e i s r e l a t e d t o phenomena due to age s t r u c t u r e and t h e i r e f f e c t s on p o p u l a t i o n f l u c t u a t i o n s . I f t h e r e a r e d i f f e r e n c e s b e t w e e n a n i m a l s b o r n a t d i f f e r e n t t i m e s i n t h e i r m o t h e r ' s l i f e c y c l e , t h e n a p o p u l a t i o n a r i s i n g m a i n l y f r o m y o u n g a d u l t s would h a v e v e r y d i f f e r e n t q u a l i t a t i v e c h a r a c t e r i s t i c s t h a n w o u l d a p o p u l a t i o n p r o d u c e d m a i n l y by o l d a d u l t s . I n t h a t f i l i a l g e n e r a t i o n , t h e f o r m e r p o p u l a t i o n w o u l d c o n s i s t m a i n l y o f e a r l y -b o r n , and t h e l a t t e r m a i n l y o f l a t e - b o r n i n d i v i d u a l s . The a d u l t age s t r u c t u r e m i g h t be a f f e c t e d i n a number o f w a y s . A p o p u l a t i o n t h a t was r a p i d l y e x p a n d i n g wou ld h a v e a much h i g h e r p r o p o r t i o n o f y o u n g a d u l t s t h a n a p o p u l a t i o n t h a t was f l u c t u a t i n g n e a r i t s c a r r y i n g c a p a c i t y . A p o p u l a t i o n w h i c h was m a i n t a i n i n g i t s e l f u n d e r h i g h m o r t a l i t y r a t e s f o r any r e a s o n m i g h t a l s o h a v e a h i g h p r o p o r t i o n o f y o u n g a d u l t s . A c u r r e n t l y u n s o l v e d e c o l o g i c a l p r o b l e m c o n c e r n s t h e manner 12 i n w h i c h many p o p u l a t i o n s f l u c t u a t e a r o u n d an a v e r a g e d e n s i t y . Many s p e c i e s , p a r t i c u l a r l y c e r t a i n s m a l l mammals, e x h i b i t a r e g u l a r c y c l i c a l p a t t e r n ( C h i t t y , 1 9 6 0 ) , The q u e s t i o n a r i s e s , why s h o u l d t h e p o p u l a t i o n s a t e q u a l d e n s i t i e s b e h a v e d i f f e r e n t l y o n e i t h e r s i d e o f a peak o r t r o u g h ? On one s i d e , t h e p o p u l a t i o n c o n t i n u e s t o e x p a n d t o w a r d s a p e a k , on the o t h e r , i t c o n t i n u e s t o d e c r e a s e t o w a r d s a low p o i n t . Many p o s s i b l e r e g u l a t o r s have been t e s t e d , i n c l u d i n g f o o d l i m i t a t i o n s , p r e d a t o r d e n s i t i e s , and s h o c k d i s e a s e . T h e y h a v e b e e n shown t o be n e i t h e r n e c e s s a r y n o r s u f f i c i e n t c o n d i t i o n s t o a c c o u n t f o r t h e d i f f e r e n c e s . G e n e t i c c h a n g e i n t h e q u a l i t y o f t h e p o p u l a t i o n s w i t h c h a n g i n g d e n s i t y s t i l l s t a n d s a s a p o s s i b l e e x p l a n a t i o n o f t h e s e f l u c t u a t i o n s . I n d e e d , s u c h c h a n g e s h a v e been d o c u m e n t e d and c o r r e l a t e d w i t h p o p u l a t i o n c y c l e s (Myers and K r e b s , 1 9 7 1 ) . B u t a n o t h e r h y p o t h e s i s i s t h a t t h e r e may be i m p o r t a n t d i f f e r e n c e s i n a d u l t a g e - s t r u c t u r e be tween e x p a n d i n g and c o n t r a c t i n g p o p u l a t i o n s . One p o p u l a t i o n m i g h t c o n s i s t o f p r e d o m i n a n t l y young and t h e o t h e r o f o l d a d u l t s , a c o n d i t i o n e a s y t o d e t e r m i n e . (Or one p o p u l a t i o n m i g h t be p r e d o m i n a n t l y e a r l y - b o r n a n d t h e o t h e r l a t e - b o r n , a c o n d i t i o n t h a t a d m i t t e d l y may n o t be so e a s i l y m o n i t o r e d . ) S u c h d i f f e r e n c e s m i g h t a f f e c t s u r v i v a l and r e p r o d u c t i o n , and t h e r e f o r e p o p u l a t i o n l e v e l s . One a t t r a c t i v e c h a r a c t e r i s t i c o f t h e e f f e c t s o f m a t e r n a l age i s t h e p o s s i b i l i t y t h a t t h e y p r e s e n t f o r i n c r e a s i n g t h e r a n g e o f m e c h a n i s m s whereby o f f s p r i n g q u a l i t y c a n be i n f l u e n c e d . G e n e t i c a l l y , a f e m a l e c a n o n l y i n f l u e n c e the c h a r a c t e r i s t i c s o f h e r o f f s p r i n g by h e r c h o i c e o f a m a t e . T h e r e f o r e s h e can 13 s c a r c e l y e q u i p them t o r e s p o n d b e t t e r t h a n s h e t o c h a n g e s i n t h e e n v i r o n m e n t t h a t s h e may p e r c e i v e . On t h e o t h e r h a n d , s h e , o r h e r e n v i r o n m e n t , c a n e x e r t a m a j o r i n f l u e n c e on any c h a r a c t e r i s t i c s o f h e r o f f s p r i n g t h a t c a n be i n f l u e n c e d by h e r own p h y s i o l o g y . T h e r e i s c o n s i d e r a b l e p o t e n t i a l f o r t h e e v o l u t i o n a r y d e v e l o p m e n t o f means whereby c o n t r o l o v e r s u c h d i f f e r e n c e s m i g h t be r e m o v e d f r o m d i r e c t a s s o c i a t i o n w i t h t h e a g i n g p r o c e s s , and t r a n s f e r r e d t o a r e l a t e d c o n t r o l l i n g m e c h a n i s m . F o r e x a m p l e , i n i n s e c t s , c h a n g e s i n t h e o u t p u t o f h o r m o n e s by t h e m o t h e r a s s h e a g e s may be one o f t h e f a c t o r s c a u s i n g d i f f e r e n c e s among o f f s p r i n g . B u t h o r m o n a l o u t p u t c a n a l s o be a f f e c t e d by o t h e r f a c t o r s , s u c h a s p o p u l a t i o n d e n s i t y o r f o o d a v a i l a b i l i t y . I t i s e a s y t o s e e how t h e s e p a t h w a y s t o h o r m o n a l c h a n g e m i g h t b e g i n t o a f f e c t t h e c h a r a c t e r i s t i c s o f a f e m a l e ' s o f f s p r i n g i n t h e same way a s m a t e r n a l a g e , i f t h e r e s u l t h e l p e d h e r o f f s p r i n g t o d e v e l o p i n t o i n d i v i d u a l s b e t t e r a d a p t e d t o t h e p r e v a i l i n g c o n d i t i o n s . T h i s k i n d o f m e c h a n i s m c o u l d p r o v i d e a more r a p i d r e s p o n s e t o e n v i r o n m e n t a l c h a n g e s t h a n c o u l d p u r e l y g e n e t i c r e s p o n s e s b e c a u s e i t i m m e d i a t e l y a l t e r s t h e f i l i a l g e n e r a t i o n p h y s i o l o g i c a l l y i n s t e a d o f p r o g r e s s i v e l y a l t e r i n g s u c c e s s i v e g e n e r a t i o n s g e n e t i c a l l y t h r o u g h s e l e c t i v e m o r t a l i t y o r r e p r o d u c t i v e s u c c e s s . A p o i n t w o r t h m e n t i o n i n g i s t h a t t h e r e i s no r e a s o n t o assume t h a t t h e p a t t e r n o f d i f f e r e n c e s shown by one s p e c i e s n e e d be s i m i l a r t o t h a t shown by a n o t h e r s p e c i e s . T h e i m p o r t a n t c h a r a c t e r i s t i c o f m a t e r n a l - a g e e f f e c t s i s t h e e x i s t e n c e o f t h e 14 g r a d i e n t , n o t i t s d i r e c t i o n * T o r e t u r n t o t h e e x a m p l e o f d i s p e r s a l : f r o m t h e p o i n t o f v i e w o f t h e m e c h a n i s m , i t d o e s n o t m a t t e r i f one s p e c i e s t e n d s t o p r o d u c e d i s p e r s e r s e a r l y i n i t s l i f e , w h i l e a n o t h e r p r o d u c e s them l a t e r . T h i s d i f f e r e n c e may a f f e c t t h e p a r t i c u l a r r o l e w h i c h d i s p e r s e r s p l a y i n t h e p o p u l a t i o n p r o c e s s e s o f t h e two s p e c i e s , b u t i t d o e s n o t a f f e c t t h e b a s i c a d v a n t a g e t h a t may be c o n f e r r e d by t h e v a r i a t i o n . 2 .3 M a t e r n a l a g e a s a s o u r c e o f v a r i a t i o n : l i t e r a t u r e r e v i e w T h e r e i s a body o f work i n t h e l i t e r a t u r e on t h e s o r t s o f d i f f e r e n c e s t h a t o c c u r among o r g a n i s m s p r o d u c e d a t d i f f e r e n t t i m e s i n t h e p a r e n t a l r e p r o d u c t i v e p e r i o d . L i t t l e o f t h i s work a p p e a r s t o h a v e b e e n d o n e w i t h t h e e c o l o g y o f t h e s p e c i e s i n m i n d . T h u s i t i s n o t p o s s i b l e t o comment on t h e r e a l i t y o r g e n e r a l i t y o f t h e t h e o r e t i c a l i d e a s t h a t h a v e b e e n p r e s e n t e d h e r e . H o w e v e r , i t may be w o r t h w h i l e t o s u m m a r i z e t h e work done up t o t h i s t i m e i n o r d e r t o g a i n some i n s i g h t i n t o t h e s e i d e a s and how t h e y m i g h t be a p p l i e d a n d t e s t e d . T h e r e were a few e a r l y s t u d i e s , b u t i t was n o t u n t i l L a n s i n g (1954) p u b l i s h e d h i s work t h a t i n t e r e s t i n m a t e r n a l - a g e e f f e c t s b e g a n t o d e v e l o p . J e n n i n g s and L y n c h (1928) f o u n d a number o f d i f f e r e n c e s b e t w e e n l i n e s f r o m y o u n g and f r o m o l d p a r e n t s w i t h r e s p e c t t o some r e p r o d u c t i v e c h a r a c t e r i s t i c s i n a r o t i f e r . L a n s i n g e x p a n d e d on t h e i r w o r k . He f o u n d t h a t i n d i v i d u a l s f r o m c l o n e s m a i n t a i n e d w i t h y o u n g p a r e n t s were l a r g e r , had a l o n g e r p r e - r e p r o d u c t i v e p e r i o d , and a l o n g e r 15 o v e r a l l l i f e span as compared with c l o n e s maintained with middle-aged or o l d i n d i v i d u a l s . These c h a r a c t e r i s t i c s continued t o change over a number of g e n e r a t i o n s . He a l s o found that c l o n e s maintained with middle-aged or o l d i n d i v i d u a l s i n v a r i a b l y died out a f t e r a few g e n e r a t i o n s , whereas clones from young i n d i v i d u a l s p e r s i s t e d i n d e f i n i t e l y . The speed with which the c h a r a c t e r i s t i c s c o u l d be a l t e r e d by a change i n the a g e - c l a s s p e r p e t u a t i n g the l i n e , and the f a c t t h a t the s p e c i e s i s parthenogenetic, l e d him to conclude that the d i f f e r e n c e s were non-genetic. Lansing's i n t e r e s t i n the e f f e c t s of maternal age appears to have been s t i m u l a t e d by the importance of senescence as a medical and a human problem. T h i s l i n k probably a l s o e x i s t s f o r a number of other workers, p a r t i c u l a r l y those s t u d y i n g mammals, where the primary concern has been the e f f e c t s of maternal age on i n c i d e n c e of b i r t h d e f e c t s i n the o f f s p r i n g . These aspects are d e a l t with i n a symposium on the s u b j e c t (ed. Miner, 19 54) and i n a review by Parsons (1964) and w i l l not be d i s c u s s e d here. Lansing's work s t i m u l a t e d others to look f o r s i m i l a r r e s u l t s i n d i f f e r e n t organisms. A number of i n v e s t i g a t o r s working with D r g s o p h i l a s p e c i e s have looked f o r maternal age e f f e c t s . Comfort (1953) looked i n v a i n f o r a " L a n s i n g " e f f e c t i n P.*. §uJ)Obscura, b u t Wattiaux (1968a) found a number of d i f f e r e n c e s between e a r l y - and l a t e - b o r n l i n e s . These d i f f e r e n c e s became more pronounced as s e l e c t i o n continued over a number of g e n e r a t i o n s . Wattiaux found t h a t e a r l y - b o r n males were 16 much more a c t i v e s e x u a l l y t h a n l a t e - b o r n . E a r l y - b o r n f e m a l e s had l o w e r l o n g e v i t y a n d f e c u n d i t y t h a n l a t e - b o r n f e m a l e s . T h e r e were s i g n i f i c a n t d i f f e r e n c e s i n t h e d e g r e e o f h e t e r o g e n e i t y and i n c e r t a i n chromosome p a t t e r n s b e t w e e n t h e two l i n e s . W a t t i a u x (196 8b) r e p e a t e d h i s e x p e r i m e n t s w i t h D_. £ s e u d o o b s c u r a . Many o f t h e d i f f e r e n c e s s e e n i n D_. s u b o b s c u r a d i d n o t a p p e a r , b u t he d i d f i n d t h a t t h e l a t e - b o r n l i n e had g r e a t e r l o n g e v i t y . A number o f w o r k e r s h a v e i n v e s t i g a t e d m a t e r n a l - a g e e f f e c t s i ° P i l ® i ^ £ 2 3 5 § t e r . B r i d g e s (1927) f o u n d t h a t t h e amount o f c r o s s i n g - o v e r i n d e v e l o p i n g e g g s v a r i e d w i t h t h e a g e o f t h e m o t h e r . R o b e r t s o n a n d Sang (1945) a n d P a r s o n s (1962) f o u n d t h a t t h e v i a b i l i t y o f t h e e g g s v a r i e d w i t h t h e a g e o f t h e m o t h e r ; i n i t i a l l y v i a b i l i t y i n c r e a s e d a n d t h e n d e c l i n e d . R o b e r t s o n and Sang h y p o t h e s i z e d t h a t t h e low i n i t i a l v i a b i l i t y was t h e r e s u l t o f t h e e g g s h a v i n g been f o r m e d f r o m l a r v a l s t o r e s o f f o o d , i n s t e a d o f f r o m n e w l y - a c g u i r e d r e s o u r c e s . T h e age o f t h e f a t h e r had no e f f e c t on egg v i a b i l i t y . D a v i d (1962) a n d P a r s o n s (1962) f o u n d t h a t e g g l e n g t h d e c r e a s e d i n i t i a l l y and t h e n r o s e a g a i n a s t h e m o t h e r a g e d . D u r r a n t (1955) f o u n d t h a t t h e v a r i a t i o n i n t h e number o f s t e r n o p l e u r a l c h a e t a e o f t h e a d u l t v a r i e d w i t h m a t e r n a l a g e , b e i n g h i g h e s t f r o m v e r y y o u n g and o l d m o t h e r s , and l o w e s t f r o m m i d d l e - a g e d m o t h e r s . P a r s o n s (1962) c o n f i r m e d t h i s . O ' B r i a n (1961) a n d B u t z and H a y d e n (1962) f o u n d t h a t t h e o f f s p r i n g o f young p a r e n t s h a d l o n g e r a d u l t p e r i o d s a n d a l s o p r o d u c e d v i a b l e o f f s p r i n g f o r l o n g e r . T h i s t r e n d c o n t i n u e d o v e r a number o f g e n e r a t i o n s . D e l c o u r and H e u t s (1968) were i n t e r e s t e d i n t h e e f f e c t o f m a t e r n a l age on t h e r a t e o f m i t o s i s . 17 T h e y f o u n d t h a t a d u l t w i n g - s i z e v a r i e d w i t h t h e age o f t h e m o t h e r . Wing s i z e i n c r e a s e d i n i t i a l l y , was a t i t s g r e a t e s t i n f l i e s f r o m e g g s l a i d b e t w e e n d a y s 5 and 10 o f r e p r o d u c t i o n , d e c r e a s e d t o a minimum b e t w e e n d a y s 15 and 2 0 , and t h e n b e g a n t o i n c r e a s e a g a i n . A few o t h e r d i p t e r a n s h a v e b e e n s t u d i e d . R o c k s t e i n (1959) f o u n d t h a t f e m a l e l o n g e v i t y i n t h e h o u s e f l y Wusca d o m e s t i c a s t e a d i l y d e c r e a s e d w i t h i n c r e a s i n g m a t e r n a l a g e , b u t t h a t male l o n g e v i t y i n c r e a s e d s l i g h t l y . C a l l a h a n (1962) f o u n d t h a t p a r e n t a l age had no e f f e c t on t h e l e n g t h o f t h e l a r v a l p e r i o d i n l i 4°.2§§t ica . He f o u n d , h o w e v e r , t h a t t h e r e was a r e d u c t i o n i n t h e number o f e g g s l a i d i n o l d age by t h e o f f s p r i n g o f y o u n g o r o l d p a r e n t s as c o m p a r e d w i t h m i d d l e - a g e d p a r e n t s . A l s o , t h e o f f s p r i n g o f y o u n g m o t h e r s s u r v i v e d low h u m i d i t i e s and s t a r v a t i o n l o n g e r t h a n t h e o f f s p r i n g o f o l d m o t h e r s . H y l t o n (196 7 ) , w o r k i n g w i t h t h e m o s q u i t o E r e t m a p o d i t e s c h r y s t o g a g t e r , f o u n d t h a t more e g g s f r o m y o u n g f e m a l e s c o u l d w i t h s t a n d p r o l o n g e d d e s i c c a t i o n . M a t e r n a l - a g e e f f e c t s h a v e b e e n f o u n d i n a few s p e c i e s o f g r a i n b e e t l e s . A g a i n , L a n s i n g ' s r e s u l t s a p p e a r t o h a v e s t i m u l a t e d some o f t h i s r e s e a r c h , w h i c h seems t o be more o r i e n t e d t o w a r d s p h y s i o l o g y t h a n e c o l o g y . L u d w i g (1956) f o u n d t h a t a s p a r e n t a l a g e i n c r e a s e d , t h e l e n g t h o f t h e l a r v a l p e r i o d d e c r e a s e d i n T e n e b r i o m o l i t o r . T r a c y (1958) c o n f i r m e d t h e s e r e s u l t s and a l s o f o u n d t h a t t h e number o f l a r v a l m o u l t s d e c r e a s e d s l i g h t l y , a s d i d t h e a d u l t l i f e s p a n , a s m a t e r n a l age i n c r e a s e d . L u d w i g and F i o r e ( 1 9 6 0 , 1961) f o u n d t h a t t h e r e was no 18 s i g n i f i c a n t e f f e c t o f m a t e r n a l a g e on t h e c h a r a c t e r i s t i c s o f o f f s p r i n g f o r t h e f i r s t month a f t e r r e p r o d u c t i o n b e g a n . T h e i r b e e t l e s r e p r o d u c e d f o r a s many a s 80 d a y s . L u d w i g and J o n e s (1964) f o u n d c h a n g e s i n t h e c o n c e n t r a t i o n o f a number o f a m i n o a c i d s a s t h e b e e t l e s a g e d . T h e s e c h a n g e s were more p r o n o u n c e d i n b e e t l e s f r o m o l d p a r e n t s t h a n i n t h o s e f r o m y o u n g p a r e n t s . T e n e b r i o o b s c u r u s showed no c o n s i s t e n t e f f e c t o f m a t e r n a l age on t h e d u r a t i o n o f t h e l i f e c y c l e ( F i o r e , 1 9 6 0 ) , b u t m a t e r n a l age d i d a f f e c t g r o w t h r a t e s . B e e t l e s f r o m o l d p a r e n t s grew more s l o w l y t h a n t h o s e f r o m y o u n g p a r e n t s and were u s u a l l y s m a l l e r . S c h n e i d e r (1941) f o u n d t h a t i n t h e f l o u r b e e t l e , T r i b o l i u m c o n f u s u m , l a r v a e f r o m o l d p a r e n t s d e v e l o p e d f a s t e r t h a n t h o s e f r o m y o u n g p a r e n t s . In a d d i t i o n , o f f s p r i n g o f o l d p a r e n t s e n d u r e d s t a r v a t i o n l o n g e r b e f o r e d y i n g . R a y c h a u d h u r i and B u t z (1965) f o u n d a p r o n o u n c e d c y c l i c a l e f f e c t o f m a t e r n a l a g e on o f f s p r i n g d e v e l o p m e n t a l t i m e s i n t h e same s p e c i e s . H o w e v e r , Howe (1967) s e v e r e l y c r i t i c i z e d t h e i r d a t a , and i t i s p o s s i b l e t h a t t h e r e were some e x t e r n a l u n c o n t r o l l e d f a c t o r s w h i c h i n f l u e n c e d d e v e l o p m e n t a l r a t e s i n t h e i r e x p e r i m e n t s . Howe (1967) worked w i t h t h e g r a n a r y w e e v i l , S i t o g h i l u s 3 £ a n a r i u s i n w h e a t . He f o u n d t h a t a s t h e age o f t h e m o t h e r i n c r e a s e d , t h e d e v e l o p m e n t a l p e r i o d o f t h e o f f s p r i n g i n c r e a s e d p a r a b o l i c a l l y , d r o p p i n g o f f a g a i n a s t h e m o t h e r r e a c h e d o l d a g e . Some i n t e r e s t i n g and p r o b a b l y e c o l o g i c a l l y i m p o r t a n t e f f e c t s o f m a t e r n a l age h a v e b e e n d o c u m e n t e d i n a number o f p a r a s i t i c H y m e n o p t e r a . I n S ^ a l a n g i a d r o s o j a h i l a e , a f r u i t f l y p a r a s i t e , t h e i n c i d e n c e o f d i a p a u s e among o f f s p r i n g i n c r e a s e d 19 s l o w l y as maternal age i n c r e a s e d , although there was c o n s i d e r a b l e v a r i a t i o n from f a m i l y to f a m i l y (Simmonds, 1949). On the average, 20% of the o f f s p r i n g of 1-day old parents entered diapause, but 90% of the o f f s p r i n g of 20-day o l d parents d i d so. The developmental time of non-diapausing i n d i v i d u a l s i n c r e a s e d with i n c r e a s i n g maternal age. Cryptus i n o r n a t u s , another p a r a s i t e , behaved i n much the same way. Saunders (1962, 1965, 1966a, 1966b) documented a s i m i l a r response i n a s t r a i n of the p a r a s i t e , Nasonia v i t r i j g e n n i s . However, i n t h i s c ase, young females began by producing a s m a l l number of d i a p a u s i n g l a r v a e . They then switched to producing non-diapausing l a r v a e f o r a p e r i o d . At some p o i n t i n middle-age, they suddennly began to produce only diapausing l a r v a e . The age a t which t h i s l a s t switch o c c u r r e d was a f f e c t e d by temperature, photoperiod, and a v a i l a b i l i t y of the host. McNeil and Rabb (1973) d e s c r i b e d somewhat s i m i l a r e f f e c t s of maternal age on the i n c i d e n c e of diapause i n two h y p e r - p a r a s i t e s , ( Hi£0£teromalus tabacum and C a t o l a c c u s a e n e o v i r i d i s ) of the tabacco hornworm, Manduca sexta . The L e p i d o p t e r a have y i e l d e d a few important cases. Wellington (1965) and Wellington and Maelzer (1967) i d e n t i f i e d e f f e c t s comparable to maternal-age e f f e c t s i n the western tent c a t e r p i l l a r , Malacosoma p l u v i a l e . T h i s moth l a y s a l l i t s eggs i n a s i n g l e mass soon a f t e r the female has emerged and mated. The moth does not t h e r e f o r e age d u r i n g the process of l a y i n g . N e v e r t h e l e s s , the eggs are produced over a p e r i o d of time during the pupal s t a g e . Wellington found t h a t the eggs l a i d f i r s t gave 20 r i s e t o more a c t i v e , b e t t e r - o r i e n t e d l a r v a e t h a n t h o s e l a i d l a s t . T h e s e d i f f e r e n c e s a p p e a r e d t o be a t l e a s t p a r t i a l l y due t o u n e q u a l p a r t i t i o n i n g o f t h e m a t e r n a l f o o d r e s e r v e s a s egg p r o d u c t i o n c o n t i n u e d . T h e r e a l s o a p p e a r e d t o be an e f f e c t o f m a t e r n a l l a r v a l h i s t o r y on t h e e g g m a s s . W e l l i n g t o n f o u n d t h a t c o l o n i e s o f c a t e r p i l l a r s w h i c h c o n t a i n e d a m i x t u r e o f a c t i v e and i n a c t i v e l a r v a e f a r e d b e t t e r t h a n c o l o n i e s t h a t c o n s i s t e d m a i n l y o f one t y p e . T h e s e d i f f e r e n t t y p e s and t h e i r p r o p o r t i o n s i n a p o p u l a t i o n a s a w h o l e a f f e c t e d p o p u l a t i o n p r o c e s s e s f r o m y e a r t o y e a r ( W e l l i n g t o n ; 1960 , 1 9 6 4 ) . F r a n z and L a u x (1964) d e s c r i b e d s i m i l a r b e h a v i o u r a l d i f f e r e n c e s i n t h e c l o s e l y r e l a t e d M a l a c g s o m a n e u s t r i a b u t t h e y c o n s i d e r e d t h e s e t o be o f g e n e t i c o r i g i n . C a m p b e l l (1962) f o u n d t h a t i n C h o r i s t o n e u r a , t h e mean egg s i z e o f an e g g c l u s t e r d e c r e a s e d a s t h e f e m a l e ' s age i n c r e a s e d . He s u g g e s t e d t h a t t h i s was due t o i n c r e a s i n g c o m p e t i t i o n b e t w e e n o o c y t e s f o r d e c r e a s i n g m a t e r n a l n u t r i e n t s . C a m p b e l l r e f e r r e d t o some u n p u b l i s h e d work by H a r v e y on t h e same g r o u p , i n w h i c h egg s i z e was c o r r e l a t e d w i t h s u c h c h a r a c t e r i s t i c s a s t h e w i d t h o f h e a d c a p s u l e s i n f i r s t - i n s t a r a n d p r e - f e e d i n g s e c o n d - i n s t a r l a r v a e , and a l s o w i t h t h e l e n g t h o f t i m e a d i a p a u s i n g l a r v a c o u l d s u r v i v e . L e o n a r d (1970) f o u n d t h a t t h e f i r s t - l a i d e g g s o f t h e g y p s y m o t h , £ 2 £ t h e t r i a d isp_ar , were l a r g e r t h a n t h e l a s t - l a i d e g g s . T h e s e d i f f e r e n c e s a p p e a r e d t o be c o r r e l a t e d w i t h t h e number o f m o u l t s a l a r v a u n d e r w e n t . T h e s m a l l e r t h e e g g , t h e g r e a t e r t h e p r o b a b i l i t y t h a t t h e l a r v a h a t c h i n g f r o m i t wou ld go t h r o u g h an a d d i t i o n a l m o u l t . L a r v a e w i t h a d d i t i o n a l m o u l t s had a p r o l o n g e d 21 f i r s t i n s t a r and were a p p a r e n t l y more l i k e l y t o u n d e r g o d i s p e r s a l by w i n d . L e o n a r d p r o p o s e d t h i s a s a c o m p o n e n t o f a p o s s i b l e m e c h a n i s m f o r p o p u l a t i o n r e g u l a t i o n . The H e m i p t e r a h a v e p r o v i d e d t w o , p o s s i b l y t h r e e , e x a m p l e s o f m a t e r n a l - a g e e f f e c t s . B i c h a r d s and K c l d e r i e (1957) a c c i d e n t a l l y f o u n d some m a t e r n a l - a g e e f f e c t s i n O n c o g e l t u s f a s c i a t a , t h e l a r g e m i l k w e e d bug , a s a r e s u l t o f t h e way t h e y k e p t t h e i r s t o c k c u l t u r e s . T h e y t h e n d i d some c a r e f u l work t o d o c u m e n t t h e e f f e c t s . The egg w e i g h t s i n c r e a s e d f o r a b o u t 20 d a y s , r e m a i n e d c o n s t a n t f o r a n o t h e r 20 d a y s , t h e n b e g a n t o f a l l o f f a s s e n e s c e n c e a p p r o a c h e d . D e v e l o p m e n t a l r a t e s o f t h e l a r v a e m i r r o r e d t h i s , a s d i d t h e h a t c h a b i l i t y o f t h e e g g s t o some d e g r e e . C a l d w e l l and Hegmann (1969) f o u n d t h a t h e r i t a b i l i t y o f f l i g h t d u r a t i o n was h i g h e r f o r t h e f e m a l e t h a n t h e male p a r e n t i n t h e s m a l l m i l k w e e d b u g , L i a a e u s k a l m i i . T h e y were i n t e r e s t e d i n t h e g e n e t i c a s p e c t s o f d i s p e r s a l by f l i g h t , and d i d n o t i n v e s t i g a t e t h e i n f l u e n c e o f m a t e r n a l a g e . T h e g r e a t e r i n f l u e n c e o f t h e m o t h e r may i n d i c a t e a s e x - l i n k e d g e n e t i c f a c t o r , o r i t may i n d i c a t e some n o n - g e n e t i c i n f l u e n c e . M u r a i and K i r i t a n i (1970) i n v e s t i g a t e d t h e i n f l u e n c e o f p a r e n t a l age on t h e o f f s p r i n g o f t h e g r e e n r i c e l e a f h o p p e r , M e p h o t e t t i x c i n c t i c e p s . T h i s p e s t l a y s i t s e g g s i n a s e r i e s o f b a t c h e s o v e r a l o n g p e r i o d o f t i m e . I t t e n d s t o l a y e g g s i n c y c l e s , most i n d i v i d u a l s a p p a r e n t l y g o i n g t h r o u g h a b o u t f o u r c y c l e s o f e g g - l a y i n g . T h e r e was a d e c r e a s e i n p e r c e n t h a t c h e d f r o m t h e b e g i n n i n g t o e n d o f one c y c l e , and a p a r a b o l i c c h a n g e o v e r t h e whole e g g - l a y i n g p e r i o d , w i t h a maximum b e i n g a c h i e v e d 22 e a r l y i n t h e s e c o n d c y c l e . T h e r e was no a p p a r e n t t r e n d i n egg s i z e w i t h i n one c y c l e , b u t mean e g g s i z e c h a n g e d p a r a b o l i c a l l y a s t h e m o t h e r a g e d , s i z e b e i n g g r e a t e s t f r o m m i d d l e - a g e d f e m a l e s . The l e n g t h o f t h e e g g s t a g e and t h e t o t a l t i m e f r o m egg t o a d u l t i n c r e a s e d l i n e a r l y w i t h i n c r e a s i n g m a t e r n a l a g e . The s i z e o f a n e w l y - h a t c h e d l a r v a a n d i t s r e s i s t a n c e t o s t a r v a t i o n d e c r e a s e d l i n e a r l y . E a r l y - b o r n f e m a l e s had a l o n g e r p r e -o v i p o s i t i o n p e r i o d and a c o r r e s p o n d i n g l y l o n g e r a d u l t l i f e , b u t a s l i g h t l y l o w e r f e c u n d i t y a s c o m p a r e d w i t h l a t e - b o r n f e m a l e s . O l d f e m a l e s p r o d u c e d a few i n d i v i d u a l s t h a t were h i g h l y r e s i s t a n t t o s t a r v a t i o n a s a d u l t s . M u r a i and K i r i t a n i s u g g e s t e d t h a t f i r s t - b o r n i n d i v i d u a l s were t h e d i s p e r s e r s b e c a u s e o f t h e l o n g e r p r e - o v i p o s i t i o n p e r i o d and l o w e r f e c u n d i t y , b u t l a s t - b o r n i n d i v i d u a l s were i m p o r t a n t i n m a i n t a i n i n g t h e p o p u l a t i o n s t h r o u g h p e r i o d s o f a d v e r s e c o n d i t i o n s . T h e r e a r e two s i g n i f i c a n t c a s e s o f m a t e r n a l - a g e e f f e c t s among t h e O r t h o p t e r a . T h e s e may be p a r t i c u l a r l y i m p o r t a n t b e c a u s e o f t h e p h y s i o l o g i c a l s i m i l a r i t i e s t h e y b e a r t o some o f t h e c h a r a c t e r i s t i c s o f p h a s e p o l y m o r p h i s m i n l o c u s t s ( e . g . , s e e A l b r e c h t , V e r d i e r , and B l a c k i t h , 1 9 5 9 ) . C h a u v i n (1958) f o u n d t h a t some c r i c k e t s o f t h e s p e c i e s G r y l l u l u s d o m e s t i c u s i n c r e a s e d t h e i r d e v e l o p m e n t a l r a t e when r e a r e d i n g r o u p s . O n l y t h e p r o g e n y o f o l d f e m a l e s w e r e , a s he s a i d , " g r o u p s e n s i t i v e " . T h e p r o g e n y o f young f e m a l e s were " g r o u p i n s e n s i t i v e " . Van H o r n (1966) and V i s s c h e r (nee Van Horn) (1971) s t u d i e d t h e e f f e c t s o f m a t e r n a l age on t h e d e v e l o p m e n t o f t h e p r o g e n y o f A u l o c a r a e l l i o t i , a g r a s s h o p p e r . The number o f e g g s p e r pod may h a v e d e c r e a s e d as 23 m o t h e r s a g e d . T h e d e v e l o p m e n t a l r a t e s o f e m b r y o s f r o m y o u n g f e m a l e s were much s l o w e r t h a n e m b r y o s f r o m o l d e r f e m a l e s . P a r e n t a l d e n s i t i e s a f f e c t e d t h e d e v e l o p m e n t a l r a t e s o f t h e e m b r y o s . As t h e e m b r y o n i c d e v e l o p m e n t a l r a t e a f f e c t s t h e s u r v i v a l o f t h e i n d i v i d u a l d u r i n g w i n t e r d i a p a u s e , V i s s c h e r s u g g e s t e d t h a t i t w o u l d a l s o a f f e c t p o p u l a t i o n n u m b e r s t h e f o l l o w i n g y e a r . Q u i c k e n d e n and R o e m h i l d ( 1 9 6 9 ) , w o r k i n g w i t h t h e same s p e c i e s , f o u n d t h a t a s m a t e r n a l age i n c r e a s e d , b o t h g l y c o g e n l e v e l s a n d w e i g h t o f t h e e g g s i n c r e a s e d . Some w o r k e r s h a v e h y p o t h e s i s e d a g e n e t i c c a u s e f o r a l l t h e s e d i f f e r e n c e s due t o m a t e r n a l age ( e . g . Howe, 1 9 6 7 ) , s u g g e s t i n g t h a t t h e amount o f c r o s s i n g - o v e r may be r e s p o n s i b l e . I n d e e d , c r o s s i n g - o v e r h a s b e e n shown t o be a f f e c t e d by m a t e r n a l age a n d may i n f a c t be t h e c a u s e o f some o f t h e o t h e r d i f f e r e n c e s . H o w e v e r , t h e r e i s a s y e t no r e a s o n t o assume t h a t g e n e t i c c h a n g e s r e l a t e d t o m a t e r n a l age a r e t h e o n l y , o r e v e n t h e most i m p o r t a n t , c a u s a t i v e a g e n t s o f d i f f e r e n c e s among o f f s p r i n g b o r n a t d i f f e r e n t t i m e s . I f g e n e t i c c h a n g e s were most i m p o r t a n t , p a t e r n a l a g e s h o u l d be a s i m p o r t a n t a s m a t e r n a l a g e , b u t t h i s d o e s n o t a p p e a r to be t h e c a s e . Whenever p a t e r n a l age h a s b e e n i n v e s t i g a t e d , i t h a s had l i t t l e o r no e f f e c t on t h e c h a r a c t e r i s t i c s o f o f f s p r i n g . P h y s i o l o g i c a l l y - b a s e d d i f f e r e n c e s must a l s o be i n v o l v e d . M a t e r n a l age may a f f e c t t h e n u t r i e n t s g o i n g t o t h e e g g , o r o t h e r w i s e a f f e c t t h e e g g ' s d e v e l o p m e n t a l c a p a c i t i e s t h r o u g h some o f t h e more s u b t l e f a c t o r s ( e . g . , hormones) . Some o f t h e commonly r e c o r d e d d i f f e r e n c e s due t o m a t e r n a l 24 age up t o now h a v e b e e n r e l a t e d t o l e n g t h o f l i f e and d e v e l o p m e n t a l p e r i o d . F e c u n d i t y h a s been r e g u l a r l y m e n t i o n e d , a s h a s r e s i s t a n c e t o s t a r v a t i o n o r some o t h e r s t r e s s . H o w e v e r , t h e r e i s no r e a s o n t o assume t h a t t h e s t u d i e s r e v i e w e d h e r e a r e a random s a m p l e o f t h e k i n d s o f c h a r a c t e r i s t i c s w h i c h v a r y w i t h m a t e r n a l a g e . T h e y were s i m p l y t h e c h a r a c t e r i s t i c s t h a t seemed a p p r o p r i a t e f o r o b s e r v a t i o n a t t h e t i m e , a l t h o u g h many a r e o f l i t t l e p r a c t i c a l v a l u e t o an e c o l o g i s t b e c a u s e t h e i r e f f e c t s on p o p u l a t i o n p r o c e s s e s a r e t o o d i f f i c u l t t o m e a s u r e . D e s p i t e t h a t d i f f i c u l t y , h o w e v e r , i t i s p r o b a b l e t h a t many o f them a r e i m p o r t a n t a t t h e p o p u l a t i o n l e v e l . New s t u d i e s w i l l u n d o u b t e d l y r e v e a l o t h e r s i g n i f i c a n t e f f e c t s o f m a t e r n a l a g e on t h e c h a r a c t e r i s t i c s o f p r o g e n y . 25 C h a p t e r 3 V a r i a t i o n i n t h e p r o d u c t i o n o f a l a t a e i n l c . X £ J h o s i g h o n p i s u m ( H a r r i s ) , t h e pea a p h i d ( H o m o p t e r a : A p h i d i d a e ) 3 . 1 I n t r o d u c t i o n T h e q u e s t i o n i n v e s t i g a t e d i n t h i s p a r t o f t h e s t u d y i s w h e t h e r e a r l y - a n d l a t e - b o r n a p h i d s d i f f e r e d i n t h e i r t e n d e n c y t o p r o d u c e w i n g e d p r o g e n y . T h e r e were g o o d r e a s o n s f o r c h o o s i n g t h e p o l y m o r p h i c a l a t e / a p t e r o u s ( w i n g e d / w i n g l e s s ) s y s t e m i n A £ I £ t h o s i p h o n p i s u m t o s t u d y t h e e f f e c t s o f m a t e r n a l a g e . T h e p r o d u c t i o n o f t h e r i g h t number o f a l a t a e a t t h e r i g h t t i m e i s e x t r e m e l y i m p o r t a n t t o t h e l o n g - t e r m s u r v i v a l o f most a p h i d s . A s a g e n e r a l r u l e , a p h i d s a r e p l a n t p a r a s i t e s s h o w i n g s u c h a d a p t a t i o n s a s p a r t h e n o g e n e s i s , v i v i p a r i t y , t e l e s c o p i n g o f g e n e r a t i o n s , a l t e r n a t i o n o f h o s t s , and j u v e n i l e f e a t u r e s s u c h a s w i n g l e s s n e s s ( K e n n e d y and S t r o y a n , 1 9 5 9 ) . A l l t h e s e a d a p t a t i o n s a l l o w them t o m u l t i p l y v e r y r a p i d l y and make f u l l u s e o f o f t e n r a t h e r e p h e m e r a l f o o d s o u r c e s , s u c h a s h e r b a c e o u s a n n u a l s , o r t h e f o l i a g e o f woody s p e c i e s w h i c h may be s u i t a b l e f o r o n l y a s m a l l p a r t o f t h e y e a r . T h e a p h i d s o f t e n make t h e i r own f o o d s u p p l y u n s u i t a b l e by o v e r u s e . T h i s way o f l i f e means t h a t i t i s o f t e n n e c e s s a r y f o r a p h i d s t o move r e g u l a r l y t o new p l a n t s . T h e s e moves must b e a c c o m p l i s h e d by a l a t e m o r p h s . In a p h i d s , s e v e r a l f a c t o r s i n i t i a t e t h e p r o d u c t i o n o f 26 a l a t a e (see r e v i e w s b y B o n n e m a i s o n , 1951 ; H i l l e R i s L a m b e r s , 1966; L e e s , 1 9 6 6 ) , t h o u g h t h e i m p o r t a n c e o f e a c h f a c t o r may v a r y f r o m s p e c i e s t o s p e c i e s . Some o f t h e e n v i r o n m e n t a l s t i m u l i h a v e been w e l l s t u d i e d . P h o t o p e r i o d a n d t e m p e r a t u r e o f t e n i n i t i a t e t h e p r o d u c t i o n o f a l a t a e , p a r t i c u l a r l y i n s p e c i e s w i t h more t h a n one h o s t p l a n t d u r i n g t h e y e a r . I n c r e a s i n g d e n s i t y a l s o c a u s e s many s p e c i e s o f a p h i d s t o p r o d u c e a l a t a e , as d o e s d e t e r i o r a t i o n o f t h e h o s t p l a n t , t h r o u g h a d e c r e a s e i n t h e n u t r i e n t l e v e l o r t h r o u g h w i l t i n g due t o w a t e r l o s s . T h e r e i s c o n s i d e r a b l e v a r i a b i l i t y , e v e n w i t h i n a s p e c i e s , i n r e s p o n s e t o s u c h s t i m u l i . Some o f t h i s v a r i a b i l i t y i s due t o c l o n a l d i f f e r e n c e s , p r e s u m a b l y g e n e t i c i n n a t u r e , b u t much o f i t i s u n e x p l a i n e d . T h e o b v i o u s i m p o r t a n c e o f a l a t a e t o a p h i d s u r v i v a l , and t h e u n e x p l a i n e d v a r i a t i o n i n r e s p o n s e t o d i f f e r e n t s t i m u l i t h a t i n i t i a t e p r o d u c t i o n o f a l a t a e , made t h i s a s u i t a b l e s y s t e m t o i n v e s t i g a t e . M o r e o v e r , t h e p r e s e n c e o r a b s e n c e o f w i n g s i s c l e a r - c u t , s i m p l i f y i n g d a t a c o l l e c t i o n . T h e p e a a p h i d , Ac_yrt ho s i p h o n £ i s u m , was c h o s e n f o r a number o f r e a s o n s . I t i s l o c a l l y a b u n d a n t , e a s y t o r e a r , and h a s a s h o r t g e n e r a t i o n t i m e . C o n s i d e r a b l e i n f o r m a t i o n i s a v a i l a b l e on i t s b i o l o g y . In p a r t i c u l a r , t h e f a c t o r s c o n t r o l l i n g and i n f l u e n c i n g t h e p r o d u c t i o n o f a l a t a e had r e c e n t l y b e e n s t u d i e d i n g r e a t d e t a i l ( S u t h e r l a n d , 1 9 6 9 a , 1 9 6 9 b , 1 9 7 0 ) . A s a r e s u l t , much p r e l i m i n a r y work was u n n e c e s s a r y . T h e g e n e r a l t e c h n i q u e s had a l r e a d y been d e s c r i b e d a n d a number o f f a c t o r s t h a t had been shown t o h a v e - no e f f e c t c o u l d be i g n o r e d . T h e c e n t r a l e x p e r i m e n t s f o r t h i s s t u d y t h e r e f o r e c o u l d b e g i n i m m e d i a t e l y . 27 The experiments to be d e s c r i b e d here are complicated because they d e a l with at l e a s t t h r e e generations and o c c a s i o n a l l y more. To avoid c o n f u s i o n , a simple nomenclature i s used: the three g e n e r a t i o n s are c a l l e d grandparents, parents, and o f f s p r i n g . The grandparent was the aphid whose aging process over i t s r e p r o d u c t i v e p e r i o d was i n v e s t i g a t e d . The parents were e i t h e r e a r l y - b o r n or l a t e - b o r n , but were a l l d e a l t with a t the same ages. The o f f s p r i n g of the parents were monitored as e i t h e r apterae or a l a t a e . Wherever p o s s i b l e i n t h i s chapter, the three terms, grandparent, parent, and o f f s p r i n g are used e x c l u s i v e l y t o r e f e r to the experimental g e n e r a t i o n s . Other terms, such as mother, progeny, e t c . are used i n more gen e r a l d i s c u s s i o n s . The study asks the question : Does the age of a grandparent at the time of a parent's b i r t h a f f e c t the parent's a b i l i t y to respond to crowding by producing a l a t e o f f s p r i n g ? 3.2 M a t e r i a l s and Methods 3.2.1 L i f e h i s t o r y A£I£thosi£hon pisurn ( H a r r i s ) , the pea aphid, i s a widely d i s t r i b u t e d s p e c i e s found on many legumes, i n c l u d i n g the fodder crop, a l f a l f a . I t s l i f e c y c l e i s comparatively simple. In the s p r i n g , eggs overwintered on woody t i s s u e of the pr e v i o u s year hatch and the young aphids begin t o feed. The raorph hatching from the egg i s c a l l e d a f u n d a t r i x . T h i s morph reproduces p a r t h e n o g e n e t i c a l l y , g i v i n g b i r t h to l i v i n g apterous females. T h i s mode of r e p r o d u c t i o n c o n t i n u e s during the summer f o r many 28 g e n e r a t i o n s , t h e n u m b e r s o f w h i c h d e p e n d somewhat on l a t i t u d e . P e r i o d i c a l l y , f e m a l e s may g i v e b i r t h t o nymphs w h i c h a r e d e s t i n e d t o become a l a t a e . T h i s i s most o f t e n a r e s p o n s e e i t h e r t o c r o w d i n g o r t o h o s t - p l a n t d e t e r i o r a t i o n , b u t o t h e r f a c t o r s may a l s o be i n v o l v e d . In t h e f a l l , u n d e r t h e s t i m u l u s o f p h o t o p e r i o d i c and t e m p e r a t u r e c h a n g e s , t h e f e m a l e s b e g i n t o p r o d u c e s e x u a l e s , w h i c h i n c l u d e b o t h m a l e s and e g g - l a y i n g f e m a l e s . T h e s e x u a l e s mate a n d t h e f e m a l e s l a y o v e r w i n t e r i n g e g g s on t h e woody p a r t s o f l e g u m e s , o f t e n on t h e i r summer h o s t . P a r t h e n o g e n e t i c a l a t a e a n d a p t e r a e were t h e o n l y morphs u s e d i n t h i s s t u d y . B o t h go t h r o u g h f o u r n y m p h a l i n s t a r s b e f o r e r e a c h i n g a d u l t h o o d . U n d e r t h e c o n d i t i o n s o f t h i s s t u d y , r e p r o d u c t i o n i n t h e a p t e r a e b e g a n on t h e n i n t h day a f t e r b i r t h and c o n t i n u e d f o r a b o u t 14 d a y s . A f t e r t h i s p o i n t t h e a p h i d s o f t e n l i v e d and f e d f o r q u i t e l o n g p e r i o d s , o c c a s i o n a l l y e v e n g i v i n g b i r t h t o a nymph. 3 . 2 . 2 . R e a r i n g c o n d i t i o n s In a l l e x p e r i m e n t s t h e a p h i d s were r e a r e d on b r o a d b e a n s , V i c i a f a b a ( v a r . E x h i b i t i o n L o n g P o d ) . The p l a n t s were s t a r t e d u n d e r g r e e n h o u s e c o n d i t i o n s o f v a r y i n g t e m p e r a t u r e , l i g h t i n t e n s i t y , and p h o t o p e r i o d . T h e y were grown s i n g l y i n 4 - i n c h (10cm) p l a s t i c p o t s i n a m i x t u r e o f s a n d y s o i l and p e a t . U n d e r t h e s e c o n d i t i o n s t h e p l a n t s r e a c h e d t h e hook s t a g e and were r e a d y f o r u s e i n 9-11 d a y s a f t e r p l a n t i n g . By t h e t i m e t h e p l a n t s were 20 d a y s p a s t t h e h o o k s t a g e , t h e a p h i d s e i t h e r had b e e n t r a n s f e r r e d t o f r e s h y o u n g p l a n t s o r had b e e n c o u n t e d and 29 d i s c a r d e d w i t h t h e p l a n t s . T h e a p h i d s were t r a n s f e r r e d f r o m p l a n t t o p l a n t w i t h a f i n e , m o i s t e n e d p a i n t b r u s h . In u s e , e a c h p l a n t was i s o l a t e d w i t h i t s a p h i d s i n a c a g e , t h e l o w e r e d g e o f w h i c h was p r e s s e d down i n t o t h e s o i l . T h e c a g e was made f r o m a 3 2 - o u n c e (909cc) unwaxed c a r d b o a r d f r e e z e r c a r t o n w i t h i t s b o t t o m c u t o u t , a n d c l o s e d w i t h a c l e a r p l a s t i c l i d . T h e l i g h t i n t e n s i t y a t s o i l l e v e l i n s u c h c a g e s was a p p r o x i m a t e l y 100 f o o t - c a n d l e s , e n o u g h t o s u p p o r t h e a l t h y g r o w t h i n t h e y o u n g p l a n t s . The a p h i d s were r e a r e d and a l l e x p e r i m e n t s were r u n i n f o u r g r o w t h c h a m b e r s h e l d a t 2 0 ± 1 ° C w i t h a p h o t o p e r i o d o f 1 6 L:8D. U n d e r t h e s e c o n d i t i o n s t h e c l o n e r e m a i n e d i n t h e p a r t h e n o g e n e t i c mode o f r e p r o d u c t i o n . H u m i d i t y i n t h e c h a m b e r s was n e i t h e r c o n t r o l l e d n o r m o n i t o r e d , and may h a v e v a r i e d somewhat w i t h e x t e r n a l w e a t h e r c o n d i t i o n s . T h e c a g e s r e d u c e d v a r i a t i o n s , h o w e v e r , b e c a u s e t h e unwaxed c a r d b o a r d a l l o w e d m o i s t u r e t o e v a p o r a t e , t h u s a v o i d i n g e x c e s s i v e l y h i g h h u m i d i t i e s and c o n d e n s a t i o n p r o b l e m s . I t a l s o a c t e d a s a w i c k , m o v i n g m o i s t u r e up f r o m t h e s o i l t o t h e v i c i n i t y o f t h e l e a v e s , t h u s a v o i d i n g v e r y low h u m i d i t i e s . The p l a n t s were w e l l w a t e r e d when f i r s t c a g e d . F u r t h e r w a t e r i n g was r a r e l y n e c e s s a r y b e f o r e t h e p l a n t s were d i s c a r d e d a b o u t t h r e e weeks l a t e r , s h o w i n g t h a t t h e h u m i d i t y was c o n s i s t e n t l y m a i n t a i n e d a t an a c c e p t a b l y h i g h l e v e l . 30 3.2.3 Rearing techniques A s i n g l e parthenogenetic female was c o l l e c t e d i n September, 1972, from an a l f a l f a f i e l d on the OBC campus. I t was p l a c e d on a broad-bean p l a n t s e v e r a l weeks o l d and kept at 20°C and 16L:8D to found the experimental c l o n e . The i n i t i a l p o p u l a t i o n of aphids was allowed t o grow unhindered. P e r i o d i c a l l y , s e v e r a l nymphs were t r a n s f e r r e d to a h e a l t h y u n i n f e s t e d pla n t and the remainder were d i s c a r d e d . Onder these c o n d i t i o n s , a few winged aphids were produced p r i o r to each r e - e s t a b l i s h m e n t on a new p l a n t . In January, 1973, a more r e g u l a r regime was i n i t i a t e d i n p r e p a r a t i o n f o r the experiments. Onless otherwise s p e c i f i e d , a l l p l a n t s used from t h i s p o i n t on were at the hook stage and were caged as d e s c r i b e d . To e s t a b l i s h the experimental stock c u l t u r e (Figure 1), a s i n g l e f o u r t h - i n s t a r nymph was i s o l a t e d on a p l a n t and l e f t to reproduce. I t was allowed to reproduce f o r two days, during which time i t bore about 20 nymphs. I t was then t r a n s f e r r e d to a f r e s h p l a n t and allowed to reproduce f o r another two days, b e a r i n g another 20 nymphs. When these 10 aphids had reached the stage of f o u r t h - i n s t a r nymphs or t e n e r a l a d u l t s , 20 of them were i s o l a t e d on f r e s h p l a n t s . F i v e aphids were then s e l e c t e d from these a c c o r d i n g to t h e i r development, so that each was separated by one day's development from i t s nearest s i b . These f i v e were allowed t o reproduce f o r two days before being removed from t h e i r p l a n t s . The l a s t two females were t r a n s f e r r e d to f r e s h p l a n t s i n s t e a d of being d i s c a r d e d , and were allowed to reproduce 31a F i g u r e 1. Method f o r o b t a i n i n g a stock c u l t u r e of hs. £i§fii t from one i n d i v i d u a l , which would y i e l d a d a i l y supply of newly-matured f i r s t - b o r n aphids. Day 1 i s the f i r s t day of r e p r o d u c t i o n of the foundress. (0 - a d u l t aphid, o - j u v e n i l e aphid) 31 Days !_ 4 O - © O - © : - O © O O - 0 - Q ® days ] & 2 days 3 & 4 10- 12 0-^ © o - o - o o 11- i3 0 ^ ( D O — O — O O 12- iA 0-* © O - O - O O 13- i7 0 ^ © O — O - - - O O ® 0 - - 0 - - - 0 0 w-18 0— ©o-o-oo® o—o—OO 19- 2 i 0-^ © O - O - O O 20- 22 0 ^ © O - O - O O 21- 23 0-> © O - O - O O 22- 24 0-> © O - O - O O 23- 25 0 ^ © O - O - O O 24- 28 0-> © O — O - - - O O ® O - O - O O 25- 29 0 ^ © O - - - O — O O 0 O - O - O O 28- 30 0 ^ © O - O - O O 29- 31 © O - O - O O 30- 32 0-^ © 0 - - - 0 - - - 0 0 31- 33 0-^ © O - O - O O 32- 34 0-* © O - O - O O 33- 35 © O - O - O O 34- 36 0-^ © O - O - O O 35- 37 0-> © O - O - O O 36- 38 0-^ © O - O - O O 32 t h e r e f o r a f u r t h e r two d a y s . T h i s a r r a n g e m e n t r e s u l t e d i n s e v e n l i n e s o f a p h i d s . I n t h e n e x t g e n e r a t i o n , one a p h i d f r o m e a c h l i n e was c h o s e n t o p e r p e t u a t e i t , i s o l a t e d on a f r e s h p l a n t , and l e f t t o r e p r o d u c e f o r two d a y s . T h e f e m a l e s f r o m t h e l a s t two l i n e s o n c e a g a i n were a l l o w e d t o r e p r o d u c e f o r an e x t r a two d a y s . T h i s p r o c e d u r e r e s u l t e d i n n i n e s e p a r a t e l i n e s o f a p h i d s f r o m a s i n g l e common a n c e s t o r , a l l e s t a b l i s h e d by a p h i d s b o r n w i t h i n t h e f i r s t f o u r d a y s o f t h e r e p r o d u c t i v e p e r i o d , and e a c h one d a y a p a r t i n d e v e l o p m e n t . B e c a u s e o f t h e d e v e l o p m e n t a l t i m e o f t h e a p h i d s a t 2 0 ° C , n i n e l i n e s were s u f f i c i e n t t o p r o v i d e one p l a n t w i t h a p p r o x i m a t e l y 20 m a t u r i n g a p h i d s e v e r y d a y . T h e r e a f t e r , e a c h d a y , one t e n e r a l a d u l t was s e l e c t e d f r o m t h e a p p r o p r i a t e l i n e , i s o l a t e d on a f r e s h p l a n t and a l l o w e d t o r e p r o d u c e f o r two d a y s , g i v i n g b i r t h t o a b o u t 20 n y m p h s . T h i s m a i n t a i n e d t h e s t o c k c u l t u r e . H e r s i s t e r s were e i t h e r u s e d f o r a n e x p e r i m e n t o r d i s c a r d e d . T h e r e w a s , o f c o u r s e , c o n s i d e r a b l e o v e r l a p i n t h e b i r t h t i m e s o f t h e a p h i d s f r o m a d j a c e n t l i n e s . B u t a s l i g h t r a n g e i n a g e was o f no c o n s e q u e n c e t o t h e s u b s e q u e n t e x p e r i m e n t s . W i t h a l i t t l e e x p e r i e n c e , i t was a s i m p l e m a t t e r t o p i c k a p h i d s a t t h e c o r r e c t s t a g e t o k e e p t h e s t o c k - c u l t u r e l i n e s i n p r o p e r p h a s e . A f t e r s e t t i n g up t h e n i n e l i n e s , and b e f o r e u s i n g a n y o f t h e a p h i d s f r o m t h e s t o c k c u l t u r e f o r e x p e r i m e n t a l p u r p o s e s , a l l l i n e s were a l l o w e d t o r u n t h r o u g h a t l e a s t one g e n e r a t i o n . T h i s e n s u r e d t h a t a l l a p h i d s u s e d t o s t a r t an e x p e r i m e n t were among t h e i r m o t h e r ' s f i r s t 20 n y m p h s , a n d t h a t a t l e a s t t h e m o t h e r s o f 33 t h e s e a p h i d s , i f n o t t h e i r g r a n d m o t h e r s and g r e a t - g r a n d m o t h e r s , were a l s o f r o m t h e f i r s t 20 nymphs o f t h e i r r e s p e c t i v e g e n e r a t i o n s . M o r t a l i t y i n t h e s t o c k c u l t u r e was u s u a l l y a b o u t 10%, b u t o c c a s i o n a l l y r o s e a s h i g h a s 20% d u r i n g t h e n y m p h a l p e r i o d . The c a u s e o f t h i s m o r t a l i t y i s u n k n o w n . T h e r e was low m o r t a l i t y i n t h e e a r l y a d u l t p e r i o d . B u t s i n c e o n l y a p p a r e n t l y h e a l t h y a d u l t s were c h o s e n t o p e r p e t u a t e t h e i r l i n e s , m o r t a l i t y n e v e r i n t e r f e r e d w i t h m a i n t e n a n c e o f t h e s t o c k c u l t u r e . U n d e r t h e h o l d i n g c o n d i t i o n s , t h e s t o c k c u l t u r e c o n t i n u e d t o p r o d u c e a p t e r o u s p a r t h e n o g e n e t i c a p h i d s . T h r o u g h o u t t h e l i f e o f t h e c l o n e , a l a t a e a p p e a r e d i n o n l y f i v e o f t h e more t h a n 250 s e p a r a t e u n i t s u t i l i z e d . (Each u n i t c o n s i s t e d o f a p l a n t w i t h 20 a p h i d s . ) T h i s low f i g u r e i n d i c a t e s t h a t t h e a p h i d s u s u a l l y were r e m o v e d f r o m t h e s t o c k - c u l t u r e p l a n t s and i s o l a t e d on f r e s h p l a n t s p r i o r t o t h e b u i l d - u p o f a s i g n i f i c a n t c r o w d i n g s t i m u l u s . On t h e few o c c a s i o n s when a l a t a e d i d a p p e a r i n t h e s t o c k c u l t u r e , t h a t l i n e was n o t u s e d a s a s o u r c e o f e x p e r i m e n t a l a n i m a l s , b u t was d i s c a r d e d . The b l a n k l e f t i n t h e s t o c k c u l t u r e was t h e n f i l l e d by a l a t e nymph f r o m t h e p r e c e d i n g l i n e o r an e a r l y nymph f r o m t h e f o l l o w i n g l i n e . T h i s a r r a n g e m e n t o f t h e s t o c k c u l t u r e p r o v i d e d a d a i l y s u p p l y o f a p h i d s w i t h b a c k g r o u n d s a s n e a r l y i d e n t i c a l as p o s s i b l e . H o w e v e r , i t d i d c o n s i s t o f e s s e n t i a l l y n i n e s e p a r a t e b u t p a r a l l e l c l o n e s . In o r d e r t o m i n i m i z e t h e p o s s i b i l i t y o f d i f f e r e n c e s a r i s i n g among t h e s e c l o n e s t h r o u g h random g e n e t i c d r i f t , t h e s t o c k c u l t u r e was r e - e s t a b l i s h e d c o m p l e t e l y f r o m one 34 i n d i v i d u a l p r i o r t o e a c h m a j o r s e t o f e x p e r i m e n t s ( J a n u a r y 1 9 7 3 , A p r i l 1973 , and O c t o b e r 1 9 7 3 ) . 3 . 2 . 4 E x p e r i m e n t a l d e s i g n The m a j o r p r o b l e m i n d e s i g n i n g an e x p e r i m e n t w h i c h i n v e s t i g a t e s some a s p e c t o f a g i n g i n v o l v e s t h e p a s s a g e o f t i m e . I t i s n o t p o s s i b l e t o c o m p a r e e a r l y - and l a t e - b o r n y o u n g o f t h e same f e m a l e s i m u l t a n e o u s l y . Any d i f f e r e n c e s f o u n d c o u l d p o s s i b l y be a t t r i b u t e d t o e x t e r n a l i n f l u e n c e s s u c h a s c h a n g i n g w e a t h e r o r h o s t p l a n t c h a r a c t e r i s t i c s , a s w e l l a s t o i n t e r n a l d i f f e r e n c e s i n t h e m o t h e r a s s h e a g e s . A s i t i s i m p o s s i b l e t o be c e r t a i n t h a t a l l s u c h t e m p o r a l g r a d i e n t s h a v e b e e n e l i m i n a t e d , t h e e x p e r i m e n t s must be d e s i g n e d t o e x p o s e any t h a t m i g h t r e m a i n . T h u s a l l e x p e r i m e n t s were s e t up a s a s e r i e s o f r e p l i c a t e s s t a g g e r e d i n t i m e i n a s t e p w i s e m a n n e r . The f o l l o w i n g i s a d e s c r i p t i o n o f one s u c h r e p l i c a t e ( F i g u r e 2 ) . A g r a n d p a r e n t was i s o l a t e d on a p l a n t a s a l a t e f o u r t h - i n s t a r nymph o r a t e n e r a l a d u l t . A d a i l y r e c o r d was k e p t o f t h e number o f nymphs i t b o r e f r o m t h e o n s e t o f r e p r o d u c t i o n u n t i l i t s d e a t h . The g r a n d p a r e n t was t r a n s f e r r e d r e g u l a r l y t o new p l a n t s i n o r d e r t o o b t a i n f o u r t r e a t m e n t b a t c h e s o f p a r e n t a p h i d s b o r n a t d i f f e r e n t t i m e s d u r i n g i t s r e p r o d u c t i v e p e r i o d . A s a r u l e , b a t c h I p a r e n t s were b o r n on d a y s 1 and 2 o f t h e g r a n d p a r e n t ' s r e p r o d u c t i v e p e r i o d , b a t c h I I on d a y s 4 and 5 , b a t c h I I I on d a y s 7 and 8 , and b a t c h IV on d a y s 10 and 11 . O c c a s i o n a l l y b a t c h e s had t o be a c c u m u l a t e d f r o m a s l i g h t l y l o n g e r p e r i o d i f r e p r o d u c t i o n s l o w e d u n e x p e c t e d l y . T h i s 35a Figure 2. Sequence of events f o r one experimental u n i t f o r t e s t i n g the e f f e c t of g r a n d p a r ental age on p a r e n t a l a b i l i t y to produce a l a t e o f f s p r i n g . 35 B a t c h I B a t c h I I B a t c h I I I B a t c h IV Day 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 g r a n d p a r e n t s i s o l a t e d ^ , b a t c h I 4r"-~~~~^  / p a r e n t s b o r n . / b a t c h I I p a r e n t s b o r n p a r e n t s i s o l a t e d b a t c h I I I p a r e n t s b o r n p a r e n t s c r o w d e d p a r e n t s i s o l a t e d • p a r e n t s c r o w d e d o f f s p r i n g g r o u p c o m p l e t e b a t c h IV p a r e n t s b o r n 1 p a r e n t s i s o l a t e d I p a r e n t s c r o w d e d o f f s p r i n g g r o u p c o m p l e t e o f f s p r i n g c o u n t e d o f f s p r i n g g r o u p c o m p l e t e o f f s p r i n g c o u n t e d p a r e n t s i s o l a t e d I p a r e n t s c r o w d e d o f f s p r i n g g r o u p c o m p l e t e o f f s p r i n g c o u n t e d o f f s p r i n g c o u n t e d 36 c o m p r o m i s e a f f e c t e d m a i n l y b a t c h I V , w h i c h o f t e n had t o be t a k e n f r o m day 12 a s w e l l a s f r o m d a y s 10 and 1 1 . at t h i s s t a g e e a c h b a t c h c o n s i s t e d o f a t l e a s t 15 a p h i d s w h i c h were r e a r e d t o g e t h e r o n one p l a n t . T h e p a r e n t a p h i d s were r e a r e d i n g r o u p s r a t h e r t h a n s i n g l y i n o r d e r t o s a v e s p a c e , s i n c e S u t h e r l a n d (1969a) showed t h a t i n £ i s u m t h e a l a t e / a p t e r o u s p o l y m o r p h i s m i s d e t e r m i n e d j u s t p r i o r t o an a p h i d ' s b i r t h . T h u s s u b - a d u l t s a r e i n s e n s i t i v e t o c r o w d i n g s t i m u l i . As e a c h g r o u p o f 15 p a r e n t s a p p r o a c h e d m a t u r i t y , 10 h e a l t h y s p e c i m e n s were s e l e c t e d and i s o l a t e d on 10 f r e s h p l a n t s and t h e n l e f t t o b e g i n r e p r o d u c t i o n . T h e t e r m " b a t c h " r e f e r s t o t h i s s m a l l e r g r o u p o f 10 p a r e n t a p h i d s s e l e c t e d t o be c r o w d e d r a t h e r t h a n t h e o r i g i n a l g r o u p o f 15 t h a t were r e a r e d t o g e t h e r . T h e 10 p l a n t s w i t h t h e i r a p h i d s were p l a c e d t o g e t h e r on a t r a y and a s s i g n e d r a n d o m l y t o p o s i t i o n s i n t h e g r o w t h c h a m b e r s . A s t a n d a r d c r o w d i n g s t i m u l u s , w h i c h w i l l be d e s c r i b e d l a t e r , was i m p o s e d two d a y s l a t e r , when a l l had b e g u n r e p r o d u c t i o n . A t t h i s t i m e , e a c h p a r e n t o f a p a r t i c u l a r b a t c h m i g h t h a v e p r o d u c e d 1-25 n y m p h s , i n d i c a t i n g c o n s i d e r a b l e v a r i a t i o n i n t h e age o f t h e p a r e n t s w i t h i n e a c h b a t c h . B u t t h e r e was no s i g n i f i c a n t v a r i a t i o n i n mean number o f nymphs b e t w e e n b a t c h e s . The o f f s p r i n g b o r n t o a l l 10 p a r e n t a p h i d s b e f o r e t h e c r o w d i n g s t i m u l u s were p l a c e d t o g e t h e r on two p l a n t s and r e a r e d u n t i l t h e y were o l d e n o u g h t o be i d e n t i f i e d a s e i t h e r a p t e r o u s o r a l a t e . T h i s p r e c a u t i o n was n e c e s s a r y t o v e r i f y t h a t t h e p a r e n t a p h i d s were n o t i n a d v e r t e n t l y c r o w d e d p r i o r t o t h e e x p e r i m e n t a l c r o w d i n g . D u r i n g a l l t h e e x p e r i m e n t s , t h e r e were 37 o n l y f i v e o f 80 b a t c h e s i n w h i c h one o r two a l a t a e a p p e a r e d among t h e p r e - c r o w d i n g o f f s p r i n g . . T h e s e few a n o m a l o u s i n d i v i d u a l s a p p e a r e d among a l l f o u r t r e a t m e n t s , b a t c h e s I, I I , I I I , and I V , and seemed t o h a v e no e f f e c t on s u b s e q u e n t r e s p o n s e s t o c r o w d i n g . A f t e r t h e s t a n d a r d c r o w d i n g s t i m u l u s , t h e p a r e n t a p h i d s were r e t u r n e d t o t h e i r p l a n t s , r e p l a c e d i n t h e same p o s i t i o n i n t h e g r o w t h c h a m b e r s , and a l l o w e d t o r e p r o d u c e f o r f o u r d a y s . I n d i c a t i o n s f r o m p r e l i m i n a r y e x p e r i m e n t s and f r o m S u t h e r l a n d ' s (1969a) work were t h a t a l l a p h i d s c o u l d be e x p e c t e d t o h a v e c e a s e d p r o d u c i n g a l a t a e a s a r e s u l t o f t h e e x p e r i m e n t a l c r o w d i n g s t i m u l u s a f t e r f o u r d a y s , a n d t o h a v e r e t u r n e d t o p r o d u c i n g o n l y a p t e r o u s o f f s p r i n g . T h e p a r e n t a p h i d s were r e m o v e d a n d d i s c a r d e d a t t h a t t i m e . T h e i r o f f s p r i n g were l e f t on t h e p l a n t s and k e p t f o r s i x d a y s , o r u n t i l t h e y c o u l d be i d e n t i f i e d a s a p t e r a e o r a l a t a e . T h e n a r e c o r d was made o f t h e t o t a l number o f s u r v i v i n g o f f s p r i n g f r o m e a c h p a r e n t and t h e number t h a t were a l a t e . A l l t h e o f f s p r i n g were t h e n d i s c a r d e d . A l l o f f s p r i n g o f t h e p a r e n t g e n e r a t i o n were b o r n u n d e r c o n t r o l l e d c o n d i t i o n s , b u t were r e a r e d a t o r d i n a r y room t e m p e r a t u r e s and l i g h t i n t e n s i t i e s . T h i s a r r a n g e m e n t was n e c e s s a r y t o s a v e s p a c e and was p o s s i b l e b e c a u s e , i n A._ p i s u m , nymphs a r e d e t e r m i n e d a s a l a t e o r a p t e r o u s p r i o r t o b i r t h . T h e c o n d i t i o n s u n d e r w h i c h an i n d i v i d u a l i s r e a r e d do n o t a f f e c t i t s own morph ( S u t h e r l a n d , 1 9 6 9 a ) . The c r o w d i n g s t i m u l u s i n t h e s e e x p e r i m e n t s was e s s e n t i a l l y t h a t u s e d by S u t h e r l a n d (1969a) when i n v e s t i g a t i n g t h e e f f e c t o f 38 c r o w d i n g on w ing p r o d u c t i o n i n A . £ i s u m . H i s method c o n s i s t e d o f i s o l a t i n g a number o f a p h i d s i n a 1 x 2 - i n c h (2 .5x5cm) g l a s s v i a l f o r a g i v e n p e r i o d and t h e n r e t u r n i n g them t o p l a n t s t o r e p r o d u c e . He f o u n d t h a t s o l i t a r y a p h i d s p l a c e d i n s u c h a v i a l d i d n o t p r o d u c e a l a t a e . T h e r e f o r e , s t a r v a t i o n a l o n e was n o t s u f f i c i e n t t o i n i t i a t e a l a t a - p r o d u c t i o n . From o t h e r w o r k , he c o n c l u d e d t h a t i t was t h e a p h i d / a p h i d e n c o u n t e r s c a u s i n g s t i m u l a t i o n o f m e c h a n o - r e c e p t o r s , p a r t i c u l a r i l y on t h e l e g s and a n t e n n a e , t h a t c a u s e d a l a t a - p r o d u c t i o n . He t e s t e d a p h i d s a t a number o f d i f f e r e n t d e n s i t i e s f o r v a r y i n g p e r i o d s o f t i m e , and o b t a i n e d a s e r i e s o f c u r v e s f o r r e s p o n s e t o c r o w d i n g . He o b t a i n e d c l o s e t o a maximum r e s p o n s e by c r o w d i n g 10 a p h i d s f o r 24 h o u r s a t 1 5 ° C . The e x p e r i m e n t s p r e s e n t e d h e r e were a l l r u n a t 2 0 ° C . D e v e l o p m e n t a l t i m e s a t 2 0 ° C were a b o u t t w o - t h i r d s what t h e y were a t 1 5 ° C . T h u s one m i g h t e x p e c t t o o b t a i n t h e maximum r e s p o n s e a t 2 0 ° C by c r o w d i n g 10 a p h i d s f o r 12 -16 h o u r s . I t was f e l t t h a t a s t i m u l u s somewhat l e s s t h a n m a x i m a l m i g h t show any d i f f e r e n c e s i n r e s p o n s e more c l e a r l y . A few c o m p a r a t i v e t e s t s were r u n by c r o w d i n g 10 a p h i d s f o r e i g h t and 12 h o u r s . T h e r e a p p e a r e d t o be no d i f f e r e n c e b e t w e e n t h e two t r e a t m e n t s . T h e r e f o r e e i g h t h o u r s o f c r o w d i n g f o r 10 a p h i d s was c h o s e n a s t h e s t a n d a r d s t i m u l u s . The c r o w d i n g s t i m u l u s u s e d t h r o u g h o u t t h e e x p e r i m e n t s t h u s c o n s i s t e d o f g r o u p i n g 10 a d u l t a p h i d s i n a 1 x 2 - i n c h p l a s t i c v i a l i n a g r o w t h c h a m b e r a t 2 0 ° C f o r e i g h t h o u r s . The p o s i t i o n o f t h e v i a l i n t h e g r o w t h c h a m b e r was s t a n d a r d i z e d . The c r o w d i n g was a l w a y s s t a r t e d b e t w e e n 11 :00 and 1 1 : 3 0 am and e n d e d b e t w e e n 7 :00 39 and 7:30 pm. Each batch of parents was crowded f o r 8h±2.5tnin. C e r t a i n m o d i f i c a t i o n s were made i n t h i s b a s i c procedure f o r d i f f e r e n t experiments. These m o d i f i c a t i o n s w i l l be d e s c r i b e d with the r e s u l t s . In order to expose any g r a d i e n t s over time that might confound the r e s u l t s , the b a s i c procedure was r e p l i c a t e d i n a stepwise f a s h i o n . Thus, when one grandparent had completed batch I and began to produce batch I I , another grandparent was e s t a b l i s h e d to begin producing batch I, and so on. The f i n a l forms f o r each experiment were s l i g h t l y d i f f e r e n t , and w i l l be d e s c r i b e d and diagrammed with the r e s u l t s . 3.3 R e s u l t s 3.3. 1 Reproductive p a t t e r n s of apterae and a l a t a e Records were kept of the d e t a i l e d r e p r o d u c t i v e h i s t o r y of both apterae and a l a t a e during v a r i o u s p r e l i m i n a r y t e s t s and while the experimental batches were being produced. A number of r e c o r d s of l i f e - h i s t o r y data and of the r e p r o d u c t i v e p a t t e r n s of JLt Ei§um apterae are i n the l i t e r a t u r e . There are a l s o l i f e t a b l e s f o r the apterae of a number of other aphid s p e c i e s . However, there appears to be no l i f e t a b l e p u b l i s h e d f o r a l a t a e of any s p e c i e s . The data from t h i s study w i l l allow comparison of the r e p r o d u c t i v e p a t t e r n s of the two morphs. Twenty-six apterous and 13 a l a t e aphids were followed d a i l y from the onset of r e p r o d u c t i o n u n t i l t h e i r deaths. Figure 3 g i v e s the a g e - s p e c i f i c f e c u n d i t y (ox) of these females. F i g u r e 4 g i v e s t h e i r a g e - s p e c i f i c s u r v i v a l ( l x ) . I t should be pointed out 40a F i g u r e 3. Age s p e c i f i c - f e c u n d i t y of apterous and a l a t e parthenogenetic aphids. 40 ADp/6/66 _ (*w) Ajjpuroaj. D i j !>ads-a6 y 41a F i g u r e 4. A g e - s p e c i f i c s u r v i v a l o f a p t e r o u s and a l a t e p a r t h e n o g e n e t i c a p h i d s . Age-specif ic survival 42 t h a t t h e c u r v e s i n F i g u r e 4 b e g i n a f t e r t h e f i n a l m o u l t . M o r t a l i t y f i g u r e s p r i o r t o t h i s t i m e were n o t r e c o r d e d . As a r e s u l t , i t h a s h o t been p o s s i b l e t o p r o d u c e c o m p l e t e l i f e t a b l e s f o r e i t h e r m o r p h , and s t a t i s t i c s s u c h a s r m , t h e i n t r i n s i c r a t e o f i n c r e a s e , o r R o , t h e n e t r e p r o d u c t i v e r a t e , c a n n o t be c a l c u l a t e d . From r e c o r d s k e p t o f t h e s t o c k c u l t u r e , t h e p r e - a d u l t m o r t a l i t y o f a p t e r a e c a n be e s t i m a t e d a s a p p r o x i m a t e l y 10% u n d e r t h e r e a r i n g c o n d i t i o n s . P r e - a d u l t m o r t a l i t y among t h e a l a t a e i s more d i f f i c u l t t o e s t i m a t e . A l a t a e c a n n o t be d i s t i n g u i s h e d f r o m a p t e r a e u n t i l a t l e a s t t h e t h i r d i n s t a r , and n o t e a s i l y u n t i l t h e f o u r t h . C o n s e q u e n t l y , no r e c o r d was k e p t o f t h e m o r t a l i t y on p l a n t s f r o m w h i c h a l a t a e were o b t a i n e d . T h e r e a p p e a r e d t o be a f a i r l y h i g h m o r t a l i t y o f t h e a l a t a e a r o u n d t h e p e r i o d o f t h e f i n a l m o u l t . T h e r e was a l s o some m o r t a l i t y v e r y e a r l y i n t h e r e p r o d u c t i v e p e r i o d . H o w e v e r , a n y a l a t a w h i c h s u r v i v e d and r e p r o d u c e d n o r m a l l y f o r t h e f i r s t day o r two o f r e p r o d u c t i o n , c o n t i n u e d t o r e p r o d u c e f o r many d a y s . T h i s s h a r p b r e a k i n m o r t a l i t y s u g g e s t e d t h a t t h e e a r l y a d u l t m o r t a l i t y was a s s o c i a t e d w i t h t h e m o u l t o r t h e t e n e r a l p e r i o d i n some way. F i g u r e s 3 and 4 t h e r e f o r e i n c l u d e o n l y a l a t a e w h i c h r e p r o d u c e d n o r m a l l y f o r more t h a n t h r e e d a y s . T h i s r e s t r i c t i o n makes t h e c u r v e s f o r a l a t a e and a p t e r a e c o m p a r a b l e s i n c e n e i t h e r i n c l u d e s m o r t a l i t y r e s u l t i n g f r o m e v e n t s p r i o r t o t h e o n s e t o f r e p r o d u c t i o n . The b a s i c c h a r a c t e r i s t i c s o f t h e r e p r o d u c t i v e h i s t o r y o f t h e a p t e r a e c a n be o b t a i n e d f r o m F i g u r e s 3 a n d 4 , and T a b l e 1. 43 T a b l e 1. D e m o g r a p h i c d a t a f o r a p t e r o u s and a l a t e p a r t h e n o g e n e t i c JU £ i s u m f e m a l e s 1 , w i t h c o m p a r a t i v e d a t a f r o m F r a z e r (1972a) f o r a p t e r a e 2 . A p t e r a e A l a t a e A p t e r a e ( F r a z e r , 72) P r e - r e p r o d u c t i v e p e r i o d (days) 8 . 23 r a n g e , 8 - 9 10 7. 5 7 ± . 1 7 R e p r o d u c t i v e p e r i o d 3 (days) 1 4 . 8 1 ± 2 . 14 1 7 . 0 8 ± 3 . 15 11 . 2 7 ± 2 . 3 5 P o s t - r e p r o d u c t i v e p e r i o d (days ) 6 . 2 7 ± 4 . 03 4 . 3 8 + 4 . 07 6. 16+4.85 T o t a l l i f e s p a n (days ) 2 9 . 3 1 ± 3 . 90 3 1 . 4 6 ± 4 . 77 F e c u n d i t y * ( l a r v a e ) 9 5 . 1 2 ± 8 . 71 8 6 . 2 3 H 0 . 3 0 9 6 . 7 2 ± 2 2 . 9 2 Maximum mx ( e s t i m a t e d f r o m c u r v e s ) 1 0 . 0 7 . 8 11. 8 1. ± 1 s t a n d a r d d e v i a t i o n 2 . ± 1 s t a n d a r d e r r o r 3 . A p t e r a e and a l a t a e s i g n i f i c a n t l y d i f f e r e n t , , 0 1 0 < p < . 0 2 5 4 . A p t e r a e and a l a t a e s i g n i f i c a n t l y d i f f e r e n t , . 0 0 5 < p < . 0 1 0 44 i t i s p o s s i b l e to compare these data with s i m i l a r data c o l l e c t e d f o r 2U £isum apterae a t 20°C by C a r t i e r (1960), S y l v e s t e r and Richardson (1966), Murdie (1969a, 1969b), F r a z e r (1972a), and S i d d i g u i , Barlow and Randolph (1973). These data were based on c l o n e s e s t a b l i s h e d from d i f f e r e n t p a r t s of North America, and from England, and reared on a v a r i e t y of host p l a n t s . Some of F r a z e r ' s data are g i v e n i n Table 1 f o r comparison. His clone was c o l l e c t e d i n the v i c i n i t y of the one used here. Data on the other c l o n e s mentioned above were presented i n d i f f e r e n t forms, making d i r e c t comparisons d i f f i c u l t . However, some g e n e r a l i t i e s can be e x t r a c t e d . From the p o i n t of view of l i f e - h i s t o r y s t r a t e g i e s , a number of parameters are p a r t i c u l a r i l y i n t e r e s t i n g . Among these are: the time to f i r s t r e p r o d u c t i o n ; mean f e c u n d i t y ; and t h e ' maximum value f o r mean d a i l y f e c u n d i t y . The g e n e r a l shape of the mx curve (Figure 3) i s a l s o important. In s p i t e of being c o l l e c t e d from the same ge n e r a l area, F r a z e r ' s and my c l o n e s e x h i b i t e d c o n s i d e r a b l e d i f f e r e n c e s (Table 1 ) . F r a z e r ' s clone began and ceased r e p r o d u c t i o n e a r l i e r , although i t s mean f e c u n d i t y was roughly the same, due to a higher maximumum mean d a i l y f e c u n d i t y . C o n s i d e r i n g a l l c l o n e s t o g e t h e r , the p r e - r e p r o d u c t i v e p e r i o d v a r i e d from 7,57 days (Frazer, 1972a) to 8.9 days ( C a r t i e r , 1960). Mean f e c u n d i t y v a r i e d from 83.7 $>oyo ( S i d d i g u i and Barlow, 1973) to 117.0 o_o/$ (Murdie, 1969b). Maximum mean d a i l y f e c u n d i t y v a r i e d from approximately 8 <j>o/oyday ( S i d d i g u i and Barlow, 1973) to approximately 12 <j>o/o/day (Murdie, 1 9 6 9 ) . These d i f f e r e n c e s may 45 h a v e been due t o c l o n a l d i f f e r e n c e s r e s u l t i n g f r o m d i f f e r e n t b i o t y p e s ( F r a z e r , 1 9 7 2 b ) , o r t h e y may have b e e n due t o s l i g h t d i f f e r e n c e s i n r e a r i n g c o n d i t i o n s . But i n s p i t e o f t h e v a r i a t i o n , t h e mx c u r v e s e x h i b i t a g e n e r a l p a t t e r n c o n s i s t e n t f r o m c l o n e t o c l o n e . R e p r o d u c t i v e r a t e s t e n d t o r i s e g u i c k l y a t f i r s t , m a i n t a i n i n g a h i g h l e v e l f o r s i x t o e i g h t d a y s , t h e n f a l l i n g r e l a t i v e l y r a p i d l y . L i f e t a b l e s a r e a v a i l a b l e a t 2 0 ° C f o r t h e a p t e r o u s morph o f a few o t h e r s p e c i e s o f a p h i d s , a n d i t i s i n t e r e s t i n g t o c o m p a r e t h e s h a p e s o f t h e i r a g e - s p e c i f i c f e c u n d i t y c u r v e s w i t h t h a t o f A p i s u m . Myzus p _ e r s i c a e ( B a r l o w , 1962) a n d A p h i s f a b a e ( F r a z e r , 1972a) a r e v e r y s i m i l a r t o JU p i s u m , b u t t h e f e c u n d i t y o f A_. f a b a e d r o p s o f f somewhat more s l o w l y . J J a c r o s i p h u m e u p h o r b i a e ( B a r l o w , 1962) and T h e r i o a p h i s m a c u l a t a ( M e s s e n g e r , 1964) h a v e l o n g e r r e p r o d u c t i v e p e r i o d s and mx c u r v e s t h a t r i s e t o a l o n g p l a t e a u r a t h e r t h a n a s h a r p p e a k . Some c o m p a r i s o n s c a n be made b e t w e e n t h e a p t e r a e and a l a t a e i n my c l o n e . F i g u r e 4 and T a b l e 1 show t h a t t h e a l a t a e l i v e d s l i g h t l y l o n g e r t h a n t h e a p t e r a e , h a v i n g l o n g e r p r e - r e p r o d u c t i v e and r e p r o d u c t i v e p e r i o d s . The e x p l a n a t i o n f o r t h e l o n g e r p r e - r e p r o d u c t i v e p e r i o d l i e s i n t h e w i n g e d c o n d i t i o n and m i g r a t o r y t e n d e n c y o f t h e m o r p h . The a l a t a e m o u l t e d t o a d u l t s a t r o u g h l y t h e same t i m e as t h e a p t e r a e , b u t t h e y d i d n o t b e g i n t o b e a r y o u n g i m m e d i a t e l y . I n s t e a d , t h e i r b o d y v o l u m e a n d w e i g h t d e c r e a s e d c o n s i d e r a b l y , p r o b a b l y t h r o u g h w a t e r l o s s . T h e y became r e s t l e s s , a n d w a n d e r e d a n d f l e w a b o u t t h e c a g e s , e v e n t h o u g h t h e y were p r o v i d e d w i t h 46 h e a l t h y y o u n g p l a n t s . T h i s p h a s e l a s t e d f o r one o r two d a y s . E v e n t u a l l y t h e a l a t a e showed what h a s b e e n c a l l e d t h e s e t t l i n g r e s p o n s e ( J o h n s o n , 1 9 5 8 ) . T h e y b e g a n t o f e e d on t h e p l a n t s p r o v i d e d , and s o o n b o r e t h e i r f i r s t y o u n g . J o h n s o n (1957) s t u d i e d t h i s phenomenon i n a number o f s p e c i e s o f a p h i d s . The l e n g t h o f t h e m i g r a t o r y p h a s e a p p e a r s t o v a r y , d e p e n d i n g on w h e t h e r a s u i t a b l e h o s t p l a n t i s p r o v i d e d . In a l l t h e s p e c i e s J o h n s o n e x a m i n e d , t h e w i n g m u s c l e s b e g a n d e g e n e r a t i n g o n c e t h e m i g r a t o r y p h a s e was o v e r , s o t h a t f u r t h e r f l i g h t was s o o n i m p o s s i b l e . Some s p e c i e s were a b l e t o b e g i n r e p r o d u c t i o n p r i o r t o a u t o l y s i s o f t h e w i n g m u s c l e s . O t h e r s d e l a y e d r e p r o d u c t i o n u n t i l a f t e r f l i g h t had become i m p o s s i b l e . The r e p r o d u c t i v e p e r i o d o f JU £ i s u m a l a t a e was c o n s i d e r a b l y l o n g e r t h a n t h a t o f t h e a p t e r a e . T h e s h a p e s o f t h e age s p e c i f i c f e c u n d i t y c u r v e s were a l s o d i f f e r e n t ( F i g u r e 3 ) . T h e f e c u n d i t y o f t h e a p t e r a e r o s e t o a p e a k v a l u e and f e l l o f f q u i c k l y . T h a t o f t h e a l a t a e a l s o r o s e s h a r p l y , b u t t o a l o w e r l e v e l w h i c h was m a i n t a i n e d f o r 11 d a y s i n t h e a l a t a e a s c o m p a r e d w i t h s i x i n t h e a p t e r a e . In s p i t e o f t h e l o n g e r r e p r o d u c t i v e p e r i o d , a l a t a e p r o d u c e d f e w e r y o u n g t h a n a p t e r a e . K e n n e d y a n d S t r o y a n (1959) , i n t h e i r r e v i e w o f a p h i d b i o l o g y d i s c u s s e d t h e d i v i s i o n o f l a b o u r b e t w e e n t h e d i f f e r e n t m o r p h s . T h e y s u g g e s t e d t h a t t h e d i v i s i o n i s b e t w e e n a s s i m i l a t i o n and g r o w t h ( m e a s u r e d by f e c u n d i t y , g e n e r a t i o n t i m e , e t c . ) , and s e n s o r i m o t o r p r o c e s s e s ( l e a d i n g t o d i s p e r s a l a n d m i g r a t i o n ) . A c c o r d i n g t o t h e m , t h i s i s e x p r e s s e d q u i t e c l e a r l y when t h e a l a t e morph i s a l s o t h e s e x u a l m o r p h , b u t l e s s s o when t h e a l a t e 47 morph i s p a r t h e n o g e n e t i c . T h i s s t a t e m e n t i s b o r n e o u t by the f i g u r e s f o r a v e r a g e f e c u n d i t y i n t h e two £ i s u m morphs s t u d i e d h e r e . In a d d i t i o n , t h e d i f f e r e n c e i n t h e s h a p e o f t h e a g e -s p e c i f i c f e c u n d i t y c u r v e s i m p l i e s a c o n s i d e r a b l e d i f f e r e n c e b e t w e e n t h e morphs i n r e p r o d u c t i v e s t r a t e g i e s a s w e l l a s i n r e p r o d u c t i v e e f f o r t . U n f o r t u n a t e l y , i t i s n o t p o s s i b l e t o d e a l w i t h r e p r o d u c t i v e s t r a t e g i e s i n a t r u l y m e a n i n g f u l way w i t h o u t l i f e t a b l e d a t a on b o t h morphs u n d e r f i e l d c o n d i t i o n s . T h e r e i s no way o f k n o w i n g how r e a l - w o r l d m o r t a l i t y a f f e c t s t h e r e l a t i v e s h a p e s o f t h e c u r v e s . B u t by t e m p o r a r i l y i g n o r i n g t h i s d i f f i c u l t y , an i n t e r e s t i n g p r o b l e m c a n be a p p r o a c h e d . T h a t i s , why a r e t h e r e p r o d u c t i v e c h a r a c t e r i s t i c s o f t h e a l a t e morph s o d i f f e r e n t f r o m t h o s e o f t h e a p t e r o u s morph? D e c r e a s e d mean f e c u n d i t y c a n be a t t r i b u t e d t o t h e i n c r e a s e d c o s t o f l o c o m o t o r y s t r u c t u r e s , a nymph w h i c h must m a n u f a c t u r e s t r u c t u r e s s u c h a s w i n g s and w ing m u s c l e s h a s l e s s r e s o u r c e s l e f t t o p r o d u c e o o c y t e s a n d e m b r y o s . T h e d e l a y e d o n s e t o f r e p r o d u c t i o n c a n be a t t r i b u t e d t o t h e a d d e d t i m e n e c e s s a r y f o r a n e w l y - m o u l t e d a d u l t t o p r e p a r e f o r and c a r r y o u t a f l i g h t . R e a s o n s f o r t h e l o w e r mean d a i l y f e c u n d i t y and t h e l o n g e r r e p r o d u c t i v e p e r i o d a r e h a r d e r t o i m a g i n e . T h e y c o u l d be due s i m p l y t o p h y s i o l o g i c a l l i m i t a t i o n s i n a l a t a e s u c h a s d e c r e a s e d a b d o m i n a l s i z e o r number o f o v a r i o l e s . H o w e v e r , t h e s e a r e v e r y p l a s t i c c h a r a c t e r i s t i c s i n a p h i d s i n g e n e r a l . U n l e s s t h e r e were s e l e c t i v e f o r c e s a c t i v e l y m a i n t a i n i n g t h e s e c h a r a c t e r i s t i c s , the mx c u r v e o f t h e a l a t a e m i g h t be e x p e c t e d t o s h i f t s t e a d i l y t o 48 t h e l e f t , w i t h t h e a l a t a e p r o d u c i n g more and more o f t h e i r c o m p l e m e n t o f y o u n g i n t h e f i r s t few d a y s o f r e p r o d u c t i o n , j u s t a s t h e a p t e r a e d o . What p o s s i b l e a d v a n t a g e c o u l d t h e r e be t o a l o w e r mean d a i l y f e c u n d i t y and a l o n g e r r e p r o d u c t i v e p e r i o d ? One p o s s i b l e e f f e c t o f t h i s r e p r o d u c t i v e p a t t e r n m i g h t be t h e s l o w i n g down o f t h e g r o w t h r a t e o f a new p o p u l a t i o n . However t h i s i s t h e o p p o s i t e o f what i s n o r m a l l y e x p e c t e d o f a c o l o n i z e r . U n d e r t h e t h e o r y o f r and K s e l e c t i o n ( M a c A r t h u r a n d W i l s o n , 1 9 6 7 ) , c o l o n i z e r s s h o u l d h a v e h i g h r e p r o d u c t i v e r a t e s . C e r t a i n l y t h i s i s t r u e o f a p h i d s a s a t a x o n o m i c g r o u p , b u t i t i s a p p a r e n t l y u n t r u e o f t h e i r c o l o n i z i n g m o r p h . I f t h e a l a t e morph m a i n t a i n e d t h e a b i l i t y t o f l y e v e n s h o r t d i s t a n c e s , and c o u l d t h e r e b y d i s t r i b u t e i t s p r o g e n y more w i d e l y , t h e l o w e r d a i l y r a t e o f p r o d u c t i o n o v e r a l o n g e r p e r i o d o f t i m e c o u l d be a d v a n t a g e o u s . B u t i t i s u n l i k e l y t h a t t h i s s p e c i e s f l i e s a t a l l o n c e i t b e g i n s r e p r o d u c t i o n . I t m i g h t be t h a t t h e a l a t e morph i s g e n e r a l l y more a c t i v e a f t e r i t s e t t l e s , d i s t r i b u t i n g i t s o f f s p r i n g t o a g r e a t e r e x t e n t t h a n i t s a p t e r o u s s i s t e r s on t h e p l a n t on w h i c h i t s e t t l e s t o r e p r o d u c e . Whether t h i s i s t h e c a s e i s unknown, b u t i t c o u l d e a s i l y be t e s t e d . T h e g e n e r a l s h a p e o f t h e mx c u r v e o f JU p i s u m a l a t a e i s s i m i l a r t o t h a t o f a p t e r a e o f T h e r i o a p h i s m a c u l a t a and M £ E ° s i p h u i § u p h o r b i a e i n t h a t i t shows more o f a p l a t e a u t h a n a p e a k . F r a z e r (1972a) h a s s u g g e s t e d t h a t T. . m a c u l a t a may show t h i s p a t t e r n b e c a u s e o f i t s p r i m i t i v e s t a t u s . T h a t i s , i t has n o t s p e c i a l i z e d r e p r o d u c t i v e l y t o t h e same d e g r e e a s o t h e r 49 s p e c i e s . The a l a t e morph i s g e n e r a l l y c o n s i d e r e d t o be t h e l e s s s p e c i a l i z e d o r more p r i m i t i v e f o r m . T h u s t h e same e x p l a n a t i o n c o u l d be p u t f o r w a r d f o r A._ £ i s u m a l a t a e . H o w e v e r , e u p h o r b i a e w h i c h e x h i b i t s t h e same p a t t e r n i s n o t c o n s i d e r e d t o be i n a p r i m i t i v e g r o u p , w e a k e n i n g t h e a r g u m e n t somewhat . L i t t l e e l s e c a n be s a i d a b o u t t h i s p r o b l e m w i t h o u t the a d d i t i o n o f more d a t a . E c o l o g i c a l and b e h a v i o u r a l d a t a on t h e two morphs o f ju_ £ i s u m a r e e s s e n t i a l , a s a r e s i m i l a r d a t a on a number o f o t h e r s p e c i e s , b e f o r e a n y g e n e r a l p a t t e r n c a n be d e t e c t e d . T h i s i s an e v o l u t i o n a r y q u e s t i o n w h i c h m i g h t p r o v i d e g e n e r a l i n s i g h t s i n t o r e p r o d u c t i v e s t r a t e g i e s . 3 . 3 . 2 E f f e c t o f g r a n d p a r e n t a l a g e on p a r e n t a l a b i l i t y t o p r o d u c e a l a t e o f f s p r i n g 3 . 3 . 2 . 1 A p t e r o u s g r a n d p a r e n t s The f i r s t i n t h i s s e r i e s o f e x p e r i m e n t s was d e s i g n e d t o e s t a b l i s h w h e t h e r o r n o t t h e r e were d i f f e r e n c e s be tween e a r l y -and l a t e - b o r n a p h i d s i n t h e i r a b i l i t y t o p r o d u c e a l a t a e . A g r a n d p a r e n t f o r t h i s e x p e r i m e n t was an a p t e r o u s i n d i v i d u a l i s o l a t e d f r o m t h e s t o c k c u l t u r e a s a f o u r t h - i n s t a r nymph. F o u r b a t c h e s o f p a r e n t a p h i d s were o b t a i n e d f r o m t h i s g r a n d p a r e n t t h r o u g h o u t i t s l i f e , t r e a t e d i n t h e manner a l r e a d y d e s c r i b e d , and t h e i r o f f s p r i n g t h e n s c o r e d a s e i t h e r a p t e r o u s o r a l a t e . T h i s p r o g r a m was r e p e a t e d s e q u e n t i a l l y f o r e i g h t d i f f e r e n t g r a n d p a r e n t a p h i d s . T h e p r o g r a m i n v o l v e d t h e c r o w d i n g o f e i g h t o f e a c h o f t h e f o u r t r e a t m e n t s , b a t c h e s I, I I , I I I , and I V , o r a 50 t o t a l o f 320 p a r e n t a p h i d s , a n d t h e s c o r i n g o f a b o u t 1 2 , 8 0 0 a p t e r o u s o r a l a t e o f f s p r i n g . The s t r u c t u r e o f t h e e x p e r i m e n t i s i l l u s t r a t e d i n F i g u r e 5 . Two i t e m s were r e c o r d e d f o r e a c h p a r e n t , t h e t o t a l number o f o f f s p r i n g b o r n d u r i n g t h e f o u r d a y s a f t e r c r o w d i n g , and t h e number o f t h e s e t h a t were a l a t e . T a b l e 2 shows some o f t h e c h a r a c t e r i s t i c s o f t h e r e s p o n s e s o f t h e f o u r b a t c h e s . The number o f o f f s p r i n g p r o d u c e d o v e r t h e f o u r - d a y p e r i o d a f t e r t h e c r o w d i n g s t i m u l u s was r e m a r k a b l y c o n s i s t e n t t h r o u g h o u t t h e e x p e r i m e n t , a l t h o u g h , a s T a b l e 2 s h o w s , b a t c h I I d i f f e r e d i n s i g n i f i c a n t l y f r o m t h e o t h e r s . T h e u n i f o r m i t y o f t h e r e p r o d u c t i v e p a t t e r n o v e r t h e f o u r b a t c h e s i n d i c a t e s t h a t w h a t e v e r e l s e may be i n v o l v e d , t h e r e i s no g r e a t d i f f e r e n c e i n a d u l t s u r v i v a l o r i n r e p r o d u c t i v e p o t e n t i a l e i t h e r be tween e a r l y - and l a t e - b o r n a p h i d s , o r o v e r t h e p e r i o d o f t h e e x p e r i m e n t . H o w e v e r , t h e r e s u l t s f o r p r o d u c t i o n o f a l a t a e i n d i c a t e t h a t t h e r e was c o n s i d e r a b l e v a r i a t i o n w i t h i n t h e e x p e r i m e n t a l s y s t e m . T h i s i s i l l u s t r a t e d by t h e s i z e o f t h e s t a n d a r d e r r o r o f t h e mean number o f a l a t a e p e r p a r e n t and t h e r a n g e o f r e s p o n s e p e r b a t c h o f 10 p a r e n t s among t h e e i g h t g r a n d p a r e n t s ( T a b l e 2 ) . In e a c h t r e a t m e n t b a t c h t h e r e was a t l e a s t one b a t c h o f 10 p a r e n t s t h a t p r o d u c e d no a l a t a . In no s i n g l e b a t c h d i d a l l 10 p a r e n t a p h i d s become a l a t a p r o d u c e r s . In s p i t e o f t h a t v a r i a t i o n , t h e r e was no d e t e c t a b l e t r e n d i n t h e d a t a r u n n i n g o v e r t h e e x t e n t o f t h e e x p e r i m e n t . T h e r e f o r e any d i f f e r e n c e s b e t w e e n t h e f o u r t r e a t m e n t b a t c h e s must be 51a Fi g u r e 5. Experimental design f o r i n v e s t i g a t i n g the e f f e c t of g r a n d p a r e ntal age a t time of p a r e n t a l b i r t h on p a r e n t a l a b i l i t y t o produce a l a t e o f f s p r i n g : apterous grandparents. Day 1 i s the day the f i r s t grandparent was i s o l a t e d from the stock c u l t u r e . Each poin t r e p r e s e n t s one crowding o p e r a t i o n . Grandparent number — NJ W <Jt O N 00 T 1 1 1 1 1 1 r • • • • • • • • • • • a O 52 T a b l e 2 . C h a r a c t e r i s t i c s o f r e p r o d u c t i o n o f p a r e n t a p h i d s a f t e r a s t a n d a r d c r o w d i n g s t i m u l u s : a p t e r o u s g r a n d p a r e n t s . B a t c h I B a t c h I I B a t c h I I I B a t c h IV Mean number o f o f f s p r i n g p e r p a r e n t * 4 0 . 3 5 ± 6 . 6 8 4 2 . 5 5 ± 6 . 1 6 4 0 . 7 5 ± 5 . 6 2 4 0 . 5 4 ± 6 . 6 7 Mean number o f a l a t a e p e r p a r e n t z 3 . 6 6 (0 .88 ) 2 . 0 3 (0 .69) 1. 41 ( 0 . 4 6 ) 1. 1 5 ( 0 . 3 0 ) Mean number o f a l a t a e p e r p a r e n t r e s p o n d i n g 1 1 . 7 2 7 . 0 4 6 . 2 8 4 . 6 0 Maximum number o f a l a t a e f r o m a n y one p a r e n t 38 25 25 13 Range o f r e s p o n s e b e t w e e n g r a n d p a r e n t s ( b a t c h t o t a l s ) 0 -141 0 - 5 3 . 0 - 4 5 0 -22 1 . ± 1 s t a n d a r d d e v i a t i o n 2 . ( s t a n d a r d e r r o r o f t h e mean) 53 a t t r i b u t e d to e f f e c t s of maternal age. I t i s reasonable to suppose t h a t even though a given treatment batch might vary c o n s i d e r a b l y i n response from grandparent to grandparent, the fo u r treatment batches from one grandparent should bear a c o n s i s t e n t r e l a t i o n s h i p to one another, A comparison of the i n t e n s i t y of response among the d i f f e r e n t batches of parents from the same grandparent suggested a p a t t e r n , but t h i s was not obvious. I t was p a r t i a l l y obscured by v a r i a t i o n from an unknown source. The data f o r t h i s and succeeding experiments are presented untreated i n Appendix I. A d i s c u s s i o n of p o s s i b l e sources of v a r i a t i o n i n the data i s presented i n Appendix I I . In s p i t e of v a r i a t i o n , the data show a c l e a r - c u t e f f e c t of maternal age on p r o d u c t i o n of a l a t a e . From Table 3 i t i s obvious that the d i f f e r e n c e s i n numbers of a l a t e o f f s p r i n g among the d i f f e r e n t batches are h i g h l y s i g n i f i c a n t , and t h a t batch I parents gave by f a r the g r e a t e s t response. Table 4 shows the r e l a t i o n s h i p of grandparental age to the p a r e n t a l response. Although d i f f e r e n c e s here are not s i g n i f i c a n t , the tr e n d i s f o r s l i g h t l y more e a r l y - than l a t e -born parents to respond to crowding, and f o r more of the former t o produce high numbers of a l a t e o f f s p r i n g . F i g u r e 6 shows t h i s c l e a r l y . The d i s t r i b u t i o n s of p a r e n t a l response f o r batches I I , I I I , and IV are s i m i l a r , with more than 60% of the a l a t a -producing parents b e aring 1-5 a l a t a e . In c o n t r a s t , l e s s than H0% of the responding parents i n batch I produced 1-5 a l a t a e , but 20% produced more than 21, s t r e t c h i n g out the t a i l of the 54 T a b l e 3 . E f f e c t o f g r a n d p a r e n t a l age a t t i m e o f p a r e n t a l b i r t h on t h e number o f a l a t e o f f s p r i n g p r o d u c e d i n r e s p o n s e t o c r o w d i n g : a p t e r o u s g r a n d p a r e n t s . B a t c h I B a t c h I I B a t c h I I I B a t c h IV Number o f a l a t e o f f s p r i n g p r o d u c e d 293 162 113 92 Number o f a p t e r o u s o f f s p r i n g p r o d u c e d 2935 3242 3147 3151 T o t a l number o f o f f s p r i n g p r o d u c e d 3228 3404 3260 3243 X 2 t e s t f o r i n d e p e n d e n c e o f o f f s p r i n g morph v s . b a t c h X 2 = 1 6 0 . 6 9 p « . 005 55 T a b l e 4 . E f f e c t o f g r a n d p a r e n t a l age a t t i m e o f p a r e n t a l b i r t h o n p a r e n t a l r e s p o n s e t o c r o w d i n g : a p t e r o u s g r a n d p a r e n t s . B a t c h I B a t c h I I B a t c h I I I B a t c h IV Number o f p a r e n t s p r o d u c i n g 0 a l a t a e 55 57 62 60 Number o f p a r e n t s p r o d u c i n g 1-5 a l a t a e 9 14 12 13 Number o f p a r e n t s p r o d u c i n g >5 a l a t a e 16 9 6 7 T o t a l number o f p a r e n t s t e s t e d 80 80 80 80 X2 t e s t f o r i n d e p e n d e n c e o f p a r e n t a l r e s p o n s e v s . b a t c h X2=8.08 .10<p< .25 56a F i g u r e 6. The number of parents producing d i f f e r e n t numbers of a l a t e o f f s p r i n g i n each batch, expressed as a percent of the parents from each batch that responded: apterous grandparents. 56 60 40 20 0 nn I ' I I Batch I n=25 c a i. o a. © O) o c a u w a> a. 60 - i—i 40 -20 -i i — i " r 60 -40 -20 -I I r 1 • • • Batch I n=23 Batch III n=18 60 40 20 ~\ r iO O CM n CO o «o 2 £ £ ? 7 7 ? Y Y i f Batch IV n=20 Number of alate offspring 57 d i s t r i b u t i o n . I t appears t h a t e a r l y - b o r n parents respond more s t r o n g l y than l a t e - b o r n parents t o an experimental crowding s t i m u l u s by producing a l a t e o f f s p r i n g . The parents born i n the f i r s t few days of the grandparent's r e p r o d u c t i v e p e r i o d (batch I) respond very s t r o n g l y , and the i n t e n s i t y of p a r e n t a l response appears to d i m i n i s h as the grandparent ages. However, the decrease i n response does not appear to be l i n e a r , and batches I I , I I I , and IV are more s i m i l a r i n response than batches I and I I . 3.3.2.2 A l a t e grandparents T h i s experiment was e s s e n t i a l l y a r e p e t i t i o n of the prev i o u s one, except t h a t the grandparents themselves were a l a t e not apterous. The t e s t was designed to e s t a b l i s h whether or not the e a r l y - and l a t e - b o r n progeny of a l a t a e d i f f e r e d i n t h e i r a b i l i t y to produce a l a t a e ; and i f so, whether t h i s p a t t e r n d i f f e r e d from that produced by the progeny of apterae. The r a t i o n a l e f o r t h i s experiment and the two th a t f o l l o w came from the r e s u l t s of the previous experiment and t h e i r r e l a t i o n s h i p to work by Sutherland (1970) on A._ £isum documenting a process which he b e l i e v e d s i m i l a r to what Lees (1960, 1966) has c a l l e d an " i n t e r v a l t i m e r " . The concept of the i n t e r v a l timer was f i r s t put forward by Lees (1960) to e x p l a i n some o b s e r v a t i o n s he had made on the process of se x u a l morph det e r m i n a t i o n i n Mejjoura v i c i a e , Many s p e c i e s of aphids reproduce p a r t h e n o g e n e t i c a l l y under long-day, or summer, photoperiods. In the f a l l , when the photoperiod 58 s h o r t e n s , t h e y s w i t c h t o s e x u a l r e p r o d u c t i o n a n d p r o d u c e o v e r w i n t e r i n g e g g s . U s u a l l y , i f p a r t h e n o g e n e t i c a p h i d s a r e p l a c e d i n a r t i f i c i a l s h o r t - d a y c o n d i t i o n s , t h e y i m m e d i a t e l y s w i t c h t o s e x u a l r e p r o d u c t i o n . H o w e v e r , i t h a s l o n g been r e c o g n i z e d t h a t , f o r many s p e c i e s , n e i t h e r t h e f u n d a t r i x ( the p a r t h e n o g e n e t i c morph h a t c h i n g f r o m t h e o v e r w i n t e r e d egg) n o r t h e f i r s t few g e n e r a t i o n s a f t e r t h e f u n d a t r i x w i l l r e s p o n d t o s h o r t p h o t o p e r i o d s . T h e y r e m a i n p a r t h e n o g e n e t i c u n d e r a l l c o n d i t i o n s . E c o l o g i c a l l y t h i s i s n o t s u r p r i s i n g , s i n c e t h e p h o t o p e r i o d s d u r i n g t h e s p r i n g may be s h o r t e n o u g h t o s w i t c h summer f o r m s b a c k t o s e x u a l r e p r o d u c t i o n . Some m e c h a n i s m , o t h e r t h a n p h o t o p e r i o d , i s t h e r e f o r e n e c e s s a r y t o keep t h e a p h i d s r e p r o d u c i n g p a r t h e n o g e n e t i c a l l y u n t i l t h e p h o t o p e r i o d l e n g t h e n s b e y o n d t h e c r i t i c a l s p a n . U n t i l L e e s b e g a n h i s w o r k , t h i s m e c h a n i s m was a l w a y s a s s u m e d t o be s i m p l y t h e p a s s a g e o f a g i v e n number o f g e n e r a t i o n s ( B o n n e m a i s o n , 1 9 5 1 ) . L e e s (1960) r e a r e d two l i n e a g e s , a f i r s t - b o r n and a l a s t -b o r n l i n e , f r o m s i x f u n d a t r i c e s o f M e g o u r a v i c i a e u n d e r s h o r t -day c o n d i t i o n s and o b s e r v e d t h e t i m e and g e n e r a t i o n number a t w h i c h s e x u a l morphs a p p e a r e d . He c o n c l u d e d f r o m h i s d a t a t h a t s e x u a l e s a p p e a r e d i n t h e two l i n e a g e s a t t h e same t i m e , r a t h e r t h a n a f t e r t h e same number o f g e n e r a t i o n s . A s an a l t e r n a t i v e t o t h e s i m p l e g e n e r a t i o n - e f f e c t h y p o t h e s i s , he p r o p o s e d t h e i n t e r v a l - t i m e r h y p o t h e s i s . L e e s 1 i n t e r v a l t i m e r i s a p h y s i o l o g i c a l " c l o c k " o f some s o r t w h i c h r u n s down a t a c o n s t a n t r a t e and i s n o t a f f e c t e d by d i l u t i o n s i n c e t h e number o f i n t e r v e n i n g g e n e r a t i o n s (and h e n c e 59 t h e number o f c e l l d i v i s i o n s ) d o e s n o t a f f e c t i t . In L e e s * own words ( 1 9 6 0 , p 1 7 3 ) , " T h e c o n t r o l l i n g m e c h a n i s m seems t o p o s s e s s u n i q u e f e a t u r e s , t h e p r o p e r t y o f t r a n s f o r m a t i o n w i t h t i m e b e i n g a p p a r e n t l y c o m b i n e d w i t h a c a p a c i t y f o r s e l f r e p l i c a t i o n . " S i n c e L e e s f i r s t p u t f o r w a r d t h e t h e o r y , D i x o n (1971) h a s d e s c r i b e d an i n t e r v a l t i m e r i n s e x u a l morph d e t e r m i n a t i o n i n a n o t h e r s p e c i e s . I t i s g e n e r a l l y r e c o g n i z e d t h a t a p t e r o u s m o t h e r s p r o d u c e more a l a t a e t h a n d o a l a t e m o t h e r s u n d e r s i m i l a r c i r c u m s t a n c e s . I n some s p e c i e s t h e i n h i b i t i o n h a s b e e n r e p o r t e d t o go b e y o n d t h e a l a t a e t o t h e n e x t g e n e r a t i o n . I n 1 9 6 6 , L e e s p o s t u l a t e d a s i m i l a r i n t e r v a l t i m e r t o e x p l a i n t h e p r o d u c t i o n o f a l a t a e by a l i n e f o u n d e d by an a l a t a i n H e g o u r a v i c i a e . S u t h e r l a n d (1970) r e p o r t e d t h a t l i k e K r v i c i a e , A;, p i s u m a l a t a e a l s o r e g a i n e d t h e a b i l i t y t o p r o d u c e a l a t a e a f t e r a f i x e d l e n g t h o f t i m e i n s t e a d o f number o f g e n e r a t i o n s . A c c o r d i n g t o h i m , t h e c a p a c i t y t o p r o d u c e a l a t a e d i d n o t r e t u r n c o m p l e t e l y u n t i l t h e t h i r d g e n e r a t i o n a f t e r a n a l a t e a n c e s t o r o f f i r s t - b o r n l i n e s , but t h e c a p a c i t y r e t u r n e d i n t h e s e c o n d g e n e r a t i o n o f l a s t - b o r n l i n e s . The d a t a p r e s e n t e d by L e e s and S u t h e r l a n d s u g g e s t e d t o me t h a t m a t e r n a l - a g e e f f e c t s w e r e , a t t h e v e r y l e a s t , i n t i m a t e l y i n v o l v e d i n t h e i n t e r v a l - t i m e r m e c h a n i s m i n t h e a l a t e / a p t e r o u s p o l y m o r p h i c s y s t e m . O r , a t m o s t , t h e y a l o n e m i g h t be s u f f i c i e n t t o e x p l a i n t h e e a r l i e r o b s e r v a t i o n s w i t h o u t t h e n e e d t o i n v o k e t h e c o m p l e x i d e a o f an i n t e r v a l t i m e r , A s e r i e s o f e x p e r i m e n t s was d e s i g n e d t o t e s t t h i s i d e a and f u r t h e r t o d o c u m e n t m a t e r n a l - a g e e f f e c t s . The f i r s t t e s t s i m p l y i n v e s t i g a t e d t h e e f f e c t o f a g i n g o f an a l a t a on t h e t e n d e n c y o f 60 i t s p r o g e n y t o p r o d u c e a l a t a e t h e m s e l v e s . F o r e a c h r e p l i c a t e i n t h i s e x p e r i m e n t i t was n e c e s s a r y t o o b t a i n an a l a t e g r a n d p a r e n t . T o do t h i s , 10 a p h i d s ( g r e a t - g r a n d p a r e n t s ) were t a k e n f r o m t h e s t o c k c u l t u r e , c r o w d e d i n t h e u s u a l m a n n e r , and p l a c e d s i n g l y on p l a n t s t o r e p r o d u c e f o r f o u r d a y s . T h e i r p r o g e n y were r e a r e d u n t i l t h e y r e a c h e d t h e f o u r t h i n s t a r , when a l a t a e c o u l d be e a s i l y r e c o g n i z e d . An a p h i d was t h e n c h o s e n f r o m among t h e o l d e s t h e a l t h y p r e - a l a t a e and t r e a t e d a s a g r a n d p a r e n t i n t h e manner a l r e a d y d e s c r i b e d . T h i s e x p e r i m e n t was p e r f o r m e d w i t h s i x a l a t e g r a n d p a r e n t s s o t h a t s i x o f e a c h o f b a t c h e s I, I I , I I I , a n d I V , o r 240 p a r e n t a p h i d s , were c r o w d e d . T h e s e p a r e n t s p r o d u c e d some 9 , 6 0 0 a l a t e and a p t e r o u s o f f s p r i n g . T h e r e p l i c a t e s were t i m e d s e q u e n t i a l l y t o e x p o s e o v e r - r i d i n g g r a d i e n t s , b u t b e c a u s e o f s p a c e l i m i t a t i o n s t h e y were c a r r i e d o u t i n t h r e e v p a i r s o f two r e p l i c a t e s a s shown i n F i g u r e 7 . T a b l e 5 shows t h e g e n e r a l p a t t e r n o f r e s p o n s e f o r e a c h b a t c h . T h i s t a b l e (and T a b l e s 6 and 7) i s d i v i d e d i n t o two s e c t i o n s . O f t h e b a t c h e s o f p a r e n t s f r o m t h e s i x g r a n d p a r e n t s , f i v e s e t s b e h a v e d i n t h e same m a n n e r , and one s e t b e h a v e d d i f f e r e n t l y . The d a t a a r e p r e s e n t e d f i r s t f o r a l l s i x g r a n d p a r e n t s , and t h e n f o r o n l y f i v e , e x c l u d i n g t h o s e f o r t h e a b e r r a n t g r a n d p a r e n t . As i n t h e p r e v i o u s e x p e r i m e n t , t h e f o u r t r e a t m e n t b a t c h e s o f p a r e n t s g a v e r i s e t o r o u g h l y t h e same number o f o f f s p r i n g . And o n c e a g a i n t h e r e was c o n s i d e r a b l e v a r i a t i o n i n l e v e l o f p r o d u c t i o n o f a l a t a e f r o m one g r a n d p a r e n t a l s e t t o a n o t h e r . 61a F i g u r e 7 . E x p e r i m e n t a l d e s i g n f o r i n v e s t i g a t i n g t h e e f f e c t o f g r a n d p a r e n t a l a g e a t t i m e o f p a r e n t a l b i r t h on p a r e n t a l a b i l i t y t o p r o d u c e a l a t e o f f s p r i n g : a l a t e g r a n d p a r e n t s ( o ) ; a l a t e g r e a t -g r a n d p a r e n t s / a p t e r o u s g r a n d p a r e n t s ( • ) . Day 1 i s t h e day t h a t t h e 10 a p h i d s u s e d t o p r o d u c e t h e f i r s t a l a t e g r a n d p a r e n t were i s o l a t e d f r o m t h e s t o c k c u l t u r e . E a c h p o i n t r e p r e s e n t s one c r o w d i n g o p e r a t i o n . Grandparent number — 10 w o> o —i 1 1 1 1 r L9 62 T a b l e 5 , C h a r a c t e r i s t i c s o f r e p r o d u c t i o n o f p a r e n t a p h i d s a f t e r a s t a n d a r d c r o w d i n g s t i m u l u s : a l a t e g r a n d p a r e n t s . B a t c h I B a t c h I I B a t c h I I I B a t c h IV 6 g r a n d p a r e n t s Mean number o f o f f s p r i n g p e r p a r e n t ^ 3 7 . 9 0 ± 6 . 4 7 3 9 . 4 7 ± 7 . 0 0 4 1 . 7 0 ± 5 . 6 3 3 8 . 2 3 ± 7 . 7 3 Mean number o f a l a t a e p e r p a r e n t 2 1 .58 (0 .69 ) 2 . 15 (0 .61) 2 . 17 (0 .73) 1. 32 (0 .45) Mean number o f a l a t a e p e r p a r e n t r e s p o n d i n g 1 3 . 5 7 5 . 3 8 7 . 2 2 4 . 1 6 Maximum number o f a l a t a e f r o m a n y one p a r e n t - 27 22 23 23 Range o f r e s p o n s e b e t w e e n g r a n d p a r e n t s ( b a t c h t o t a l s ) 0 - 9 5 3 - 4 2 0 - 4 7 0 -24 63 T a b l e 5 . C o n t i n u e d 5 g r a n d p a r e n t s Mean number o f o f f s p r i n g p e r p a r e n t * 3 7 . 0 2 ± 6 . 2 8 4 0 . 1 2 ± 6 . 8 3 4 1 . 7 8 ± 5 . 5 9 3 7 . 7 6 ± 7 . 1 4 Mean number o f a l a t a e p e r p a r e n t 2 0 . 0 0 2 . 5 0 (0 .72) 2 . 4 6 (0 .87 ) 1. 26 (0. 50) Mean number o f a l a t a e p e r p a r e n t r e s p o n d i n g 0 . 0 0 5 . 9 5 8. 20 4 . 2 0 Maximum number o f a l a t a e f r o m any one p a r e n t 22 23 23 Range o f r e s p o n s e b e t w e e n g r a n d p a r e n t s ( b a t c h t o t a l s ) 3 -42 0 - 4 7 0 -24 1. ± 1 s t a n d a r d d e v i a t i o n 2 . ( s t a n d a r d e r r o r o f t h e mean) 64 however, there was no g r a d i e n t i n response running throughout the experiment. Therefore any o v e r a l l d i f f e r e n c e s among treatment batches can be a t t r i b u t e d to maternal age d i f f e r e n c e s . Tables 6 and 7 i n d i c a t e that there are s i g n i f i c a n t d i f f e r e n c e s i n the responses of the parents of d i f f e r e n t batches when c o n s i d e r i n g both the f i v e and the s i x r e p l i c a t e s . C o n s i d e r i n g f o r the moment only the data f o r f i v e r e p l i c a t e s , i t i s obvious t h a t p r o d u c t i o n of a l a t a e by the batch I parents from an a l a t e grandparent was completely suppressed. T h i s suppression i s i n sharp c o n t r a s t to batch I parents from an apterous grandparent, which i n the previous experiment, showed a maximum response. The a b i l i t y of parents from a l a t e grandparents to respond was completely r e s t o r e d by the time batch I I parents were born, t h a t i s , by days 4 and 5 of the grandparent's r e p r o d u c t i v e sequence. A comparison of the f i g u r e s f o r mean number of a l a t a e per parent from a l a t e grandparents (Table 5) and from apterous grandparents (Table 2) i n d i c a t e s t h a t the responses of batches I I , I I I , and IV from a l a t e grandparents are much the same as those from apterous grandparents. F i g u r e 8 f u r t h e r emphasizes t h i s p o i n t . The d i s t r i b u t i o n s show t h a t the response of parents to crowding i s very s i m i l a r f o r batches I I , I I I , and IV. Bore than 70% of the responding parents produced one to f i v e a l a t a e , with s m a l l numbers of parents producing more. T h i s i s s i m i l a r to the response by parents of batches I I , I I I , and IV from apterous grandparents where more than 60$ of the responding parents produced only one to f i v e a l a t a e (Figure 6). 65 T a b l e 6 . E f f e c t o f g r a n d p a r e n t a l a g e a t t i m e o f p a r e n t a l b i r t h on t h e number o f a l a t e o f f s p r i n g p r o d u c e d i n r e s p o n s e t o c r o w d i n g : a l a t e g r a n d p a r e n t s . Batch I Batch I I Batch I I I Batch IV 6 g r a n d p a r e n t s Number o f a l a t e o f f s p r i n g p r o d u c e d 95 129 130 79 Number o f a p t e r o u s o f f s p r i n g p r o d u c e d 2174 2239 2372 2209 T o t a l number o f o f f s p r i n g p r o d u c e d 2269 2368 2502 2288 X 2 t e s t f o r i n d e p e n d e n c e o f o f f s p r i n g morph v s . b a t c h X 2 = 13 .66 . 010<p< .025 5 g r a n d p a r e n t s Number o f a l a t e o f f s p r i n g p r o d u c e d 0 125 123 63 Number o f a p t e r o u s o f f s p r i n g p r o d u c e d 1846 1881 1966 1819 T o t a l number o f o f f s p r i n g p r o d u c e d 1846 2006 2089 1882 X 2 t e s t f o r i n d e p e n d e n c e o f o f f s p r i n g morph v s . b a t c h X 2 = 1 2 5 . 1 1 p « . 005 66 T a b l e 7 . E f f e c t o f g r a n d p a r e n t a l a g e a t t i m e o f p a r e n t a l b i r t h on p a r e n t a l r e s p o n s e t o c r o w d i n g : a l a t e g r a n d p a r e n t s . B a t c h I B a t c h I I B a t c h I I I B a t c h IV 6 g r a n d p a r e n t s Number o f p a r e n t s p r o d u c i n g 0 a l a t a e 53 36 42 41 Number o f p a r e n t s p r o d u c i n g 1-5 a l a t a e 2 17 13 15 Number o f p a r e n t s p r o d u c i n g >5 a l a t a e 5 7 5 4 T o t a l number o f p a r e n t s t e s t e d 60 60 60 60 X 2 t e s t f o r i n d e p e n d e n c e o f p a r e n t a l r e s p o n s e v s . b a t c h X 2 = 1 5 . 9 6 . 0 1 0 < p < . 0 2 5 5 g r a n d p a r e n t s Number o f p a r e n t s p r o d u c i n g 0 a l a t a e 50 29 35 35 Number o f p a r e n t s p r o d u c i n g 1-5 a l a t a e 0 14 10 12 Number o f p a r e n t s p r o d u c i n g >5 a l a t a e 0 7 5 3 T o t a l number o f p a r e n t s t e s t e d 50 50 50 50 X 2 t e s t f o r i n d e p e n d e n c e o f p a r e n t a l r e s p o n s e v s . b a t c h X 2=26.49 p<.005 67 In other words, the i n h i b i t i o n of the production of a l a t a e by an a l a t e aphid i s very b r i e f , no more than three days. By the f o u r t h day of i t s r e p r o d u c t i v e p e r i o d , an a l a t e aphid i s producing young t h a t are no d i f f e r e n t i n t h e i r response to crowding from the corresponding young of an a p t e r a . The one exception to t h i s p a t t e r n of response by the progeny of a l a t a e i s worth n o t i n g . Batch I parents from one of the s i x grandparents d i d respond to the crowding s t i m u l u s . In f a c t , t h e i r response was among the s t r o n g e s t recorded in any of the experiments or the p r e l i m i n a r y work. Seven of the 10 parents produced a t o t a l of 95 a l a t e o f f s p r i n g . The^ d i s t r i b u t i o n of p a r e n t a l response i s shown i n F i g u r e 8, even though i t i s i n a p p r o p r i a t e t o c a l c u l a t e percentages with a sample s i z e of seven. The d i s t r i b u t i o n of these seven batch I parents resembles the d i s t r i b u t i o n of batch I parents from an apterous grandparent (Figure 6). T h i s i m p l i e s that the mechanism pr e v e n t i n g a l a t a -p r o d u c t i o n by e a r l y - b o r n progeny of winged aphids may be a d i f f e r e n t mechanism from the one l e a d i n g to high p r o d u c t i o n of a l a t a e by the e a r l y - b o r n progeny of wingless aphids. When the mechanism suppessing a l a t a - p r o d u c t i o n went amiss, production immediately jumped to the high l e v e l found among the e a r l y - b o r n progeny of apterae. The data presented here place the v a l i d i t y of the i n t e r v a l t i mer, as i t r e l a t e s t o A._ £isum , i n some doubt. T h i s question w i l l be d i s c u s s e d f u r t h e r i n the f o l l o w i n g s e c t i o n s . 68a F i g u r e 8. The number of parents producing d i f f e r e n t numbers of a l a t e o f f s p r i n g i n each batch, expressed as a percent of the parents from each batch that responded: a l a t e grandparents. 68 60 -40 -20 on Batch I n = 7 60 -40 -20 a n , , , Batch I n = 24 60 h 40 20 m m a 1~—I r Batch n = 18 60 40 20 a n 1 T 1 l , r o V) o >o V) ~ •— CN CN I I I I I — o — -o O IO o r> co Y !N S n S Batch IV n = 19 Number of alate offspring 69 3 . 3 . 2 . 3 A l a t e g r e a t - g r a n d p a r e n t s / a p t e r o u s g r a n d p a r e n t s T o f u r t h e r e l i m i n a t e t h e p o s s i b i l i t y o f i n v c l v m e n t o f a n i n t e r v a l t i m e r a s e n v i s a g e d by L e e s , i t a p p e a r e d n e c e s s a r y t o r e p e a t t h e e x p e r i m e n t on t h e s e c o n d g e n e r a t i o n a f t e r a n a l a t e a p h i d . A c c o r d i n g t o S u t h e r l a n d ( 1 9 7 0 ) , t h e s e c o n d g e n e r a t i o n o f a f i r s t - b o r n l i n e s t i l l showed a l o w e r e d r e s p o n s e t o c r o w d i n g and t h e f u l l c a p a c i t y t o r e s p o n d d i d n o t r e t u r n u n t i l t h e t h i r d g e n e r a t i o n a f t e r a n a l a t e a p h i d . I n o t h e r w o r d s , n o r m a l numbers o f a l a t a e o n l y r e a p p e a r e d i n t h e f o u r t h g e n e r a t i o n a f t e r an a l a t a . To t e s t t h i s c l a i m , t h e e x p e r i m e n t was r e p e a t e d u s i n g as t h e g r a n d p a r e n t one o f t h e f i r s t few a p h i d s b o r n e by e a c h a l a t e g r a n d p a r e n t . T h e a l a t e g r a n d p a r e n t s o f t h e p r e v i o u s e x p e r i m e n t t h u s became g r e a t - g r a n d p a r e n t s , and s i x a p t e r o u s g r a n d p a r e n t s were u s e d a s r e p l i c a t e s . E a c h o f t h e s e was b o r n d u r i n g t h e f i r s t two d a y s o f r e p r o d u c t i o n o f a d i f f e r e n t a l a t e a p h i d . The d e s i g n o f e a c h r e p l i c a t e was a s p r e v i o u s l y d e s c r i b e d . T h e t i m i n g o f the s i x r e p l i c a t e s i s shown i n F i g u r e 7 . T a b l e 8 shows t h e g e n e r a l p a t t e r n o f r e s u l t s f o r t h i s e x p e r i m e n t . The same r e s p o n s e was e x h i b i t e d h e r e a s was shown by a p h i d s w i t h no a l a t a i n t h e i r r e c e n t a n c e s t r y . T h e r e f o r e t h e r e was no c a r r y o v e r o f i n h i b i t i o n t o t h e s e c o n d g e n e r a t i o n a f t e r a n a l a t a . In f a c t , t h e r e s p o n s e s t o c r o w d i n g were s l i g h t l y s t r o n g e r i n t h i s e x p e r i m e n t t h a n i n t h e o r i g i n a l e x p e r i m e n t w i t h a p t e r o u s g r a n d p a r e n t s . In t h i s c a s e t h e r e a r e s i g n i f i c a n t d i f f e r e n c e s among t h e t r e a t m e n t b a t c h e s i n n o t o n l y t h e f r e q u e n c i e s o f the two morphs ( T a b l e 9) b u t a l s o o f t h e p a r e n t a l r e s p o n s e s ( T a b l e 70 T a b l e 8 . C h a r a c t e r i s t i c s o f r e p r o d u c t i o n o f p a r e n t a p h i d s a f t e r a s t a n d a r d c r o w d i n g s t i m u l u s : a l a t e g r e a t - g r a n d p a r e n t s / a p t e r o u s g r a n d p a r e n t s . B a t c h I B a t c h I I B a t c h I I I B a t c h IV Mean number o f o f f s p r i n g p e r p a r e n t * 3 4 . 1 7 + 6 . 8 6 4 0 . 2 5 + 5 . 3 8 3 7 . 6 0 + 1 0 . 6 1 3 6 . 6 0 ± 8 . 1 2 Mean number o f a l a t a e p e r p a r e n t * 4 . 9 7 (1 .03) 1 .38 (0 .42) 2 . 3 2 ( 0 . 7 5 ) 2 . 7 7 (0. 93) Mean number o f a l a t a e p e r p a r e n t r e s p o n d i n g 1 0 . 2 8 4 . 8 8 6 . 3 2 9 . 2 2 Maximum number o f a l a t a e f r o m any one p a r e n t 30 19 30 39 Range o f r e s p o n s e b e t w e e n g r a n d p a r e n t s ( b a t c h t o t a l s ) 9 -84 5 -38 2 -64 1-97 1. ± 1 s t a n d a r d d e v i a t i o n 2 . ( s t a n d a r d e r r o r o f t h e mean) 71 Table 9. E f f e c t of grandparental age at time of p a r e n t a l b i r t h on t h e number of a l a t e o f f s p r i n g produced i n response to crowding: a l a t e great-grandparents/apterous grandparents. Batch I Batch I I Batch I I I Batch IV number of a l a t e o f f s p r i n g produced 298 83 139 166 Number of apterous o f f s p r i n g produced 1752 2335 2115 2030 T o t a l number o f o f f s p r i n g produced 2050 2418 2254 2196 X2 t e s t f o r o f f s p r i n g independence of morph vs. batch X2=204.48 p « . 0 0 5 72 Tabl e 10. E f f e c t of grandparental age a t time of p a r e n t a l b i r t h on p a r e n t a l response t o crowding: a l a t e great-grandparents/apterous grandparents. Batch I Batch I I Batch I I I Batch IV Number of parents producing 0 a l a t a e 31 43 38 42 Number of parents producing 1-5 a l a t a e 11 12 15 11 Number of parents producing >5 a l a t a e 18 ' 5 7 7 T o t a l number of parents t e s t e d 60 60 60 60 t e s t f o r independence of p a r e n t a l response vs. batch X2=14.51 .010<p<.025 73 10). Batch I parents gave a st r o n g e r response than any other batch. Figure 9 i l l u s t r a t e s the d i s t r i b u t i o n of p a r e n t a l response. As i n p r e v i o u s experiments, batches I I , I I I , and IV are s i m i l a r to one another, with more than 60% of responding parents f a l l i n g i n t o the f i r s t c l a s s , and producing only one to f i v e a l a t e o f f s p r i n g . The somewhat longer t a i l shown f o r batch IV i s due to the response of a s i n g l e s e t of parents, from one grandparent, i n which f o u r produced 97 a l a t e o f f s p r i n g . T h i s batch has i n f l a t e d a l l measures of a l a t a p r oduction f o r batch IV i n t h i s experiment. Once again, batch I parents showed a d i s t r i b u t i o n very d i f f e r e n t from the other batches. Less than 40% . of responding parents produced one to f i v e a l a t a e , and a c o n s i d e r a b l e p r o p o r t i o n of parents produced high numbers of a l a t a e . These r e s u l t s c o nfirm that t h e r e i s no ge n e r a t i o n c a r r y o v e r of the s u p r e s s i o n of the a b i l i t y to produce a l a t a e . I n h i b i t i o n i n t h i s c l o n e l a s t e d no longer than the f i r s t two or three days of the r e p r o d u c t i v e p e r i o d of the a l a t a i t s e l f . The r e s u l t s a l s o reconfirmed the c o n c l u s i o n s a r i s i n g out of the o r i g i n a l experiment. Aphids born i n the f i r s t two or three days of an aptera's r e p r o d u c t i v e p e r i o d are more l i k e l y to respond s t r o n g l y to a standard crowding s t i m u l u s than aphids born l a t e r i n i t s l i f e . 74a F i g u r e 9. The number of parents producing d i f f e r e n t numbers of a l a t e o f f s p r i n g i n each batch, expressed as a percent o f the parents from each batch that responded: a l a t e great-grandparents/apterous grandparents. 74 60 40 20 Batch I n=29 T 1 1— 1—~-r C o Q. a> co o c © 60 -40 -20 • 60 40 20 a D n Batch I n = 17 Batch III n = 22 T — " - i — 1— — i r 60 |-40 20 I- a . ~ o « o o « o o £> Q 7 T T ? T ? i T Batch IV n = 18 Number of alate offspring 75 3 . 3 . 3 E f f e c t o f a g i n g o f a l a t a e on t h e i r a b i l i t y t o p r o d u c e a l a t e p r o g e n y T h e r e was o n e f u r t h e r t e s t n e c e s s a r y b e f o r e r e j e c t i n g t h e c o n c e p t o f an i n t e r v a l t i m e r f o r a p t e r o u s / a l a t e morph d e t e r m i n a t i o n . I t c o u l d be a r g u e d t h a t t h e i n t e r v a l t i m e r i n t h i s c l o n e s i m p l y r a n down s o o n e r t h a n i t d i d i n S u t h e r l a n d ' s c l o n e . P a r e n t a p t e r a e b o r n on t h e f o u r t h day o f an a l a t e g r a n d p a r e n t ' s r e p r o d u c t i v e p e r i o d were a b l e t o r e s p o n d t o c r o w d i n g and p r o d u c e a l a t a e . In o t h e r w o r d s , a b o u t 14 d a y s a f t e r an a l a t a b e g a n r e p r o d u c t i o n , a l a t a e were b o r n i n i t s l i n e a g e . I f t h e i n t e r v a l t i m e r t h e o r y were c o r r e c t , a l a t e g r a n d p a r e n t s t h e m s e l v e s s h o u l d t h e r e f o r e r e s p o n d t o c r o w d i n g by day 14 o f t h e i r r e p r o d u c t i v e p e r i o d , a n d p r o d u c e more a l a t a e . H o w e v e r , a s T a b l e 1 and F i g u r e 3 s h o w , t h e a v e r a g e r e p r o d u c t i v e p e r i o d o f an a l a t a i s o n l y 17 d a y s l o n g . I n a d d i t i o n , i t i s known t h a t a s a p t e r a e age t h e i r p r o d u c t i o n o f a l a t a e i n r e s p o n s e t o c r o w d i n g d e c r e a s e s , p r o b a b l y b e c a u s e t h e i r l o w e r a c t i v i t y l e v e l r e s u l t s i n f e w e r a p h i d / a p h i d e n c o u n t e r s ( S u t h e r l a n d , 1 9 6 9 a ) . In f a c t , o l d pea a p h i d s , w h e t h e r a p t e r o u s o r a l a t e , move a b o u t v e r y l i t t l e . T h e c o m b i n a t i o n o f t h e s e two d i f f i c u l t i e s p r e v e n t e d t h e c r i t i c a l e x p e r i m e n t o f c r o w d i n g a l a t a e a t day 14 o f t h e i r r e p r o d u c t i v e p e r i o d . P r o b a b l y t h e v e r y l a t e s t t h a t s u c h an e x p e r i m e n t c o u l d be c a r r i e d o u t would be n e a r day 9 o f t h e r e p r o d u c t i v e p e r i o d . T h a t i s t h e l a t e s t day w h i c h wou ld e n s u r e an a d e q u a t e number o f p o s t -c r o w d i n g p r o g e n y f o r s c o r i n g a s a p t e r o u s o r a l a t e . By day 9 o f i t s r e p r o d u c t i v e p e r i o d , an a l a t a h a s b e e n p r o d u c i n g a p t e r a e 76 f u l l y competent to produce a l a t a e f o r a t l e a s t f i v e days. T h e r e f o r e , i f the same mechanism t h a t prevented i t s e a r l y - b o r n progeny from producing a l a t a e , a l s o prevented i t from producing a l a t a e (as the i n t e r v a l - t i m e r theory proposes), i t should now a l s o respond by producing a l a t a e i t s e l f . T h i s h y p o t h e s i s seemed reasonable, because Johnson and B i r k s (1960) found t h a t a l a t e *£lii§ c r a c c i v o r a were at f i r s t i n c a p a b l e of producing a l a t a e , but a f t e r a few days became f u l l y competent to do so. That change appeared to them to be c o r r e l a t e d with the completion of h i s t o l y s i s of the f l i g h t muscles, a process which i n many s p e c i e s of aphids begins soon a f t e r an a l a t a s e t t l e s and begins to f e e d (Johnson, 1957) . l e e s (1966) commented on these r e s u l t s by s u g g e s t i n g that the i n t e r v a l timer was very s h o r t . To t e s t f o r r e s t o r a t i o n of the a b i l i t y t o produce a l a t a e i n o l d a l a t a e , the experiment shown i n F i g u r e 10 was c a r r i e d out. Ten i n d i v i d u a l s from the stock c u l t u r e were i s o l a t e d , allowed to begin r e p r o d u c t i o n , then crowded and allowed t o reproduce f o r four days. When the nymphs reached the f o u r t h i n s t a r and could be i d e n t i f i e d , 20 p r e - a l a t a e were i s o l a t e d on 20 p l a n t s and allowed to begin r e p r o d u c t i o n . As soon as r e p r o d u c t i o n had begun, 10 of these a l a t a e were crowded i n the usual manner, retu r n e d to t h e i r p l a n t s , and allowed to reproduce f o r f o u r days. When t h e i r progeny were o l d enough, they were scored as apterae or a l a t a e . The other 10 a l a t a e were l e f t t o reproduce and grow o l d . Each day t h e i r progeny were counted and removed from t h e i r p l a n t s . On the n i n t h day a f t e r the beginning of r e p r o d u c t i o n , they were crowded and then returned to f r e s h 77a F i g u r e 10. Experimental design f o r i n v e s t i g a t i n g the e f f e c t of aging i n a l a t e aphids on t h e i r a b i l i t y to produce a l a t a e ( a- a s i n g l e r e p l i c a t e b- t i m i n g of the r e p l i c a t e s ) . 77 Day 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 i s o l a t e 10 apterae r crowd 10 apterae (e) i s o l a t e 20 a l a t e progeny of the apterae crowd 10 young a l a t a e (•) i progeny group from young a l a t a e complete crowd 10 o l d a l a t a e (o) count progeny of young a l a t a e progeny group from o l d a l a t a e complete count progeny of o l d a l a t a e e 0) JQ £ c a> o u ~5_ a> 3 2 e h e 10 20 Day 30 78 p l a n t s to reproduce f o r f o u r days. These progeny were a l s o r e a r e d and scored as apterae or a l a t a e . T h i s procedure was repeated s e q u e n t i a l l y f o u r times, r e s u l t i n g i n the crowding of 40 young and 40 o l d a l a t a e and the s c o r i n g of t h e i r progeny. The r e s u l t s of t h i s experiment were unequivocal. There was not a s i n g l e a l a t a found among the progeny of any of these 80 a l a t a e . The a l a t a e d i d not r e g a i n the a b i l i t y to produce a l a t a e as they aged, even though they were no longer i n h i b i t i n g t h e i r progeny from producing a l a t a e . In the l i g h t of these experiments i t i s p o s s i b l e to re-e v a l u a t e the r e s u l t s obtained by Lees (1966) f o r Megoura v i c i a e and by Sutherland (1970) f o r A.. £isum i n r e l a t i o n to the i n t e r v a l timer and maternal-age e f f e c t s . Lees presented very l i t t l e a c t u a l data on M_. v i c i a e . But those he d i d present appear to f i t a simple model of maternal-age e f f e c t s j u s t as well as they f i t h i s complex model of an i n t e r v a l timer. I t i s p o s s i b l e to examine Sutherland's data and experimental design more c r i t i c a l l y . F igure 11 i s a r e p r o d u c t i o n of h i s f i g u r e summarizing h i s data. As can be seen, h i s data are very s i m i l a r i n c h a r a c t e r to my r e s u l t s . In the f i r s t g e neration a f t e r an a l a t a , f i r s t - b o r n progeny pro'duced no a l a t a and l a s t -born progeny produced a few. Then, on average, i n a l l succeeding g e n e r a t i o n s , f i r s t - b o r n aphids ( l i n e s 1 and 3) responded r e l a t i v e l y s t r o n g l y to crowding and l a s t - b o r n aphids ( l i n e s 2 and 4) r e l a t i v e l y weakly. However, t h i s i s not the way S u t h e r l a n d i n t e r p r e t e d h i s data. He b e l i e v e d t h a t a l a t a -p roduction had not r e t u r n e d to normal u n t i l a time roughly 79a F i g u r e 11 . R e s p o n s e t o a s t a n d a r d c r o w d i n g s t i m u l u s o f s u c c e s s i v e g e n e r a t i o n s o f a p h i d s o f t h e g r e e n s t r a i n o f A._ p_isum , A l l a p h i d s t e s t e d were a p t e r a e w i t h t h e e x c e p t i o n o f g e n e r a t i o n 0 . V e r i c a l l i n e s r e p r e s e n t t h e l i f e s p a n o f t h e i n s e c t s ( a p p r o x i m a t e l y 14 d a y s o f p r e - r e p r o d u c t i v e a n d 14 d a y s o f r e p r o d u c t i v e l i f e ) . H o r i z o n t a l l i n e s i n d i c a t e w h e t h e r t h e a p h i d s o f a p a r t i c u l a r g e n e r a t i o n were b o r n e a r l y o r l a t e i n t h e r e p r o d u c t i v e p e r i o d o f t h e i r p a r e n t s . G e n e r a t i o n numbers a r e g i v e n i n roman n u m e r a l s . T h e p e r c e n t a g e s o f a l a t a - p r o d u c e r s i n t h e t e s t e d f e m a l e s o f e a c h g e n e r a t i o n a r e g i v e n i n a r a b i c n u m e r a l s . ( r e d r a w n f r o m S u t h e r l a n d , 1 9 7 0 ) . 79 SXD Q 80 e q u i v a l e n t t o day 45 o f h i s e x p e r i m e n t . T h e r e a r e two m a j o r r e a s o n s f o r t h e d i f f e r e n c e b e t w e e n h i s i n t e r p r e t a t i o n and m i n e . F i r s t , S u t h e r l a n d t e s t e d o n l y two b a t c h e s , a f i r s t - b o r n and a l a s t - b o r n (as d i d L e e s ) , i n s t e a d o f f o u r b a t c h e s s p a c e d t h r o u g h o u t t h e r e p r o d u c t i v e p e r i o d . He t h e n made two i m p o r t a n t a s s u m p t i o n s a b o u t t h e r e l a t i o n s h i p b e t w e e n t h e s e two b a t c h e s , n e i t h e r o f w h i c h h a s t u r n e d o u t t o be v a l i d . He a s s u m e d t h a t when t h e r e i s no a l a t a i n an a p h i d ' s i m m e d i a t e a n c e s t r y , f i r s t -b o r n and l a s t - b o r n w o u l d r e s p o n d i d e n t i c a l l y t o c r o w d i n g . A s a r e s u l t , he n e i t h e r e x p e c t e d n o r r e c o g n i z e d t h e l o w e r r e s p o n s e s o f l i n e s 2 a n d 4 a s c o m p a r e d w i t h l i n e s 1 a n d 3 , w h i c h were a p p a r e n t i n g e n e r a t i o n I I and l a t e r . He a l s o a s s u m e d a l i n e a r i n c r e a s e i n i n t e n s i t y o f r e s p o n s e f r o m f i r s t - b o r n t o l a s t - b o r n p r o g e n y a s t h e t i m i n g m e c h a n i s m r a n down. He t h e r e f o r e came t o t h e c o n c l u s i o n t h a t t h e a b i l i t y t o p r o d u c e any a l a t a e o n l y r e t u r n e d v e r y l a t e i n g e n e r a t i o n I, r a t h e r t h a n a l m o s t i m m e d i a t e l y , a s i s t h e c a s e . The s e c o n d r e a s o n f o r S u t h e r l a n d ' s d i f f e r e n t i n t e r p r e t a t i o n i s r e l a t e d t o t h e l e v e l o f r e p l i c a t i o n . He d i d n o t s t a t e how many r e p l i c a t e s c o n t r i b u t e d t o e a c h p e r c e n t a g e i n F i g u r e 1 1 , b u t f o r most o t h e r work he u s e d b e t w e e n 20 and 40 a p h i d s . N o r d o e s he a p p e a r t o have r e p l i c a t e d t h e whole t e s t s e q u e n t i a l l y i n t i m e . As a r e s u l t , he a p p e a r s t o h a v e been u n a w a r e o f t h e l e v e l o f v a r i a t i o n p o s s i b l e i n t h e r e s p o n s e , and h a s p l a c e d f a r more c o n f i d e n c e i n h i s p e r c e n t a g e s t h a n may be w a r r a n t e d . T h a t t h e v a r i a t i o n i n h i s c l o n e was a s g r e a t a s i t was i n my c l o n e i s i l l u s t r a t e d by a c o m p a r i s o n o f t h e p e r c e n t a g e o f a l a t a - p r o d u c e r s 81 from g e n e r a t i o n I I I and l a t e r i n l i n e s 1 and 3 when an i n t e r v a l timer would no longer be i n v o l v e d . Sutherland has no e x p l a n a t i o n f o r t h i s v a r i a t i o n . The only data p o i n t i n F i g u r e 11 that supports the the i d e a of a ti m i n g mechanism such as an i n t e r v a l timer i s the s o - c a l l e d low value of 15% a l a t a producers i n the f i r s t - b o r n l i n e of gen e r a t i o n I I . But unless t h a t percentage i s the r e s u l t of many r e p l i c a t e s i t cannot be accepted as adeguate proof of the theor y . The data from my experiments, and a r e - a n a l y s i s of Sutherland's and Lees' work do not support the hypothesis that t h e r e i s an i n t e r v a l timer i n v o l v e d i n the c o n t r o l of the production of a l a t a e i n A_. pisum or i n Megoura v i c i a e . On the c o n t r a r y , the e x p l a n a t i o n o f t h a t c o n t r o l i s probably q u i t e simple, i n v o l v i n g changes with age i n the a l a t a which a f f e c t the c h a r a c t e r of the embryos i t i s c a r r y i n g , but have no f u r t h e r e f f e c t on the progeny a f t e r b i r t h . The q u e s t i o n of an i n t e r v a l timer i n the c o n t r o l of sexual morph dete r m i n a t i o n has not been addressed here. I t i s not p o s s i b l e to r e - a n a l y s e Lees' (1960) data to ev a l u a t e the p o s s i b i l i t y of the involvment of maternal-age e f f e c t s i n that system. In any case, there i s c e r t a i n l y more than a simple maternal-age e f f e c t i n v o l v e d i n the sexual morph dete r m i n a t i o n system, as i n h i b i t i o n can continue f o r as many as f i v e or s i x gen e r a t i o n s . 82 3.3.4 E f f e c t of grandparental age on p a r e n t a l a b i l i t y to produce a l a t e o f f s p r i n g : apterous grandparents, caged popu l a t i o n s A f t e r e s t a b l i s h i n g that there were p h y s i o l o g i c a l l y important d i f f e r e n c e s between e a r l y and l a t e - b o r n aphids i n t h e i r a b i l i t y to produce a l a t a e , I was i n t e r e s t e d to see whether such d i f f e r e n c e s might be e c o l o g i c a l l y important. The crowding s t i m u l u s u n t i l now had been a h i g h l y a r t i f i c i a l s t i m u l u s used by some p h y s i o l o g i s t s s t u d y i n g the a l a t e / a p t e r o u s polymorphism. The r e l a t i o n s h i p between t h i s s t i m u l u s and the s t i m u l u s an aphid may respond to i n nature i s unknown. The experiment shown i n Figure 12 was t h e r e f o r e designed to t e s t whether these same p h y s i o l o g i c a l d i f f e r e n c e s between e a r l y - and l a t e - b o r n progeny c o u l d a f f e c t aphids r e c e i v i n g more n a t u r a l crowding. The b a s i c approach was to put e a r l y - and l a t e - b o r n f i r s t -i n s t a r aphids on l a r g e bean p l a n t s and allow them to grow and reproduce i n t h i s l e s s a r t i f i c i a l environment. T h e i r progeny c o u l d then be scored as e i t h e r apterous or a l a t e . Only two batches of parents were used i n t h i s experiment i n s t e a d of the u s u a l f o u r . Batch 1 parents s e l e c t e d f o r t h i s t e s t were born on days 1 and 2 o f a grandparent's r e p r o d u c t i v e p e r i o d , and the batch 2 parents were born on days 5 or 6, midway between the p r e v i o u s batches I I and I I I . In a d d i t i o n , i n s t e a d of using a s i n g l e grandparent, each r e p l i c a t e used 10 grandparents i s o l a t e d from the stock c u l t u r e as f o u r t h - i n s t a r nymphs or t e n e r a l a d u l t s . Ten f i r s t - i n s t a r nymphs were c o l l e c t e d from each of the 10 grandparents from days 83a F i g u r e 12. Experimental design f o r i n v e s t i g a t i n g the e f f e c t of g r a n d p a r e ntal age at time of p a r e n t a l b i r t h on p a r e n t a l a b i l i t y to produce a l a t e o f f s p r i n g : apterous grandparents, caged p o p u l a t i o n s . ( a- a s i n g l e r e p l i c a t e b- timing of the r e p l i c a t e s ) . 83 10 grandparents i s o l a t e d batch 1 parents born 100 batch 1 parents placed i n cage (•) batch 2 parents born 100 batch 2 parents placed i n cage (o) batch 1 parents begin r e p r o d u c t i o n batch 2 parents begin r e p r o d u c t i o n batch 1 cage dismantled batch 2 cage dismantled batch 1 o f f s p r i n g counted batch 2 o f f s p r i n g counted ® o 10 20 30 Day • -batch 1 o-batch 2 ©-discarded 84 1 and 2 of r e p r o d u c t i o n , and again from days 5 and 6. Each batch of 100 parent nymphs was placed on the h i g h e s t growing t i p of a group of bean p l a n t s . T h i s group c o n s i s t e d of four 20-day-old bean p l a n t s 40 to 50cra t a l l i n one 6-inch (15cm) p l a s t i c pot c o n t a i n i n g a mixture of s o i l , peat, and f e r t i l i z e r . The f o u r p l a n t s were t i e d t o gether around a s i n g l e c e n t r a l s t a k e . Once the f i r s t i n s t a r parents were s e t t l e d , the pot was placed i n s i d e a l a r g e cage, 2x2x3ft (61x61x91cm). The cage was kept i n a greenhouse under a photoperiod of 16L:8D, maintained by n a t u r a l l i g h t i n g supplemented with f l u o r e s c e n t lamps. The temperature f l u c t u a t e d around 20°C. The p l a n t s and t h e i r aphids were l e f t u n t i l the parents had reached adulthood and reproduced f o r f o u r or f i v e days. At t h i s time, the remaining parent aphids were removed and counted. A l l p l a n t m a t e r i a l above the ground l e v e l was saved and i t s dry weight was l a t e r recorded. The o f f s p r i n g were c o n f i n e d , i n groups of approximately 50, to s e e d l i n g s caged i n the usual manner. They were l e f t to mature u n t i l they could be s c o r e d as a l a t a e or apterae. T h i s experimental u n i t was r e p l i c a t e d f o u r times. However, due to space l i m i t a t i o n s , o n l y t h r e e s e t s of r e p l i c a t e s c ould be maintained. From e a r l i e r work i t appeared t h a t d a i l y d i f f e r e n c e s would be g r e a t e r than grandparental d i f f e r e n c e s . In a d d i t i o n , each r e p l i c a t e was the product of 10, i n s t e a d of one grandparent and t h i s should have decreased d i f f e r e n c e s between a p a i r of r e p l i c a t e s . T h e r e f o r e , batch 1 of r e p l i c a t e 1 and batch 2 of r e p l i c a t e 4 were d i s c a r d e d . T h i s procedure r e s u l t e d i n three p a i r s of treatment batches as shown i n F i g u r e 12. 85 Once t h e young p a r e n t s were e s t a b l i s h e d , t h e y were n o t d i s t u r b e d a g a i n u n t i l the who le c o l o n y was d i s m a n t l e d . N e v e r t h e l e s s , a number o f q u a l i t a t i v e o b s e r v a t i o n s were made, and a n a v e r a g e s e q u e n c e o f e v e n t s c a n be d e s c r i b e d f o r t h e p e r i o d be tween e s t a b l i s h i n g t h e a p h i d s and d i s m a n t l i n g t h e c o l o n y . Mos t o f t h e 10 0 n y m p h a l p a r e n t s s e t t l e d down v e r y q u i c k l y a t o r n e a r t h e p o i n t where t h e y were f i r s t d e p o s i t e d . T h e r e was r e l a t i v e l y l i t t l e f u r t h e r movement t h r o u g h o u t t h e n y m p h a l p e r i o d . By t h e l a t e f o u r t h i n s t a r , t h e r e f o r e t h e p a r e n t a p h i d s were v e r y c l o s e t o g e t h e r and were u s i n g o n l y a s m a l l p e r c e n t a g e o f t h e a v a i l a b l e p l a n t s u r f a c e . N e a r t h e t i m e t h a t r e p r o d u c t i o n was b e g i n n i n g , t h e p a r e n t a p h i d s b e g a n m o v i n g a b o u t c o n s i d e r a b l y , a l t h o u g h most r e m a i n e d on t h e p l a n t s . As r e p r o d u c t i o n p r o g r e s s e d , more and more o f t h e p a r e n t a p h i d s b e g a n t o c r a w l a r o u n d , and l a r g e n u m b e r s c o u l d be s e e n w a l k i n g o f f t h e p l a n t s and d y i n g a b o u t t h e e d g e s o f t h e c a g e . A s m a l l p r o p o r t i o n o f t h e o f f s p r i n g s u f f e r e d t h e same f a t e . At t h i s t i m e , t h e p l a n t s s t i l l a p p e a r e d t o be h e a l t h y and g r o w i n g i n s p i t e o f t h e h e a v y i n f e s t a t i o n . T h e r e a r e a number o f i m p o r t a n t ways i n w h i c h t h i s e x p e r i m e n t s t i l l d i f f e r e d f r o m a n a t u r a l c r o w d i n g e x p e r i e n c e . F i r s t , t h e i n i t i a l l o c a l d e n s i t y o f p a r e n t a p h i d s was v e r y h i g h c o m p a r e d w i t h a n a t u r a l s i t u a t i o n , w h i c h may e x p l a i n why s o many a n i m a l s w a l k e d o f f t h e p l a n t . S e c o n d l y , t h e p a r e n t s t h a t l e f t t h e p l a n t had no o p p o r t u n i t y t o f i n d a new p l a n t u n d e r t h e e x p e r i m e n t a l c o n d i t i o n s b e f o r e d y i n g f r o m d e h y d r a t i o n . T h o s e t h a t l e f t m i g h t h a v e b e e n t h e o n e s t h a t r e c e i v e d t h e s t r o n g e s t 86 s t i m u l u s to produce a l a t a e . The important d i f f e r e n c e s between batches 1 and 2 t h e r e f o r e c o u l d have been expressed i n these animals r a t h e r than i n the ones that stayed on the p l a n t s . But without o f f s p r i n g , t h a t i n f o r m a t i o n i s not a v a i l a b l e . In a d d i t i o n , the aphid/aphid i n t e r a c t i o n s on a pot-grown broad-bean may be very d i f f e r e n t from those on a normal host i n the f i e l d . Legumes such as c l o v e r and a l f a l f a are the most common n a t u r a l host of t h i s s p e c i e s i n North America. On a host such as a l f a l f a , the aphids are concentrated on the growing t i p s , the only part of the p l a n t s u i t a b l e f o r them. On broad-bean any p a r t of the p l a n t i s s u i t a b l e f o r s u p p o r t i n g aphid growth and r e p r o d u c t i o n . T h i s means that the s u r f a c e area a v a i l a b l e and the aphid d i s t r i b u t i o n are very d i f f e r e n t on the experimental p l a n t s from the u s u a l s i t u a t i o n , i n the r e a l world, which could produce a d i f f e r e n t r a t e of aphid/aphid encounters and thereby a f f e c t the r e s u l t i n g p r o duction of a l a t a e . The r e s u l t s of the experiment are presented i n Table 11. The number of parents remaining a t the end of the experiment ranged from 24 to 32. The number of o f f s p r i n g ranged from about 1600 to n e a r l y 2300. The dry weight of the p l a n t s v a r i e d from 3.01gm to 4.02gm. None of these parameters was s i g n i f i c a n t l y d i f f e r e n t between the two treatment batches. The percentage of a l a t a e produced was low and v a r i e d c o n s i d e r a b l y between r e p l i c a t e s . Batch 2 produced a s l i g h t l y higher p r o p o r t i o n of a l a t a e than batch 1 i n a l l three c a s e s , but none of these d i f f e r e n c e s were s t a t i s t i c a l l y s i g n i f i c a n t . There i s a 12.5% p r o b a b i l i t y of the same batch being l a r g e r i n a l l three 87 Table 11. E f f e c t of age of apterous grandparents on the number of a l a t e o f f s p r i n g produced i n response t o a more n a t u r a l crowding s t i m u l u s . Batch 1 R e p l i c a t e number 1-2 2-3 3-4 I n i t i a l number of parents 100 100 100 F i n a l number of parents 27 29 32 T o t a l number of o f f s p r i n g 1 897 1652 2120 Number of a l a t e o f f s p r i n g 3 113 52 F i n a l dry weight of p l a n t (gm) 3.01 3.93 4.02 Pl a n t m a t e r i a l per parent (gm/o) 0. 11 0.14 0.13 Number of a l a t a e per s u r v i v i n g parent 0.11 3.89 1. 63 % a l a t a e 0.2 6.8 2.5 Batch 2 R e p l i c a t e number 1-2 2-3 3-4 I n i t i a l number of parents 100 100 100 F i n a l number of parents 31 24 24 T o t a l number of o f f s p r i n g 2297 1722 2290 Number of a l a t e o f f s p r i n g 46 171 64 F i n a l dry weight of p l a n t (gm) 3.53 3. 12 3. 68 P l a n t m a t e r i a l per parent (gm/<j)) 0. 12 0.13 0. 15 Number of a l a t a e per s u r v i v i n g parent 1 .48 7. 12 2.67 % a l a t a e 2.0 9.9 2.8 88 r e p l i c a t e s i f batch V and batch 2 are i n f a c t the same. The f a c t t h at the d i r e c t i o n of d i f f e r e n c e s between the batches was the reverse of th a t found i n e a r l i e r work may be noteworthy. However, t h i s experiment was f a r l e s s " n a t u r a l " than had a n t i c i p a t e d , so i t i s d i f f i c u l t to know how to i n t e r p r e t the r e s u l t s . That they are c o n s i s t e n t l y i n the same d i r e c t i o n suggests t h a t the three batch 1 r e p l i c a t e s are more s i m i l a r to one another than they are to the three batch 2 r e p l i c a t e s . The switch i n d i r e c t i o n of the e f f e c t , i f i t e x i s t s i n the data a t a l l , could be i n t e r p r e t e d i n a number of ways. Sutherland (1969a) found t h a t as the number of aphid/aphid i n t e r a c t i o n s i n c r e a s e d , the number of a l a t a e produced i n c r e a s e d . The d i f f e r e n c e between e a r l y - and l a t e - b o r n aphids could be simply a d i f f e r e n c e i n a c t i v i t y l e v e l , e a r l y - b o r n aphids being on t h e average more a c t i v e . T h i s d i f f e r e n c e would e x p l a i n both s e t s of data. When c o n f i n e d i n a small v i a l , the more a c t i v e e a r l y - b o r n aphids would experience more aphid/aphid encounters. When placed on a l a r g e p l a n t , the a c t i v e e a r l y - b o r n aphids would spread over the p l a n t f a s t e r and thus have fewer aphid/aphid encounters than the l e s s a c t i v e l a t e - b o r n i n d i v i d u a l s . However i f a c t i v i t y l e v e l were a l l that were i n v o l v e d , batch 2 would be expected to have more a d u l t s remaining on the p l a n t at the end of the experiment because o f i t s lower a c t i v i t y l e v e l . T h i s was not the case. In f a c t , the numbers of parents remaining were s u r p r i s i n g l y uniform. ' The production of a l a t a e i n a l l cages was very low i n r e l a t i o n to the i n t e n s i t y of the crowding e x p e r i e n c e . I t seems 89 , l i k e l y t h a t the important i n d i v i d u a l s i n both batches were those aphids t h a t l e f t the p l a n t s . They were the i n d i v i d u a l s responding most to the s t i m u l u s , and t h e r e f o r e might have been the ones t h a t would produce the most a l a t a e . As p r e v i o u s l y noted, i t would be necessary to know the p r o p o r t i o n s of a l a t a e produced by these i n d i v i d u a l s b e f o r e we can answer the o r i g i n a l g u e s t i o n . A very much more complex experimental system would be necessary f o r d e f i n i t i v e t e s t s o f the e c o l o g i c a l aspects of maternal-age e f f e c t s . 3.4 D i s c u s s i o n Before beginning a d i s c u s s i o n of the p o s s i b l e importance of v a r i a t i o n s i n p r o d u c t i o n of a l a t a e due to maternal-age e f f e c t s , i t may be u s e f u l to summarize the r e s u l t s of the major experiments. A l a r g e p r o p o r t i o n of aphids born i n the f i r s t two or three days of an a p t e r a ' s r e p r o d u c t i v e p e r i o d respond to a standard crowding treatment by producing a l a t e aphids. A s m a l l e r p r o p o r t i o n of those born during the r e s t of the aptera's l i f e respond thus (Figure 13a and 13b). E a r l y - b o r n aphids which respond to crowding tend to produce a much high e r number of a l a t a e than those born l a t e r (Figure 14a and 14b). T h i s d i f f e r e n c e r e s u l t s i n approximately twice as many a l a t a e being produced by groups of e a r l y - b o r n aphids than by s i m i l a r groups of l a t e r - b o r n aphids (Figure 15a and 15b). In other words, w i t h i n the context of the standard crowding s t i m u l u s , an aphid 90a Figure 13. E f f e c t of grandparental age at the time of p a r e n t a l b i r t h on the p r o p o r t i o n of parents producing a l a t e o f f s p r i n g : a) and b) apterous grandparents; c) a l a t e grandpaents (o o 6 r e p l i c a t e s , o o 5 r e p l i c a t e s ) . Proportion of parents producing alate offspring 91a Figure 14. E f f e c t of grandparental age at the time of p a r e n t a l b i r t h on the mean number of a l a t e o f f s p r i n g produced per parent responding: a) and b) apterous grandparents; c) a l a t e grandparents (o o 6 r e p l i c a t e s , o o 5 r e p l i c a t e s ) . Mean number of alatae per parent respond ing 92a F i g u r e 1 5 . E f f e c t o f g r a n d p a r e n t a l age a t t h e t i m e o f p a r e n t a l b i r t h on t h e t o t a l p r o p o r t i o n o f a l a t e o f f s p r i n g p r o d u c e d : a) and b) a p t e r o u s g r a n d p a r e n t s ; c) a l a t e g r a n d p a r e n t s (o o 6 r e p l i c a t e s , o o 5 r e p l i c a t e s ) . 92 93 bears most of i t s a l a t a - p r o d u c e r s i n the f i r s t few days of i t s r e p r o d u c t i v e p e r i o d , bearing mainly aptera-producers t h e r e a f t e r . a l a t a e u s u a l l y produce only aptera-producers i n t h e i r f i r s t two or three days of r e p r o d u c t i o n . However, they soon begin to give b i r t h to a l a t a - p r o d u c e r s a t roughly the same low l e v e l as apterae do l a t e r i n t h e i r r e p r o d u c t i v e p e r i o d (Figures 13c, 14c, and 15c). I t may be worth noting from F i g u r e s 13, 14, and 15, t h a t , f o r both apterae and a l a t a e , the products of r e p r o d u c t i o n from the f o u r t h day on, although v a r i a b l e , behave i n much the same way (compare a l s o , Figures 6, 8, and 9 f o r batches I I , I I I , and IV) . These d i f f e r e n c e s c o u l d be extremely h e l p f u l to the p h y s i o l o g i s t i n i n v e s t i g a t i n g the mechanisms c o n t r o l l i n g the a l a t e / a p t e r o u s polymorphism i n aphids. Animals with p a r t i c u l a r t e n dencies to produce a l a t a e can be e a s i l y obtained i n order to study the d i f f e r e n c e s i n t h e i r hormonal and .reproductive p h y s i o l o g y . From the i n f o r m a t i o n a l r e a d y a v a i l a b l e , i t appears that j u v e n i l e hormone i s i n t i m a t e l y i n v o l v e d i n the p r o d u c t i o n of a l a t a e i n aphids (Johnson, 1959; Lees, 1961, 1966; White, 1968, 1971; White and Gregory, 1972). The presence of j u v e n i l e hormone at t h e a p p r o p r i a t e time d i s r u p t s the normal pr o d u c t i o n of wings. In a d d i t i o n , changes i n the a c t i v i t y of the corpus alatum, the g l a n d ' r e l e a s i n g j u v e n i l e hormone, seem to be c o r r e l a t e d with wing p r o d u c t i o n . However, the whole process i s s t i l l p o o r l y understood. Depending on the s p e c i e s , an aphid may be determined as an a l a t a before i t i s born, a f t e r i t i s born, or at e i t h e r 94 time (Lees, 1966). In d i f f e r e n t i n s t a n c e s , t h e r e f o r e , i t might be j u v e n i l e hormone produced by the mother, by the unborn embryo, by the young aphid, or by a l l t h r e e , t h a t i s important t o the p r o d u c t i o n of an a l a t a . One can o n l y s p e c u l a t e how the hormonal changes might r e l a t e to maternal-age e f f e c t s . Almost a l l aphids have s m a l l wing buds j u s t p r i o r to and j u s t a f t e r b i r t h (Johnson and Birks,1960). In aphids determined as a l a t a e , these continue to develop, whereas i n a l l others they degenerate. White (1971) found t h a t the s i z e of these wing buds p r i o r to b i r t h v a r i e d , depending on whether the mother was newly moulted or was o l d e r , and whether i t was a l a t e or apterous. She found that these d i f f e r e n c e s i n s i z e were c o r r e l a t e d with changes i n the s i z e of the corpus alatum of both the a d u l t and the unborn nymph. I t may be t h a t i n a d d i t i o n to a f f e c t i n g the s i z e of wing buds i n an embryo, the mother's age a l s o i n f l u e n c e s the development and c h a r a c t e r i s t i c s of the hormonal system of the embryo, i n c l u d i n g the p o t e n t i a l of the corpus alatum. The r e s u l t i n g e f f e c t s could a f f e c t the embryo's l a t e r a b i l i t y to produce a l a t a e . The d i f f e r e n c e s r e s u l t i n g from b i r t h order may be e v o l u t i o n a r y a r t i f a c t s of other p h y s i o l o g i c a l processes, or they may be s p e c i f i c a l l y evolved a d a p t a t i o n s . In e i t h e r case, they may have e c o l o g i c a l as w e l l as p h y s i o l o g i c a l import, and merit d i s c u s s i o n i n terms of aphid ecology. There are two ways to approach b i r t h o r d e r d i f f e r e n c e s . They can be c o n s i d e r e d i n r e l a t i o n to the s u r v i v a l of one i n d i v i d u a l and that i n d i v i d u a l ' s descendants (and t h e r e f o r e i t s genes). T h i s i s the long-term or 95 e v o l u t i o n a r y p o i n t o f v i e w . O r t h e y c a n be c o n s i d e r e d t h r o u g h t h e i r e f f e c t s on p o p u l a t i o n p r o c e s s e s f r o m one g e n e r a t i o n t o the n e x t . T h i s i s a s h o r t - t e r m v i e w , A u s e f u l c o n c e p t t o keep i n mind when d i s c u s s i n g b o t h v i e w p o i n t s i s den B o e r ' s (1968) " s p r e a d i n g o f r i s k " , (den B o e r h a s h y p o t h e s i z e d t h a t v a r i a t i o n w i t h i n n a t u r a l p o p u l a t i o n s , i n s t e a d o f b e i n g s i m p l y t h e r e s u l t o f i m p e r f e c t l y a d a p t e d i n d i v i d u a l s , i s a c t u a l l y a d a p t i v e , d i m i n i s h i n g t h e r i s k o f e x t i n c t i o n and l e s s e n i n g t h e a m p l i t u d e o f p o p u l a t i o n f l u c t u a t i o n s . ) An a p t e r o u s a p h i d a p p e a r s t o s p r e a d r i s k among i t s p r o g e n y i n r e l a t i o n t o d i s p e r s a l i n a t l e a s t two w a y s . F i r s t , i t s r e s p o n s e t o c r o w d i n g o r p l a n t d e t e r i o r a t i o n v a r i e s o v e r i t s l i f e t i m e . S u t h e r l a n d (1969a) f o u n d i n A._ £ i s u m t h a t p r o d u c t i o n o f a l a t a e was l o w e r e d i n o l d a p h i d s . B o n n e m a i s o n (1951) r e p o r t e d a d e c r e a s e i n a b i l i t y t o p r o d u c e a l a t a e a s t h e a g e o f t h e m o t h e r i n c r e a s e d i n B r e y i c r y n e b r a s s i c a e , b u t he f o u n d i n MY.zus E g r s i c a e t h a t a b i l i t y t o p r o d u c e a l a t a e i n c r e a s e d t o a maximum midway t h r o u g h t h e r e p r o d u c t i v e p e r i o d , t h e n d e c r e a s e d a g a i n . An a p h i d d u r i n g i t s r e p r o d u c t i v e l i f e t h u s w i l l p r o d u c e some b a l a n c e o f d i s p e r s e r s a n d n o n - d i s p e r s e r s among i t s p r o g e n y . The r a t i o o f t h e two morphs i t p r o d u c e s w i l l be r e l a t e d t o t h e i n t e n s i t y o f t h e s t i m u l i ' e l i c i t i n g p r o d u c t i o n o f a l a t a e . S e c o n d l y , a s shown f o r A . £ i s u m , an a p h i d may s p r e a d r i s k among i t s a p t e r o u s p r o g e n y by p r o d u c i n g i n d i v i d u a l s t h a t v a r y i n t h e i r a b i l i t y t o p r o d u c e d i s p e r s e r s when t h e y r e c e i v e t h e a p p r o p r i a t e s t i m u l u s . I n o t h e r w o r d s , e a c h a p h i d p r o d u c e s some i n d i v i d u a l s w h i c h t e n d t o be a l a t a - p r o d u c e r s and o t h e r s t h a t 96 tend to be aptera-producers. These two maternal types are produced a t d i f f e r e n t times i n the aphid's r e p r o d u c t i v e p e r i o d : the former i n the f i r s t few days; the l a t t e r over the remainder of t h e r e p r o d u c t i v e p e r i o d . The r e s u l t of t h i s d i v i s i o n i s that the aphid's d i r e c t , apterous descendants i n c l u d e i n d i v i d u a l s that can respond very s t r o n g l y to s t i m u l i and ot h e r s that w i l l respond weakly, i f a t a l l . In t h i s manner each aphid produces descendants to meet a l l e v e n t u a l i t i e s ; to d i s p e r s e i n l a r g e numbers as w e l l as to remain i n the l o c a l i t y f o r another ge n e r a t i o n . By producing a range of progeny types an aphid may i n c r e a s e the l i k e l i h o o d that a t l e a s t one l i n e of descendants w i l l s u r v i v e and maintain that p a r t i c u l a r genotype. In terms of pr o d u c t i o n of d i s p e r s e r s , a l a t e aphids appear to be much more r e s t r i c t e d than apterous ones. Al a t a e of EiUM™ a n c * many other s p e c i e s (Lees, 1966) are unable to produce a l a t a e a t a l l , although other s p e c i e s do r e g a i n t h i s a b i l i t y l a t e r i n l i f e (Johnson and Birks,1960). In a d d i t i o n , a l a t a e are somewhat r e s t r i c t e d i n t h e i r a b i l i t y to bear a l a t a -producers. A._ pisum a l a t a e can produce a range of types among t h e i r progeny. But i n s t e a d of ranging from s t r o n g to weak a l a t a -producers, the types of progeny range from non-producers of a l a t a e to weak a l a t a - p r o d u c e r s . T h i s d i f f e r e n c e between a l a t a e and apterae may be due to some p h y s i o l o g i c a l c o n s t r a i n t a s s o c i a t e d with the a l a t e c o n d i t i o n . The hormonal s t a t e necessary t o develop f u n c t i o n a l wings may be in c o m p a t i b l e with the s t a t e necessary to produce young t h a t are a l a t a e or w i l l be a l a t a - p r o d u c e r s . The su p p r e s s i o n of course may be an adaptation 97 t h a t i s advantageous to an a l a t e aphid and i t s progeny. I t s e x i s t e n c e c e r t a i n l y ensures that a t l e a s t one complete g e n e r a t i o n remains on a new host p l a n t before l a r g e numbers of new a l a t a e are produced to d i s p e r s e f u r t h e r . Any disadvantages r e s u l t i n g from t h i s temporary i n a b i l i t y to d i s p e r s e may be compensated f o r by the s e l e c t i v i t y of the a l a t a e i n choosing a p l a n t on which to s e t t l e and reproduce. It has been observed (way and Cammell, 1970) t h a t a l a t e aphids w i l l s e t t l e b r i e f l y on and then leave many appa r e n t l y s u i t a b l e p l a n t s before f i n a l l y s e l e c t i n g one. The s e l e c t i o n of a h i g h l y s u i t a b l e p l a n t should minimize the need f o r d i s p e r s e r s d u r i n g the f i r s t few g e n e r a t i o n s a f t e r a l o c a l p o p u l a t i o n i s e s t a b l i s h e d by an a l a t a . When i t f i n a l l y s e t t l e s on a s u i t a b l e p l a n t , an aphid may stay f o r the r e s t of i t s l i f e and produce a l a r g e , l o c a l colony (e.g., Aphis fabae ; Way and Banks, 1967) or i t may give b i r t h to a few l a r v a e , then f l y t o another host p l a n t (e.g., H^zus p e r s i c a e ; Johnson, 1957). In the l a t t e r case, the groups of aphids d e p o s i t e d would a l l be from the same part of the mother's l i f e c y c l e and would show s i m i l a r responses to a l a t a - p r o d u c i n g s t i m u l i . I t would be e s p e c i a l l y important to the groups deposited while the mother was i n h i b i t i n g the production of a l a t a - p r o d u c e r s that the chosen host p l a n t continue to be s u i t a b l e f o r a t l e a s t two g e n e r a t i o n s . I t would be i n t e r e s t i n g t o know i f the aphid's, s e a r c h i n g behaviour and c r i t e r i a f o r h o s t - p l a n t s e l e c t i o n change i n any measurable way a f t e r i t begins producing aphids t h a t can i n t u r n produce a l a t a e . 98 There may be other ways i n which an aphid spreads r i s k among i t s progeny i n r e l a t i o n t o d i s p e r s a l . One such aspect has been r e p o r t e d by Shaw (1970a, 1970b, and 1970c). I t was long assumed t h a t a l l a l a t e aphids flew before they s e t t l e d on p l a n t s and began to reproduce; i . e . , t h a t the polymorphism was a complete dimorphism, so t h a t aphids were e i t h e r d i s p e r s e r s or n o n - d i s p e r s e r s . Shaw has shown t h a t t h i s assumption i s i n c o r r e c t . While s t u d y i n g Ajp_his fabae , he found t h a t some a l a t a e showed the expected towering behaviour; f l y i n g up i n t o the a i r stream and d i s p e r s i n g over long d i s t a n c e s . But he a l s o found a l a t a e t h a t flew only s h o r t d i s t a n c e s from p l a n t to p l a n t i n t he same l o c a l i t y , and some t h a t would not f l y at a l l . He found t h a t the p r o p o r t i o n s of these three groups i n a l a b o r a t o r y p o p u l a t i o n that was producing a l a t a e was a f f e c t e d by the c o n d i t i o n s under which the presumptive a l a t a e developed. Thus, i f the c o n d i t i o n s which i n i t i a l l y caused the pr o d u c t i o n of a l a t a e improved - i f , f o r example, aphid d e n s i t i e s decreased, or pla n t c o n d i t i o n s improved- then fewer of the subsequent a l a t a e developed i n t o the f i r s t c ategory. Consequently the l e v e l of long-range d i s p e r s a l would be lower, and the p r o p o r t i o n of l o c a l or n o n - f l i e r s higher. I t would be worth i n v e s t i g a t i n g whether maternal age might i n f l u e n c e the p r o p o r t i o n s of the d i f f e r e n t groups. For example, are more l o n g - d i s t a n c e d i s p e r s e r s produced by aphids of a p a r t i c u l a r age, or by aphids born at a p a r t i c u l a r p o i n t i n t h e i r mother's r e p r o d u c t i v e period? S p e c u l a t i o n on the p o p u l a t i o n consegences of d i f f e r e n c e s i n a l a t a - p r o d u c t i o n due to b i r t h order r a i s e s many q u e s t i o n s . When 99 a s i n g l e a l a t e aphid e s t a b l i s h e s i t s e l f on an u n i n f e s t e d p l a n t i t begins to produce apterous aphids, b u i l d i n g a l o c a l p o p u l a t i o n . Depending on the s p e c i e s , the aphids may remain c l o s e t o g e t h e r , e v e n t u a l l y forming a l a r g e , dense aggregation (e.g.. Aphis fabae ; Way and Banks, 1967: Brevicoryne b r a s s i c a e ; Hughes, 1963), or they may spread themselves f u r t h e r around the p l a n t (e.g., i£I£ihosiphon pisum : Myzus p e r s i c a e ; van Emden e t a l , 1969). In both c a s e s , a l a t e i n d i v i d u a l s are produced as p o p u l a t i o n s b u i l d up. In study a f t e r study t h i s a l a t a - p r o d u c t i o n has been shown to be a response mainly to crowding and h o s t - p l a n t d e t e r i o r a t i o n (e.g., Bonnemaison, 1951; Lees, 1961, 1967; Johnson, 1965, 1966; Sutherland, 1969a, 1969b), but the a l a t a e may perform s l i g h t l y d i f f e r e n t f u n c t i o n s i n the two d i f f e r e n t types of s p e c i e s . The aphids which form l a r g e aggregations cause severe damage which can r e s u l t i n the death of the host p l a n t . But these s p e c i e s are u s u a l l y very p a t c h i l y d i s t r i b u t e d i n the environment, l e a v i n g many p l a n t s u n i n f e s t e d . They seem to r e l y on producing a s i n g l e peak i n numbers of a l a t a e , i n order to d i s p e r s e to new p l a n t s p r i o r to the complete c o l l a p s e of t h e i r host, thereby a v o i d i n g e x t i n c t i o n . On the other hand, the s p e c i e s of aphids which space themselves to a g r e a t e r degree appear to r e l y on the production of a l a t a e to keep p o p u l a t i o n s at lower d e n s i t i e s and thus to preserve the host p l a n t longer. To do so, however, these s p e c i e s need a mechanism r e n d e r i n g them p a r t i c u l a r i l y s e n s i t i v e to s m a l l changes i n d e n s i t y . I t would appear t h e r e f o r e t h a t the production of a l a t a e may 100 prevent l o c a l e x t i n c t i o n of the aphid p o p u l a t i o n when the host p l a n t i s destroyed (e.g., Aphis fabae ; Way and Cammell, 1972), or i t may s t a b i l i z e l o c a l p o p u l a t i o n s at a more t o l e r a b l e l e v e l (e.g., My_zus £ersicae ; Wyatt, 1965). Examination of s p e c i e s d i s p l a y i n g these two p a t t e r n s with r e s p e c t to the e f f e c t s of maternal age on a l a t a - p r o d u c t i o n might prove rewarding. Age-r e l a t e d d i f f e r e n c e s could help to provide a p o p u l a t i o n with a s e n s i t i v e mechanism f o r the d e n s i t y c o n t r o l necessary i n such s p e c i e s as Acyrthosiphon £isum , which spread themselves over t h e i r host p l a n t s i n s t e a d of c l u s t e r i n g l i k e the aggregated s p e c i e s . For s p e c i e s l i k e A._ £isum , there may be very important p o p u l a t i o n advantages i n having some i n d i v i d u a l s that produce high and some that produce low numbers of a l a t a e , i n s t e a d of having a l l females producing a l a t a e at the same r a t e . I f a l l i n d i v i d u a l s responded e q u a l l y to s t i m u l i to produce a l a t a e , then the r e s u l t would be a sudden very l a r g e peak of d i s p e r s e r s . Such a peak co u l d have two e f f e c t s on the p o p u l a t i o n . F i r s t , i t could reduce the l o c a l p o p u l a t i o n f a r below the l e v e l necessary to prevent p l a n t damage, perhaps a l s o l e a v i n g i t v u l n e r a b l e t o predators and p a r a s i t e s . Secondly, i f a ca t a s t r o p h e such as a p e r i o d of bad weather c o i n c i d e d with the peak, a l l the d i s p e r s e r s might be l o s t . But i f only a f r a c t i o n of the i n d i v i d u a l s of each g e n e r a t i o n responded very s t r o n g l y to the s i g n a l to produce a l a t a e the production would be spread over a much longer p e r i o d , thus a v o i d i n g both these d i f f i c u l t i e s . Some hypotheses a r i s e from a c o n s i d e r a t i o n of the p o s s i b l e 101 e f f e c t s of d i f f e r e n c e s i n a d u l t a g e - s t r u c t u r e i n d i f f e r e n t p o p u l a t i o n s of aphids. A p o p u l a t i o n c o n s i s t i n g predominantly of young a d u l t s might respond to environmental c o n d i t i o n s very d i f f e r e n t l y from one with a l a r g e component of middle-aged or o l d a d u l t s . In the f o l l o w i n g g e n e r a t i o n , the former p o p u l a t i o n would c o n t a i n a much l a r g e r p r o p o r t i o n of e a r l y - b o r n aphids than the l a t t e r , and that d i f f e r e n c e could r e s u l t i n the production of very d i f f e r e n t numbers of a l a t a e by the two p o p u l a t i o n s . This a g e - r e l a t e d d i f f e r e n c e might be one way i n which a p o p u l a t i o n c o u l d r e g u l a t e i t s d i s p e r s a l r a t e i n r e l a t i o n to c o n d i t i o n s i n the h a b i t a t . D i f f e r e n c e s i n a d u l t a g e - s t r u c t u r e c o u l d r e s u l t from v a r i o u s kinds of s e l e c t i v e m o r t a l i t y , such as s e l e c t i v e p r e d a t i o n or p a r a s i t i s m ; g r e a t e r s e n s i t i v i t y of an age group to f a c t o r s such as unfavourable temperatures, or heavy r a i n s t h a t knock aphids o f f p l a n t s (Dunn and Wright, 1955). Nothing i s known of the r e l a t i o n s h i p of age t o these m o r t a l i t y processes 3.5 Summary 1. Apterous and a l a t e JU pisum were found to d i f f e r i n t h e i r r e p r o d u c t i v e p a t t e r n s . A l a t a e began r e p r o d u c t i o n l a t e r than apterae and produced fewer progeny over a longer p e r i o d of time. 2. Among apterous aphids, progeny born i n the f i r s t two or three days of the r e p r o d u c t i v e p e r i o d l a t e r produced higher numbers of a l a t a e i n response to a standard crowding sti m u l u s than d i d the progeny born during the remainder of 102 the r e p r o d u c t i v e p e r i o d . For a l a t e aphids, progeny born i n the f i r s t two or three days of the r e p r o d u c t i v e p e r i o d u s u a l l y produced no a l a t a i n response t o the standard crowding s t i m u l u s , whereas the progeny born l a t e r i n the r e p r o d u c t i v e p e r i o d could produce numbers comparable t o those born to s i m i l a r progeny of apterae when both were s u b j e c t e d t o crowding. A l a t e aphids d i d not respond to the standard crowding s t i m u l u s , and never produced a l a t a e . An " i n t e r v a l t i m e r " d i d not e x p l a i n the manner i n which a l i n e a g e regained the a b i l i t y to produce a l a t a e a f t e r i n i t i a l e s t ablishment of the l i n e by an a l a t e aphid. T h i s recovery was s u f f i c i e n t l y accounted f o r by maternal-age e f f e c t s a c t i n g a l o n e . P r e l i m i n a r y s t u d i e s of aphid p o p u l a t i o n s i n l a r g e cages were i n c o n c l u s i v e and d i d not allow statements t o be made about maternal-age e f f e c t s under a more n a t u r a l crowding s t i m u l u s than the standard s t i m u l u s o f t e n a p p l i e d . C o n s i d e r a t i o n of some of the i n f o r m a t i o n a v a i l a b l e on aphid p h y s i o l o g y and ecology suggested a number of ways i n which maternal-age e f f e c t s might be important to an understanding of aphid p o p u l a t i o n b i o l o g y . 103 Chapter 4 V a r i a t i o n i n i n c i d e n c e of diapause and growth c h a r a c t e r i s t i c s i n Eriocamp_a ovata L, the red-backed sawfly (Hymenoptera:Tenthredinidae) 4.1 I n t r o d u c t i o n The p r i n c i p l e q u e s t i o n asked i n the f o l l o w i n g study of the red-backed sawfly, Eriocampa ovata , was whether maternal age a t time of egg-laying a f f e c t e d the tendency of the progeny to enter diapause. Other c h a r a c t e r i s t i c s of the l i f e c y c l e , such as developmental times and numbers of i n s t a r s , were monitored s i m u l t a n e o u s l y . There appeared to be good reason to suspect an i n f l u e n c e of maternal age on the onset of diapause, at some stage, diapause i s e s s e n t i a l t o most temperate-zone i n s e c t s i n order to avoid c o l d , j u s t as i t i s e s s e n t i a l f o r the t r o p i c a l i n s e c t s exposed t o drought. T h e r e f o r e , the mechanism of c o n t r o l , and any f a c t o r s which i n f l u e n c e t h a t mechanism, w i l l have an impact on the b i o l o g y of the s p e c i e s . An i n f l u e n c e of maternal age on the i n c i d e n c e of diapause has a l r e a d y been documented f o r a number of the p a r a s i t i c Hymenoptera (Simmonds, 1949; Saunders, 1961, 1965, 1966a, 1966b; McNeil and Rabb, 1973). C o n s i d e r a b l e v a r i a t i o n , even w i t h i n one f a m i l y , i n the d u r a t i o n or p r o l o n g a t i o n of diapause, has a l r e a d y been observed i n s a w f l i e s (Prebble, 1941). But the cause of t h i s v a r i a t i o n i s as yet 104 unknown, although Pschorn-Walcher (1970) found t h a t the l a t e r i n the season a cocoon was spun, the gr e a t e r the p r o b a b i l i t y that the prepupa would enter prolonged diapause. A l l of these f a c t o r s suggested t h a t i n c i d e n c e of diapause i n s a w f l i e s might be a v a r i a b l e c h a r a c t e r i s t i c capable of being i n f l u e n c e d by maternal age. Ms. ov^ia w a s chosen as the s p e c i e s to study f o r s e v e r a l reasons. I t was common i n the v i c i n i t y and easy t o re a r at a l l stages. I t feeds on red a l d e r , Alnus rubra , which i s very common l o c a l l y and t h e r e f o r e o b t a i n a b l e i n l a r g e q u a n t i t i e s . The sawfly can have more than one ge n e r a t i o n a n n u a l l y , e x h i b i t i n g a f a c u l t a t i v e r a t h e r than an o b l i g a t e diapause. 4.2 M a t e r i a l s and methods 4.2.1 A d u l t s and eggs To o b t a i n eggs, a d u l t s had to be provided with l i v e a l d e r p l a n t s . Because of t h i s n e c e s s i t y , n e i t h e r the a d u l t nor the egg stage were kept under c o n t r o l l e d c o n d i t i o n s . To obt a i n the necessary p l a n t s , s e e d l i n g s about 1 f t (30cm) high were potted each year i n the very e a r l y s p r i n g , p r i o r to l e a f i n g , i n 8-inch (20cm) p l a s t i c pots i n a mixture of sandy s o i l and peat. They were f e r t i l i z e d p e r i o d i c a l l y with Osmocote 14-14-14, a slow-r e l e a s e f e r t i l i z e r . The p l a n t s were maintained o u t s i d e i n a sunny exposure, and watered d a i l y , or as necessary. They grew well under these c o n d i t i o n s , so t h a t by mid-summer i t was necessary to prune them. 105 To o b t a i n eggs of known h i s t o r y , an a l d e r p l a n t with no sawfly eggs or l a r v a e on i t was placed i n a cage 2x2x3ft (61x61x91cm) covered with f i b e r g l a s s screening and kept e i t h e r outdoors i n a sunny l o c a t i o n , or i n a greenhouse. A newly-emerged female was r e l e a s e d i n the cage. Each morning before the temperature was high enough f o r e g g - l a y i n g , the p l a n t was removed from the cage and checked f o r eggs l a i d the previous day. A r e c o r d was kept of the date each egg was d e p o s i t e d . A s m a l l p i e c e of tape was numbered and placed around the p e t i c l e of t h e l e a f . A drawing was made of the l e a f and a r e c o r d kept of the p o s i t i o n and number of eggs and the dates they were l a i d . A l l l e a v e s c o n t a i n i n g eggs were checked d a i l y f o r newly-hatched l a r v a e . These were removed as soon as they were found. Each sawfly was l e f t i n i t s cage u n t i l i t d i e d . I t was then p l a c e d i n Kahle's s o l u t i o n f o r l a t e r d i s s e c t i o n . Normally a sawfly was a l l o t t e d a p l a n t on emergence and was l e f t with t h a t p l a n t u n t i l i t d i e d . At no time d i d there appear to be a shortage of s u i t a b l e e g g - l a y i n g s i t e s as the female continued to reproduce. On a few o c c a s i o n s , an a l d e r p l a n t was changed part way through a female's r e p r o d u c t i v e p e r i o d , but the behaviour and r e p r o d u c t i v e p a t t e r n of those females was no d i f f e r e n t from that of females c o n f i n e d to one p l a n t . 106 4,2.2 Larvae For most purposes, l a r v a e were obtained from eggs of experimental females caged with young a l d e r s as p r e v i o u s l y d e s c r i b e d . These l a r v a e were removed from the p l a n t s to l a b o r a t o r y c o n d i t i o n s w i t h i n a few hours of h a t c h i n g . However, on some o c c a s i o n s , l a r v a e were obtained from eggs l a i d i n the f i e l d . Leaves with eggs were e i t h e r marked with b r i g h t tape and observed d a i l y u n t i l the l a r v a e hatched or, i f they were n e a r l y ready to hatch l e a v e s were sometimes brought i n t o the l a b o r a t o r y and kept watered u n t i l hatching o c c u r r e d . Two d i f f e r e n t r e a r i n g methods were used f o r the study. In one, a s i n g l e stem of a l d e r comprising from one to three leaves was placed i n a 50x15mm g l a s s v i a l f u l l of water. A plug of c o t t o n wadding was used to prevent water leakage from the v i a l . E i t h e r one or two of these stems was placed i n a 32-ounce (909cc) unwaxed cardboard f r e e z e r c a r t o n . The sawfly l a r v a e were pla c e d on the f o l i a g e and the c o n t a i n e r was c l o s e d with a c l e a r p l a s t i c l i d . The f o l i a g e was r e p l a c e d every second or t h i r d day. This method of r e a r i n g was used p a r t i c u l a r l y f o r experiments where more than one l a r v a per c o n t a i n e r might be r e q u i r e d . Larvae reared by t h i s method were u s u a l l y reared a t room temperatures and l i g h t i n t e n s i t i e s . The second r e a r i n g method was adopted so t h a t l a r g e numbers of l a r v a e c o u l d be re a r e d s i n g l y without the need f o r l a r g e amounts of space. Paper t o w e l l i n g was c u t to f i t , m o i s t e n e d , and placed i n the bottom of a 100x15mm p l a s t i c p e t r i d i s h . An a l d e r l e a f , c u t to s i z e , was placed on the paper, with the under-107 s u r f a c e upward. A l a r v a was placed on that s u r f a c e and the l i d put i n p l a c e . The p e t r i d i s h was then i n v e r t e d . Using t h i s method, the l e a f remained f r e s h and unwilted and the l a r v a was able to feed i n i t s normal p o s i t i o n on the underside of the l e a f . The p e t r i d i s h was checked d a i l y to see t h a t the paper remained moist, the l e a f was r e p l a c e d every second or t h i r d day. At which time any f r a s s was removed and the d i s c of t o w e l l i n g was renewed. T h i s regime was necessary to avoid problems with mould growth. A d a i l y r e c o r d was kept of l a r v a l moults. Larvae reared i n p e t r i dishes were reared i n a growth chamber at a temperature of 20±1°C and a photoperiod of 16L:8D from the day they hatched u n t i l they emerged as a d u l t s . Feeding r a t e s were measured on l a r v a e reared i n the cardboard c o n t a i n e r s . Area of l e a f consumed was the measure taken. Larvae were fed l e a v e s t h a t had e i t h e r been t r a c e d i n o u t l i n e or cut to known s i z e . The leaves were r e p l a c e d r e g u l a r l y and the area of l e a f consumed measured on m i l l i m e t r e - s q u a r e d paper. 4.2.3 Prepupae and cocoons The f i n a l l a r v a l i n s t a r r e q u i r e s a s o i l - l i k e medium i n which to s p i n i t s cocoon. T h i s was provided i n the cardboard c o n t a i n e r s by packing down a few t e a s p o o n s f u l of moist peat on the bottom of the c o n t a i n e r . In the p e t r i d i s h e s i t was provided by p r e s s i n g one or two t e a s p o o n s f u l of the peat i n t o a mound i n the c e n t r e of the l i d of the p e t r i d i s h . The bottom of the p e t r i d i s h , with the l e a f and the l a r v a , was then r e p l a c e d i n the 108 i n v e r t e d p o s i t i o n over t h i s mound. The peat was i n t r o d u c e d as d soon as the f i n a l moult took p l a c e . A few days a f t e r the l a r v a had entered the peat, the cocoon was removed and placed i n a c o n t a i n e r where a d u l t emergence cou l d be more e a s i l y observed. In the i n d i v i d u a l r e a r i n g s , the cocoons were t r a n s f e r r e d to s m a l l p l a s t i c p e t r i d i s h e s , 35x10mm, each of which c o n t a i n e d a s m a l l p i e c e of paper t o w e l l i n g to absorb the meconium when the a d u l t sawfly emerged from the cocoon. Without the paper, the a d u l t s f r e q u e n t l y became entangled by the f l u i d , o f t e n s t i c k i n g t h e i r wings together. The cocoons were checked d a i l y f o r a d u l t emergence. 4.3 R e s u l t s 4.3.1 L i f e h i s t o r y and h a b i t s 4.3.1. 1 Adults I*. 2y.ii£§. a d u l t s are medium s i z e d (8mm), predominantly black s a w f l i e s . The dorsum of the thorax i s a deep red> and there are a few white maculations on the l e g s . The a d u l t s f i r s t appear duri n g May i n Southwestern B r i t i s h Columbia, but the date v a r i e s c o n s i d e r a b l y , depending on the weather. In Eastern Canada, emergence seems to be s l i g h t l y l a t e r (Bouchard, 1960). In B r i t i s h Columbia, the s p e c i e s apparently reproduces e x c l u s i v e l y on red a l d e r , whereas i n the east i t a t t a c k s only Alnus rugosa var. americana , speckled a l d e r . During the study, a d u l t s were observed to spend most of 109 t h e i r time on and around young a l d e r t r e e s and s e e d l i n g s . They were very seldom seen on a l d e r t h a t had matured enough to begin f r u i t i n g , but seemed to conc e n t r a t e mainly on younger p l a n t s . When not a c t i v e , the a d u l t s c o u l d be found perching on the upper s u r f a c e s of the lea v e s of such p l a n t s . The a d u l t s d i d not appear to f e e d , although they spent c o n s i d e r a b l e time i n v e s t i g a t i n g l e a f s u r f a c e s with t h e i r mouthparts. They were only o c c a s i o n a l l y found on the f o l i a g e of other p l a n t s p e c i e s , and were never seen v i s i t i n g f l o w e r s . The a d u l t s are h e a t - l o v i n g . U s u a l l y they would not begin e g g - l a y i n g i n the cages unless the temperature was above a t l e a s t 1 6 ° C , and the sun s h i n i n g . Except during periods of egg-l a y i n g , when they are very a c t i v e and a l e r t , they are q u i t e s l u g g i s h and slow-moving, and co u l d even be caught by hand with l i t t l e t r o u b l e . As they normally perch on the upper s u r f a c e of the leaves, they are very conspicuous. T h e i r exposure coupled with t h e i r i n a c t i v i t y suggests that they are not s e r i o u s l y t r o u b l e d by p r e d a t o r s . T h i s s p e c i e s i s e n t i r e l y p a r t h e n o g e n e t i c , and no male was seen during the e n t i r e study. In f a c t , a male has been d e s c r i b e d only once ( E n s l i n , 1912; i n Bouchard, 1960). The females lay t h e i r eggs i n the v e i n s of a l d e r l e a v e s . Normally the eggs are l a i d only i n the main v e i n , but very o c c a s i o n a l l y , p a r t i c u l a r l y under l a b o r a t o r y c o n d i t i o n s , females w i l l l a y a few eggs i n the secondary v e i n s of l a r g e l e a v e s . U s u a l l y eggs are found i n a s i n g l e row, end to end along the main v e i n , s t a r t i n g w e l l down from the t i p , and c o n t i n u i n g almost to the p e t i o l e i t s e l f . But 110 o c c a s i o n a l l y , both i n the f i e l d and i n the l a b o r a t o r y , a second set of eggs may be l a i d i n a v e i n p a r a l l e l to the f i r s t s e t . T h i s suggects that females may not be capable of r e c o g n i z i n g the p r i o r presence of eggs. If the l e a f i n g u e s t i o n i s a l a r g e one, with a l a r g e main v e i n , both s e t s of eggs may hatch. If i t i s not l a r g e , however, only one s e t of eggs u s u a l l y can complete development. The e g g - l a y i n g behaviour of the female i s very c o n s i s t e n t , and has been d e s c r i b e d b r i e f l y by Borden and Dean (1971). The female appears to s e l e c t an o v i p o s i t i o n s i t e g u i t e c a r e f u l l y . I t may spend c o n s i d e r a b l e time i n v e s t i g a t i n g l e a v e s thoroughly with i t s antennae and i t s o v i p o s i t o r . On s e l e c t i n g a s i t e , i t takes up a p o s i t i o n on the upper s u r f a c e of the l e a f , p a r a l l e l to and immediately to one s i d e of the main v e i n , f a c i n g the p e t i o l e . I t c u t s a s l i t about 1mm long with i t s o v i p o s i t o r through the l e a f epidermis about 1mm to one s i d e of the main v e i n and then a t an angle down i n t o the v e i n . There i t d e p o s i t s an egg s l i g h t l y longer than 1mm, and withdraws i t s o v i p o s i t o r . I t may next step forward a few m i l l i m e t e r s and repeat the process, or i t may begin to search f o r a new s i t e . G e n e r a l l y only some half-dozen eggs are l a i d i n immediate s u c c e s s i o n , but the female may remain i n the v i c i n i t y and l a y more eggs on the same l e a f . In s e l e c t i n g an e g g - l a y i n g s i t e , the female appears to respond to the damaged t i s s u e . a r o u n d a s l i t , but not to the egg i t s e l f . T h i s may be why a second row of eggs i s somtimes l a i d i n a v e i n , because eggs may be i n s e r t e d from e i t h e r the l e f t or the r i g h t s i d e of a v e i n . 111 There appeared to be a c e r t a i n amount of i n t e r a c t i o n between females. In the f i e l d , although d e n s i t i e s of s a w f l i e s on a p a r t i c u l a r patch of a l d e r might be g u i t e high, more than one a d u l t was seldom seen on the same l e a f , or even on adjacent l e a v e s . When two or more a d u l t s were caged together, egg-laying was reduced and the a d u l t l i f e span shortened compared with s o l i t a r y females. In such cages, females could o f t e n be seen i n t e r f e r i n g with one another during attempts a t e g g - l a y i n g , and even when they were j u s t wandering about. However, i t i s not p o s s i b l e to say whether these were a g g r e s s i v e encounters, or whether they were merely a c c i d e n t a l i n t e r f e r e n c e . Reproductive c h a r a c t e r i s t i c s of 32 a d u l t s allowed to reproduce s i n g l y i n cages u n t i l they died are presented i n Table 12. Under caged c o n d i t i o n s , where leaves could be marked and e a s i l y observed, the females appeared to make a c o n s i d e r a b l e number of s l i t s i n t o which they d i d not d e p o s i t eggs. Whether t h i s i s normal, or merely an a r t i f a c t of caging i s unknown. Although the females l i v e d f o r an average of 7.5±3.72 days, they l a i d most of t h e i r eggs i n the f i r s t two or three days. But they continued to l a y a few eggs almost every day u n t i l the day they d i e d . Bouchard (1960) observed much lower l e v e l s of r e p r o d u c t i o n and s h o r t e r a d u l t l i f e spans. But many of h i s females died with l a r g e numbers of eggs s t i l l i n t h e i r o v a r i e s , and i t seems l i k e l y t h a t e i t h e r the cage or the f o l i a g e he used were inadeguate. 112 Table 12. Reproductive' c h a r a c t e r i s t i c s of 32 I*, ovata females and the developmental c h a r a c t e r i s t i c s of t h e i r o f f s p r i n g . A d u l t c h a r a c t e r i s t i c s Length of a d u l t l i f e (days) 1 7.50±3.72 Number of s l i t s made 83.40129.02 Number of eggs l a i d 76.29+26.83 Number of u n l a i d eggs 11.21110.45 Number of l a r v a e hatched 55.25+21.42 L a r v a l c h a r a c t e r i s t i c s T o t a l number of l a r v a e reared 1134 P r o p o r t i o n of l a r v a e s u c c e s s f u l l y r e a c h i n g cocoon stage .85 P r o p o r t i o n of l a r v a e with 5 i n s t a r s .04 P r o p o r t i o n of l a r v a e with 6 i n s t a r s .64 P r o p o r t i o n of l a r v a e with 7 i n s t a r s .32 P r o p o r t i o n of l a r v a e d i a p a u s i n g .09 Mean length of l a r v a l stage (days) 21.9312.38 Mean l e n g t h of cocoon stage (days) 22.8311.94 1. ±1 standard d e v i a t i o n 113 4 .3.1.2 Eggs The eggs, l y i n g i n the main veins of the l e a v e s , s w e l l the veins c o n s i d e r a b l y as they develop. There i s a l s o some s c l e r o t i z a t i o n of the l e a f t i s s u e i n the v i c i n i t y of the i n c i s i o n . There i s some m o r t a l i t y a t the egg stage (Table 12), a part of which may be due to the l e a f s c l e r o t i z a t i o n . Eouchard (1960) suggested t h a t many hatching l a r v a e were unable to break through t h i s tough p l a n t t i s s u e and t h e r e f o r e d i e d . However, u n l i k e the l a r v a e i n Quebec, which emerged on the upper s u r f a c e of the l e a f , l a r v a e i n B r i t i s h Columbia emerge on the lower s u r f a c e of the l e a f where there i s l e s s s c l e r o t i z e d t i s s u e . Not s u r p r i s i n g l y , the developmental time of the eggs i s temperature dependent. C a r e f u l measurements were not kept of temperatures f o r the eggs and a d u l t s . But at temperatures averaging 16°C, the eggs hatched i n about 11 days, whereas at temperatures around 21°C they hatched i n about f i v e days. T h i s r e s u l t i s i n sharp c o n t r a s t to Bouchard's (1960). He r e p o r t e d developmental times of 11 to 12 days f o r the s p r i n g generation, but 25 days f o r the summer g e n e r a t i o n . Borden and Dean (1971) found t h a t eggs kept at 24°C hatched i n 10 to 11 days. 4.3.1.3 Larvae Peak emergence of the l a r v a e occurs i n the e a r l y morning. The newly-hatched l a r v a e leave the v e i n by chewing a hole through i t j u s t where i t j o i n s the lower s u r f a c e of the l e a f . They immediately begin to feed on the underside of the l e a f . On hatching they are about 2-3mm long, and whitish-green , but they 114 darken as they f e e d . Soon a f t e r s t a r t i n g to f e e d , they begin to s e c r e t e a white waxy substance over the whole body, which soon transforms them i n t o white, woolly " c a t e r p i l l a r s " . The l a r v a e go through f i v e to seven ( r a r e l y eight) i n s t a r s during 21.93±2.38 days, with those with more moults averaging s l i g h t l y longer. Mature l a r v a e may grow as long as 20ram. As Table 12, and Borden and Dean (1971) a l s o show, i n B r i t i s h Columbia by f a r the m a j o r i t y mature i n s i x or seven i n s t a r s . In Quebec, however, the g e n e r a l r u l e i s a 5- or 6 - i n s t a r l a r v a l p e r i o d (Bouchard, 1960). A l l s t a d i a except the f i n a l one are f e e d i n g stages, and are pale green but covered by the white waxy s e c r e t i o n . T h i s c o v e r i n g i s l o s t with each moult but regrows and elongates throughout each i n s t a r . The shed s k i n s with t h e i r waxy c o v e r i n g are l e f t behind i n a very c h a r a c t e r i s t i c f a s h i o n on the v e i n s on the undersides of the l e a v e s , and may remain there f o r a long time. In the f i r s t two or three i n s t a r s , the l a r v a e feed by e a t i n g s m a l l h o l e s i n the l e a v e s . As they grow, they consume more and more of the leaves u n t i l , e v e n t u a l l y , i f they are numerous, they l e a v e only the main vein and some of the secondary v e i n s . F i g u r e 16 shows the average d a i l y consumption of l a r v a e as they develop. The t o t a l area of f o l i a g e consumed by one l a r v a over i t s l i f e was 1881.7±413.4mm 2. T h i s compares with Bouchard's (1960) value of 16cm 2 f o r a l a r v a i n Quebec. When the l a r v a e hatch on a l e a f they feed on t h a t l e a f and normally do not change d u r i n g the f i r s t i n s t a r . T h e r e f o r e , s i n c e the eggs are clumped, the f i r s t - i n s t a r l a r v a e are a l s o clumped. A f t e r the f i r s t moult, the l a r v a e u s u a l l y move to a new l e a f and 115a F i g u r e 16. Average d a i l y food consumption of an E. ovata l a r v a (n=12). Leaves were r e p l a c e d and measured every second day. Daily food consumption - mm 9LL 116 t h e r e f o r e spread out more. However, i n the l a s t few i n s t a r s , the l a r v a e again appeared to be q u i t e clumped. Because of t h i s c a s u a l o b s e r v a t i o n , so.me experiments were c a r r i e d out to i n v e s t i g a t e the e f f e c t of l a r v a l grouping on f e e d i n g . The r e s u l t s show t h a t i n d i v i d u a l consumption i s highest at i n t e r m e d i a t e d e n s i t i e s ( F i g u r e 17). An attempt was a l s o made to i n v e s t i g a t e any g r e g a r i o u s t e n d e n c i e s of free-moving l a r v a e by p u t t i n g known numbers of newly-hatched l a r v a e on potted a l d e r s i n both clumped and d i s p e r s e d c o n f i g u r a t i o n s . Observations of the l a r v a e were made d a i l y as they matured. The r e s u l t s were i n c o n c l u s i v e . Although the l a r v a e appeared to feed b e t t e r i n groups i n l a b o r a t o r y t r i a l s , i t i s not c l e a r i f they a c t u a l l y congregate or i n t e r a c t with one another i n any way when l e f t u n r e s t r i c t e d on p l a n t s . The l a r v a e appear to p r e f e r high humidity. Normally they feed on the underside of a l e a f , even i f t h a t happens to be the upper su r f a c e b o t a n i c a l l y , as was o f t e n the case i n the cardboard r e a r i n g c o n t a i n e r s . However, t h i s p o s i t i o n does not seem to be the r e s u l t of a simple need to be upside-down, f o r when reared i n the p e t r i d i s h e s , they f r e q u e n t l y chose to feed between the moist paper and the l e a f s u r f a c e , where the humidity must c e r t a i n l y have been near 100%. But t h i s meant that they were fe e d i n g on the upper s u r f a c e , speaking p h y s i c a l l y as well as b o t a n i c a l l y . I t i s p o s s i b l e that feeding on the underside of the l e a f may be an a d a p t a t i o n to humidity . Older l a r v a e i n the f i e l d and on potted p l a n t s were f r e q u e n t l y observed to chew out a s m a l l p i e c e of the main v e i n of a l e a f , though normally they 117a Figure 17. T o t a l food consumption (with standard d e v i a t i o n s ) of l a r v a e reared a t d i f f e r e n t d e n s i t i e s . 117 3000 £ £ I c o a £ => m C o no O O «+-c o 2000 n=5xl0 n = 5x5 n=2xl5 n = 16x2 n = 21x1 10 15 Density of larvae 1 1 8 do not feed on the v e i n s . T h i s meant that the d i s t a l p a r t of the l e a f dropped down, forming an angle i n which the l a r v a could o f t e n be found c u r l e d up. Whether or not the c u t t i n g was done i n response to humidity c o n d i t i o n s , the e f f e c t would be to r a i s e the humidity i n the angle between the s u r f a c e s . The waxy s e c r e t i o n may a l s o be an a d a p t a t i o n f o r ma i n t a i n i n g high h u m i d i t i e s around the l a r v a e , thereby d e c r e a s i n g water l o s s . When f e e d i n g , the l a r v a e s t r e t c h out l o n g i t u d i n a l l y . When not f e e d i n g , they are c o i l e d . They undergo a s h i f t i n behaviour i n response to d i s t u r b a n c e s as they mature. U n t i l about the t h i r d i n s t a r , a d i s t u r b e d l a r v a simply holds onto the l e a f t i g h t l y . But d u r i n g the l a t e r i n s t a r s , i f d i s t u r b e d i t w i l l c u r l i n t o a b a l l , drop o f f the l e a f and remain motionless on the ground f o r a while before t r y i n g t o r e l o c a t e a p l a n t . Presumably the danger to a s m a l l l a r v a from f a l l i n g o f f a tre e i s much gr e a t e r than the danger of i t s being s u c c e s s f u l l y d i s l o d g e d . Predators and p a r a s i t e s of t h i s sawfly are po o r l y known. Bouchard (1960) observed the r a r e event of a hymenopteran p a r a s i t i z i n g the l a r v a e . In t h i s study, an u n i d e n t i f i e d s p e c i e s of hymenopterous p a r a s i t e emerged from t h r e e f i e l d - c o l l e c t e d cocoons. S p i d e r s have a l s o been seen taking very s m a l l l a r v a e . Vespid wasps were b e l i e v e d to be f e e d i n g on the c i d e r l a r v a e on one o c c a s i o n . There may be some p r e d a t i o n on the other l i f e s t ages as w e l l . Earwigs were suspected of e a t i n g the eggs on some of the caged a l d e r s , but t h i s was never observed i n the f i e l d . The cocoons appear to be s u b j e c t to p r e d a t i o n , probably from s m a l l mammals, f o r many empty cocoons were found t h a t did 119 not have normal emergence holes. M o r t a l i t y i n l a b o r a t o r y - r e a r e d animals was 155? (Table 12). T h i s m o r t a l i t y took place almost e x c l u s i v e l y at the time of the f i r s t , and j u s t a f t e r the l a s t moult. The e a r l y m o r t a l i t y appeared to be due to i n a b i l i t y to shed the s k i n , while the l a t e r m o r t a l i t y was due to f a i l u r e t o s p i n a cocoon. S u r v i v a l throughout the r e s t of the l a r v a l p e r i o d was very high. 4.3.1.4 Prepupae and cocoons The l a s t l a r v a l i n s t a r i s non-feeding and i s very d i f f e r e n t i n appearance from the preceding stages. I t does not s e c r e t e the waxy c o v e r i n g and i s c o l o u r e d b r i g h t green. Soon a f t e r moulting t o t h i s stage, the l a r v a walks o f f i t s p l a n t i n search o f a s u i t a b l e s i t e to s p i n i t s cocoon. Most o f t e n t h i s i s c l o s e to the base of the stem of the p l a n t on which i t was f e e d i n g . Large numbers of cocoons, a l l cemented to one another, can o f t e n be found j u s t a t or under the s o i l s u r f a c e at the foot of young a l d e r s a p l i n g s . The cocoon, about 8mm long, i s double l a y e r e d , and the outer l a y e r i n c o r p o r a t e s b i t s of the s u b s t r a t e . In the l a b o r a t o r y , i t was necessary to provide the prepupal i n s t a r s with a moist, f i r m l y packed medium i n order to have them s p i n cocoons s u c c e s s f u l l y . Otherwise they simply d i e d , unable to pupate. Very o c c a s i o n a l l y a l a r v a would s u c c e s s f u l l y form a cocoon using a l e a f as the s u b s t r a t e . Also very o c c a s i o n a l l y , an i n d i v i d u a l t h at d i d not s p i n a cocoon would s u c c e s s f u l l y complete i t s development. 120 Some i n d i v i d u a l s continue to develop a f t e r s p i n n i n g a cocoon, and so g i v e r i s e to a second annual g e n e r a t i o n both i n B r i t i s h Columbia (Borden and Dean, 1971) and i n Quebec (Bouchard, 1960). In B r i t i s h Columbia, i t i s appa r e n t l y not always p o s s i b l e to complete a second g e n e r a t i o n . In the three years that iE_. ovata was f o l l o w e d f o r t h i s study, there was an a p p r e c i a b l e second generation l o c a l l y only i n 1972, and that was very l a t e i n s t a r t i n g so th a t the l a r v a e probably d i d not manage to complete t h e i r development. However, i t i s easy to o b t a i n a second or even a t h i r d g e n e r a t i o n a n n u a l l y i n the l a b o r a t o r y . At 20°C, non-diapausing i n d i v i d u a l s emerged i n 22.83+1.94 days a f t e r s p i n n i n g the cocoon. Although length of time as l a r v a e v a r i e d between 6 - i n s t a r and 7 - i n s t a r i n d i v i d u a l s , length o f time i n the cocoon d i d not. Two i n d i v i d u a l s t h a t developed without a cocoon, moulted t o the pupal stage i n 13 and 14 days, and then reached adulthood i n e i g h t and nine more days r e s p e c t i v e l y . Some i n d i v i d u a l s d i d not continue developing a f t e r s p i n n i n g a cocoon, but entered diapause. T h i s i s the r e s t i n g stage i n which they would normally pass the winter. The s t i m u l i c o n t r o l l i n g the i n i t i a t i o n of diapause i n t h i s s p e c i e s are unknown. Photoperiod and temperature are the most common s t i m u l i i n o ther s p e c i e s of i n s e c t s (Beck, 1967). Some p r e l i m i n a r y experiments on E.. ovata suggested t h a t humidity i s an important f a c t o r . In order t o determine whether the p e t r i d i s h e s produced humidity too high f o r the l a r v a e to t h r i v e , f o u r s e r i e s of l a r v a e were e s t a b l i s h e d . Twenty-five l a r v a e were reared s i n g l y 121 i n each of two s e t s of c o n d i t i o n s (cardboard c o n t a i n e r s or p e t r i d i s h e s a t 20°C and 16L:8D), and 50 l a r v a e i n each of another two s e t s of c o n d i t i o n s (cardboard c o n t a i n e r s or p e t r i dishes at room temperature and 16L:8D). As the t e s t was not designed to i n v e s t i g a t e humidity p r e c i s e l y , no d e t a i l e d humidity measurement was made. The cardboard c o n t a i n e r s were much d r i e r than the p e t r i d i s h e s . An unwatered l e a f would w i l t and wither i n a cardboard c o n t a i n e r i n a matter of hours. In a p e t r i d i s h , i t would stay f r e s h f o r days. Thus the r e a r i n g s presented treatments of 75 l a r v a e each i n two very d i f f e r e n t moisture regimes. The r e s u l t s (Table 13) i n d i c a t e t h a t not only d i d the l a r v a e t h r i v e i n high humidity, but t h a t fewer i n d i v i d u a l s entered diapause when reared i n high humidity. Time of emergence from diapause i s a h i g h l y v a r i a b l e c h a r a c t e r i s t i c . In a sample of more than 200 cocoons c o l l e c t e d i n the s p r i n g of 1973 from a r e s t r i c t e d area i n the f i e l d , a d u l t s continued t o emerge over a p e r i o d of s e v e r a l weeks. By the end of the summer, a c o n s i d e r a b l e p r o p o r t i o n of these cocoons had s t i l l not emerged. On being opened, they proved to c o n t a i n a p p a r e n t l y healthy prepupae. These i n d i v i d u a l s were no doubt i n prolonged diapause, a common phenomenon i n s a w f l i e s , and would have emerged i n subseguent summers. L i t t l e i s known about the i n i t i a t i o n or t e r m i n a t i o n of prolonged diapause. 122 Table 13. E f f e c t o f humidity ( r e a r i n g c o n t a i n e r ) on m o r t a l i t y and i n c i d e n c e of diapause. High humidity ( p e t r i dishes) Low humidity (cardboard c o n t a i n e r s ) M o r t a l i t y Number of l a r v a l deaths Number of l a r v a l successes (diapause or emergence) 15 60 27 48 Incidence of diapause Number e n t e r i n g diapause Number developing d i r e c t l y 59 30 18 X 2 t e s t f o r independence of humidity c o n d i t i o n s vs. m o r t a l i t y , X 2=4.74 .025<p<.050 vs. i n c i d e n c e of diapause, X 2=23.57 p<<.005 123 4.3.2 V a r i a t i o n i n some developmental c h a r a c t e r i s t i c s T h i s part of the study was designed to look f o r p o s s i b l e e f f e c t s of maternal age i n t h i s s p e c i e s . T h i r t y - t w o s a w f l i e s were allowed to reproduce i n cages on growing a l d e r p l a n t s u n t i l they d i e d . T h e i r o f f s p r i n g were reared s i n g l y i n p e t r i d i s h e s at 20°C and a photoperiod of 16L:8D from hatching u n t i l a d u l t emergence. A l l the young from some parents were r e a r e d , but from o t h e r s , only the f i r s t few and the l a s t few were reared. T h i s was necessary to save space and maintain data c o l l e c t i o n and f e e d i n g at a f e a s i b l e l e v e l . 4.3.2.1 V a r i a t i o n with time In order to c o n t r o l f o r the passage of time over the course of the experiment, newly emerged females were placed i n cages with potted p l a n t s every few days throughout the s p r i n g and summer of 1973. Thus, a t any po i n t i n time there were always e a r l y - b o r n as w e l l as l a t e - b o r n i n d i v i d u a l s hatching. The c h a r a c t e r i s t i c s of these l a r v a e can be analysed with r e s p e c t to date only, and without regard f o r maternal age. The date used f o r a l l a n a l y s e s i s the date an egg was l a i d . Day 1 i n a l l cases was May 1, 1973. A t o t a l of 967 l a r v a e were s u c c e s s f u l e i t h e r i n reachin g adulthood d i r e c t l y or e n t e r i n g diapause i n the course of the experiment. The d i s t r i b u t i o n of these l a r v a e over the experiment i s given i n F i g u r e 18. F i g u r e 19 shows the p r o p o r t i o n e n t e r i n g diapause at d i f f e r e n t times d u r i n g the experimental p e r i o d . Although there was much v a r i a t i o n t h e r e does not appear to be 124a F i g u r e 18. Number of l a r v a e which hatched at d i f f e r e n t times during the experimental p e r i o d and were maintained and reared (n=967). 124 o > 200 Si E Z) Z 100 n Larval hatching date 125a F i g u r e 19. P r o p o r t i o n of l a r v a e hatching at d i f f e r e n t t imes dur i n g the experimental period which e v e n t u a l l y entered diapause. 125 o <s o o. K CO 1 1 1 1 o O o o o CO o o o T o o I o CN T O CM O CO T o CO Larval hatching date 126a F i g u r e 20. P r o p o r t i o n of l a r v a e h a t c h i n g at d i f f e r e n t times d u r i n g the experimental p e r i o d which e v e n t u a l l y went through 7 i n s t a r s . 126 o o o o o <s o C M C O rs 00 o r— •— 1 i 1 i T 1 1 1 o o o o o o o o o I S C O o o C N C O Larval hatching date 127 any trend with time. F i g u r e 20 shows the p r o p o r t i o n of i n d i v i d u a l s with seven i n s t a r s a t d i f f e r e n t times during the experimental p e r i o d . There appears t o be a sharp decrease i n the numbers of i n d i v i d u a l s with seven i n s t a r s towards the end of the experimental p e r i o d . F i g u r e s 21 and 22 show the r e l a t i o n s h i p of date of e g g - l a y i n g to the d u r a t i o n of the l a r v a l and cocoon stages r e s p e c t i v e l y . The l a r v a l period decreased g r a d u a l l y throughout the whole p e r i o d of the experiment, while the length of the cocoon stage i n c r e a s e d , although to a l e s s e r degree. These changes occurred f o r both 6 - i n s t a r and 7 - i n s t a r , and f o r diap a u s i n g and non-diapausing i n d i v i d u a l s . These changes c o u l d not have r e s u l t e d from i n i t i a l d i f f e r e n c e s i n the c h a r a c t e r i s t i c s of the parents due to d i f f e r e n c e s i n o r i g i n or g e n e r a t i o n number of the females. Of the 32 females used, a l l except four were from the same ge n e r a t i o n . Two of the e a r l i e s t females were animals which had overwintered themselves, 28 of the females were o f f s p r i n g of overwintered females, and two of the l a s t females were o f f s p r i n g o f females that had not overwintered. The o f f s p r i n g of the four d i d not show c h a r a c t e r i s t i c s any d i f f e r e n t from the o f f s p r i n g of the 28. The o n l y p e r i o d during which these l a r v a e were not under constant c o n d i t i o n s was the p e r i o d of egg maturation p r i o r to o v i p o s i t i o n , and the p e r i o d of i n c u b a t i o n p r i o r to hatching. There were two f a c t o r s i n the environment of the eggs that o b v i o u s l y v a r i e d i n a more or l e s s r e g u l a r f a s h i o n during the experiment, temperature and photoperiod. 128a F i g u r e 21. R e l a t i o n s h i p of d u r a t i o n of the l a r v a l stage of a) nondiapausing, and b) diapausing l a r v a e with 6 i n s t a r s ( ) and 7 i n s t a r s { ), to l a r v a l hatching date. The s l o p e s of a l l r e g r e s s i o n l i n e s are h i g h l y s i g n i f i c a n t ( p « . 0 0 5 ) . (±1 standard d e v i a t i o n ) Duration of larval stage 129a Fig u r e 22. R e l a t i o n s h i p of d u r a t i o n of the cocoon stage of 6 i n s t a r (——) and 7 i n s t a r ( ) non-diapausing l a r v a e t o l a r v a l h atching date. The s l o p e s of the r e g r e s s i o n l i n e are h i g h l y s i g n i f i c a n t (p<<.005).(±1 standard d e v i a t i o n ) 129 Y=21.356(±0.413)+0.016(±0.004) X Y= 20.144(±0.067Ko.028(i0.007) X J I I I 1 1 L 80 100 120 Larval hatching date 130 T e m p e r a t u r e i n c r e a s e d o v e r t h e s e a s o n , b u t n o t i n a l i n e a r f a s h i o n . D u r i n g t h e e a r l y p a r t o f t h e s e a s o n , up u n t i l d a y 109 , t h e t e m p e r a t u r e was low and f l u c t u a t i n g . The mean i n c r e a s e d o n l y s l i g h t l y , f r o m 1 5 ° C t o 1 7 ° C o v e r t h a t p e r i o d . On d a y 1 0 9 , t h e c a g e s were a l l t r a n s f e r r e d t o a g r e e n h o u s e . A t t h i s p o i n t t h e t e m p e r a t u r e r o s e s h a r p l y and f l u c t u a t e d a r o u n d a new mean o f a b o u t 2 1 ° C . The l e n g t h o f t i m e f r o m e g g - l a y i n g t o e g g - h a t c h i n g s h o w e d t h e same s h a r p b r e a k , b u t t h e c u r v e s f o r d u r a t i o n o f l a r v a l and c o c o o n p e r i o d show no s u c h b r e a k . T h i s s u g g e s t s t h a t t e m p e r a t u r e i s n o t t h e w h o l e e x p l a n a t i o n f o r t h e s e l a t t e r c h a n g e s . H o w e v e r , t h e c h a n g e i n t h e f r e g u e n c y o f 7 - i n s t a r l a r v a e ( F i g u r e 20) d i d o c c u r a t a b o u t t h e same t i m e a s t h e t e m p e r a t u r e c h a n g e d . B o u c h a r d (1960) a l s o f o u n d a c h a n g e i n t h e number o f i n s t a r s o v e r t h e s e a s o n . A s t h e summer p r o g r e s s e d , t h e i n c i d e n c e o f 6 - i n s t a r l a r v a e i n c r e a s e d w i t h r e s p e c t t o 5 - i n s t a r l a r v a e . I n t h i s s t u d y , t h e i n c i d e n c e o f 6 - i n s t a r l a r v a e i n c r e a s e d w i t h r e s p e c t t o 7 - i n s t a r l a r v a e . T h e r e f o r e , i n t h e one c a s e , number o f i n s t a r s i n c r e a s e d and i n t h e o t h e r c a s e , i t d e c r e a s e d , d u r i n g a t e m p e r a t u r e i n c r e a s e . No s u p p l e m e n t a l l i g h t i n g was e v e r p r o v i d e d f o r t h e a d u l t s and e g g s . T h e r e f o r e , p h o t o p e r i o d v a r i e d i n a n a t u r a l way t h r o u g h o u t t h e e x p e r i m e n t . The g r a d u a l c h a n g e t h a t o c c u r r e d i n t h e l e n g t h s o f t h e l a r v a l a n d c o c o o n s t a g e s s u g g e s t s t h a t p h o t o p e r i o d may have b e e n i n v o l v e d i n t h e s e c h a n g e s . A n o t h e r f a c t o r w h i c h v a r i e d o v e r t h e e x p e r i m e n t was t h e f o l i a g e w h i c h t h e l a r v a e were f e d . T h i s f o l i a g e was o b t a i n e d f r o m w i l d a l d e r s h r u b s . I t s c h a r a c t e r w o u l d h a v e c h a n g e d o v e r 131 t h e s e a s o n , and may h a v e a f f e c t e d l a r v a l g r o w t h . H o w e v e r , t h i s was n o t a s i m p l e n u t r i t i o n a l e f f e c t , f o r t h e l a r v a e r e a r e d l a t e i n t h e s e a s o n were j u s t a s l a r g e and h e a l t h y a s t h o s e r e a r e d e a r l i e r . 4 . 3 . 2 . 2 . V a r i a t i o n i n i n c i d e n c e o f d i a p a u s e Of t h e 32 f e m a l e s whose p r o g e n y were r e a r e d , 17 p r o d u c e d one o r more d i a p a u s i n g l a r v a e . T h e r e was no o b v i o u s d i f f e r e n c e be tween t h e f e m a l e s t h a t p r o d u c e d d i a p a u s i n g l a r v a e and t h o s e t h a t d i d n o t . I n o r d e r t o t e s t w h e t h e r m a t e r n a l age had a n y e f f e c t on t h e i n c i d e n c e o f d i a p a u s e , the o f f s p r i n g o f a l l f e m a l e s were a r b i t r a r i l y d i v i d e d i n t o two g r o u p s , t h o s e l a i d p r i o r t o t h e m i d - p o i n t o f t h e f e m a l e ' s r e p r o d u c t i v e p e r i o d ( e a r l y - b o r n ) , a n d t h o s e l a i d a f t e r t h e m i d - p o i n t o f t h e f e m a l e ' s r e p r o d u c t i v e p e r i o d ( l a t e - b o r n ) . T a b l e 14 shows t h a t t h e r e was no e f f e c t o f m a t e r n a l a g e on i n c i d e n c e o f d i a p a u s e . T a b l e 15 s h o w s t h a t t h e r e was a l s o no s i g n i f i c a n t e f f e c t o f m a t e r n a l a g e on t h e number o f i n s t a r s . H o w e v e r , i f d i a p a u s i n g a n d n o n - d i a p a u s i n g l a r v a e a r e a n a l y s e d s e p a r a t e l y ( T a b l e 1 6 ) , a weak b u t n o n - s i g n i f i c a n t t r e n d a p p e a r s . S l i g h t l y more l a t e - b o r n , n o n - d i a p a u s i n g l a r v a e t h a n w o u l d be e x p e c t e d h a v e s e v e n i n s t a r s . M o r e o v e r , i n 7 - i n s t a r a n i m a l s t h e r e i s a v e r y weak t r e n d f o r more e a r l y - b o r n t h a n l a t e - b o r n l a r v a e t o d i a p a u s e ( T a b l e 1 7 ) . F i n a l l y , t h e r e i s a h i g h l y s i g n i f i c a n t r e l a t i o n s h i p b e t w e e n number o f i n s t a r s and i n c i d e n c e o f d i a p a u s e ( T a b l e 18) . A much h i g h e r p r o p o r t i o n o f 7 - i n s t a r l a r v a e t h a n 6 -i n s t a r l a r v a e e n t e r e d d i a p a u s e . As more 7 - i n s t a r a n i m a l s 132 Table 14. R e l a t i o n s h i p between i n c i d e n c e of diapause and maternal age. E a r l y - b o r n Late-born l a r v a e l a r v a e Number of non-diapausing l a r v a e 571 307 Number of diapa u s i n g l a r v a e 59 30 X 2 t e s t f o r independence of diapause vs. maternal age X2=0.044 .75<p<.90 133 Table 15. R e l a t i o n s h i p between number of l a r v a l i n s t a r s and maternal age. E a r l y - b o r n Late-born l a r v a e l a r v a e Number of 6- i n s t a r l a r v a e 435 219 Number of 7- i n s t a r l a r v a e 193 120 X 2 t e s t f o r independence of number of i n s t a r s vs. maternal age X 2=2.17 . 10<p<.25 13a T a b l e 16. R e l a t i o n s h i p b e t w e e n number o f l a r v a l i n s t a r s and m a t e r n a l a g e f o r n o n - d i a p a u s i n g and d i a p a u s i n g l a r v a e s e p a r a t e l y . E a r l y - b o r n L a t e - b o r n l a r v a e l a r v a e H o n - d i a p a u s i n g l a r v a e Number o f 6 - i n s t a r l a r v a e ais 207 Number o f 7 - i n s t a r l a r v a e 154 102 D i a p a u s i n g l a r v a e Number o f 6 - i n s t a r l a r v a e 20 12 Number o f 7 - i n s t a r l a r v a e 39 18 X 2 t e s t f o r i n d e p e n d e n c e o f number o f i n s t a r s v s . m a t e r n a l age f o r n o n - d i a p a u s i n g l a r v a e d i a p a u s i n g l a r v a e X2=3.41 X 2 = 0 . 30 .05<p< . 10 . 50<p< .75 135 T a b l e 17. R e l a t i o n s h i p between i n c i d e n c e of diapause and maternal age f o r 6 - i n s t a r and 7 - i n s t a r l a r v a e s e p a r a t e l y . E a r l y - b o r n l a r v a e l a t e - b o r n l a r v a e 6 - i n s t a r l a r v a e Number of non-diapausing l a r v a e 415 207 Number of diapa u s i n g l a r v a e 20 12 7 - i n s t a r l a r v a e Number of non-diapausing l a r v a e 154 102 Number of diapa u s i n g l a r v a e 39 18 X 2 t e s t f o r independence of diapause vs. maternal age f o r 6- i n s t a r l a r v a e 7- i n s t a r l a r v a e X 2=0.24 X2=1.34 .50<p<.75 .10<p<.25 136 Table 18. R e l a t i o n s h i p between number of l a r v a l i n s t a r s and i n c i d e n c e of diapause. Non-diapausing diapausing l a r v a e l a r v a e Number of 6- i n s t a r l a r v a e 622 33 Number of 7- i n s t a r l a r v a e 256 56 X 2 t e s t f o r independence of number of i n s t a r s vs. i n c i d e n c e of diapause X 2=42. 12 p « . 0 0 5 137 diapaused, and as the p r o p o r t i o n of 7 - i n s t a r o f f s p r i n g v a r i e d from female to female, i t was thought that producers of high p r o p o r t i o n s of 7 - i n s t a r i n d i v i d u a l s might a l s o be the producers of d i a p a u s i n g i n d i v i d u a l s . F i g u r e 23 shows that t h i s was not the case. T h i s experiment d i d not uncover any s i g n i f i c a n t e f f e c t of maternal age on i n c i d e n c e of diapause. H a t e r n a l age may n e v e r t h e l e s s be i n v o l v e d to some degree through i t s s l i g h t e f f e c t on i n s t a r number, which c e r t a i n l y does bear a r e l a t i o n s h i p t o i n c i d e n c e of diapause. I t may be necessary to understand the f a c t o r s i n i t i a t i n g diapause much b e t t e r before experiments can be designed to i n v e s t i g a t e t h i s p o s s i b l e r e l a t i o n s h i p . Under the r e a r i n g c o n d i t i o n s , only 9% of the animals entered diapause. I t i s p o s s i b l e t h a t t h i s i s the base-l i n e l e v e l t h a t w i l l always diapause, no matter what the c o n d i t i o n s . The important group of animals to monitor may be those animals which diapause when c o n d i t i o n s are a l i t t l e l e s s f a v o u r a b l e t o u n i n t e r r u p t e d development. The response o f these animals cannot be monitored u n t i l the p r o p o r t i o n of diapausing i n d i v i d u a l s can be manipulated with reasonable p r e d i c t a b i l i t y . 4.3.2.3 V a r i a t i o n i n d u r a t i o n of the l a r v a l stage L a r v a l developmental times showed c o n s i d e r a b l e v a r i a b i l i t y . Table 19 compares the mean l a r v a l p e r i o d s f o r diapausing and non-diapausing, 6- and 7 - i n s t a r l a r v a e . S e v e n - i n s t a r l a r v a e took longer on average to reach the f i n a l moult than 6 - i n s t a r l a r v a e . Diapausing l a r v a e took longer than non-diapausing l a r v a e . When 138a F i g u r e 23. R e l a t i o n s h i p of p r o p o r t i o n of o f f s p r i n g diapausing t o p r o p o r t i o n of o f f s p r i n g with 7 l a r v a l i n s t a r s f o r 32 females. Proportion of offspring entering diapause p b o b o in o In 0 0 139 Table 19. V a r i a t i o n i n l a r v a l developmental times (days) f o r 6-i n s t a r and 7 - i n s t a r , non-diapausing and d i a p a u s i n g l a r v a e , 1 2 Hon-diapausing dia p a u s i n g l a r v a e l a r v a e 6- i n s t a r l a r v a e 21,06±1.93 22.48±2.78 7- i n s t a r l a r v a e 23.21+1.93 25.1312.18 1. ±1 standard d e v i a t i o n 2. D i f f e r e n c e s between a l l c a t e g o r i e s are h i g h l y s i g n i f i c a n t . (p«.005) 140 l a r v a l development times f o r these d i f f e r e n t c a t e g o r i e s were broken i n t o e a r l y - and l a t e - b o r n groups (Table 20), i t appeared at f i r s t as though l a t e - b o r n l a r v a e developed f a s t e r than e a r l y -born l a r v a e . However, i f a c o r r e c t i o n i s made f o r the decrease i n l a r v a l developmental p e r i o d r e l a t e d to seasonal change (Figure 21) during the p a r e n t a l r e p r o d u c t i v e p e r i o d , the d i f f e r e n c e s a l l tend to disappear. Thus, any e f f e c t of maternal age on l a r v a l developmental p e r i o d i s , at most, very s l i g h t . It was thought t h a t there might be d i f f e r e n c e s between parents t h a t had undergone s i x l a r v a l i n s t a r s , and those t h a t had undergone seven l a r v a l i n s t a r s , i n t h e i r r e p r o d u c t i v e c h a r a c t e r i s t i c s and the developmental c h a r a c t e r i s t i c s of t h e i r o f f s p r i n g . Of the 32 females whose o f f s p r i n g were reared, 16 were a l s o themselves reared under c o n t r o l l e d c o n d i t i o n s and monitored d a i l y . Table 21 d i v i d e s t h i s group i n t o 6 - i n s t a r and 7 - i n s t a r parents f o r comparison. There i s no d i f f e r e n c e between these two groups i n t h e i r r e p r o d u c t i v e performance or i n the c h a r a c t e r i s t i c s of t h e i r o f f s p r i n g . The f a c t t h a t the number of i n s t a r s of the o f f s p r i n g was not a f f e c t e d by the number of i n s t a r s of the parents suggests t h a t t h i s may not be so much a g e n e t i c a l l y c o n t r o l l e d c h a r a c t e r i s t i c as one t h a t i s e n v i r o n m e n t a l l y r e g u l a t e d . 141 Table 20. V a r i a t i o n i n l a r v a l developmental times i n non-dia p a u s i n g and diapausing, 6 - i n s t a r and 7 - i n s t a r l a r v a e i n r e l a t i o n to maternal age. 1 E a r l y - b o r n Late-born C o r r e c t i o n l a r v a e l a r v a e f a c t o r 2 Non-diapausing 6- i n s t a r l a r v a e 21.16+1.96 20.84H.84 .48 Non-diapausing 7- i n s t a r l a r v a e 23.61t1.92 22.65+1.80 .59 Diapausing 6- i n s t a r l a r v a e 22.80±2.33 21.90±3.51 .49 Diapausing 7- i n s t a r l a r v a e 25.61i2.13 24.1 H2.00 .66 1. 11 standard d e v i a t i o n 2. C o r r e c t i o n f a c t o r must be added to the mean developmental time of l a t e - b o r n l a r v a e to c o r r e c t f o r decrease i n developmental times over the experiment. Values were obtained from r e g r e s s i o n equations i n F i g u r e 21 using a mean a d u l t p e r i o d of 7.5 days. 142 Table 21. Comparison of r e p r o d u c t i v e c h a r a c t e r i s t i c s and o f f s p r i n g c h a r a c t e r i s t i c s of parents with 6 - i n s t a r and 7 - i n s t a r l a r v a l h i s t o r i e s . 1 Parents with Parents with 6 i n s t a r s 7 i n s t a r s (n=7) <n=9) Length of a d u l t l i f e 6.00±3.31 7.22±3. 63 Number of s l i t s made 88.14±24.34 89.33±24 . 12 Number of eggs l a i d 86.16+25.29 78.00±20 . 73 Number of u n l a i d eggs 7.57±4.31 9.33+7. 36 Number of l a r v a e hatched 66.33±20.33 59.22±19 .49 P r o p o r t i o n of l a r v a e r e a c h i n g cocoon stage .85 .83 P r o p o r t i o n of l a r v a e with 7 i n s t a r s .32 .34 P r o p o r t i o n of l a r v a e diapausing .07 .08 Mean length of l a r v a l stage 20.84+1.93 20.32±1. 73 Mean len g t h of cocoon stage 23.16+2.02 23.64±2. 07 1. +1 standard d e v i a t i o n 143 4.4 D i s c u s s i o n ovata showed c o n s i d e r a b l e v a r i a b i l i t y i n a number of important developmental c h a r a c t e r i s t i c s when reared under constant c o n d i t i o n s f o r almost a l l of i t s l i f e . Every female produced both of the predominant l a r v a l types, those with s i x i n s t a r s , and those with seven i n s t a r s . H a l f of the females produced a t l e a s t a few d i a p a u s i n g l a r v a e . Time spent as a l a r v a v a r i e d from group to group, as well as d e c r e a s i n g c o n s i d e r a b l y over the experimental p e r i o d . Time spent as a cocoon was q u i t e constant from i n d i v i d u a l to i n d i v i d u a l at any given time, but i n c r e a s e d s l i g h t l y during the experiment. S e v e n - i n s t a r l a r v a e entered diapause more f r e q u e n t l y t h a t 6 - i n s t a r l a r v a e . However, t h e r e was no s t r i k i n g e f f e c t of maternal age e i t h e r on the i n c i d e n c e of diapause, or on other developmental c h a r a c t e r i s t i c s . N e v e rtheless, E_. ovata does pose a number of b i o l o g i c a l q u e s t i o n s . What causes the g r a d u a l change i n developmental times over the summer and how does t h i s change a f f e c t the ecology of the s p e c i e s ? What c o n t r o l s entrance i n t o diapause? How i s humidity i n v o l v e d i n t h i s process? What c o n t r o l s the onset of prolonged diapause, or r a t h e r , what c o n t r o l s time of emergence from diapause. What c o n t r o l s i n s t a r number and how i s t h i s r e l a t e d to i n c i d e n c e of diapause? Is the s p e c i e s a c t u a l l y g r e g a r i o u s and i f so, why? There are many ques t i o n s to be asked. Moreover, E_. ovata i s a convenient l a b o r a t o r y animal. I t feeds on a s p e c i e s of p l a n t t h a t i s easy t o o b t a i n or grow and i t i s 144 easy to rear i n l a r g e numbers. 4.5 Summary 1. General o b s e r v a t i o n s of the l i f e h i s t o r y and behaviour of Eriocampa ovata were summarized. 2. L a r v a l food consumption v a r i e d with r e a r i n g d e n s i t y and was g r e a t e s t at 10 l a r v a e per c o n t a i n e r . 3. Incidence of diapause was low under humid c o n d i t i o n s 4. Developmental times of l a b o r a t o r y - r e a r e d l a r v a e decreased over the season, but the l e n g t h of the the cocoon stage of those same i n d i v i d u a l s i n c r e a s e d over the season. 5. S e v e n - i n s t a r l a r v a e took longer to develop than 6 - i n s t a r l a r v a e , and entered diapause more f r e g u e n t l y . 6. There was no observable s i g n i f i c a n t e f f e c t of maternal age on i n c i d e n c e of diapause, or on other developmental c h a r a c t e r i s t i c s . 145 Chapter 5 C o n c l u s i o n For t h i s study, two processs were s e l e c t e d and t e s t e d f o r e f f e c t s of maternal age. The one process, i n c i d e n c e of diapause i n a sawfly, showed no s i g n i f i c a n t e f f e c t of maternal age. The other process, the production of winged forms by an aphid, was i n f l u e n c e d c o n s i d e r a b l y by maternal age. I t i s s i g n i f i c a n t that one of two p r o c e s s s , s e l e c t e d a r b i t r a r i l y from among hundreds of p o s s i b l e ones, e x h i b i t e d d i f f e r e n c e s due to maternal age. E f f e c t s of maternal age are a l r e a d y known to be a common b i o l o g i c a l phenomenon, and t h i s study has shown some of t h e i r e c o l o g i c a l p o s s i b i l i t i e s . Documented cases i n which maternal age a f f e c t s c h a r a c t e r i s t i c s t h a t are e c o l o g i c a l l y important are beginning to accumulate. The next step must be to t e s t whether d i f f e r e n c e s due to maternal age are r e a d i l y expressed i n wild p o p u l a t i o n s , and, i f so, whether those d i f f e r e n c e s e x e r t important e f f e c t s on p o p u l a t i o n l e v e l s . 146 L i t e r a t u r e c i t e d A l b r e c h t , F . 0 . , V e r d i e r , M . , and B l a c k i t h , R. E . 1959 . M a t e r n a l c o n t r o l o f o v a r i o l e number i n t h e p r o g e n y o f t h e m i g r a t o r y l o c u s t . N a t u r e ( L o n d . ) 184: 1 0 3 - 1 0 4 . B a i t e n s w e i l e r , W. 1972 . T h e r e l e v a n c e o f c h a n g e s i n t h e c o m p o s i t i o n o f l a r c h bud moth p o p u l a t i o n s f o r t h e d y n a m i c s o f i t s n u m b e r s . P r o c . A d v . S t u d y I n s t . D y n a m i c s Numbers P o p u l . ( O o s t e r b e e k , 1970) 2 0 8 - 2 1 9 . B a r l o w , C . A . 1 9 6 2 . 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T h e e f f e c t s o f a g e o f male and f e m a l e p a r e n t s on t h e l i f e c y c l e o f B r o s o £ h i l a m e l a n o g a s t e r . A n n . E n t o m o l . S o c . Am. 5 5 : 6 1 7 - 6 1 8 . C a l d w e l l , R. L . a n d Hegmann, J . P . 1969. H e r i t a b i l i t y o f f l i g h t d u r a t i o n i n t h e m i l k w e e d bug i x g a e u s k a l m i i . N a t u r e ( L o n d . ) 2 2 3 : 9 1 - 9 2 . C a l l a h a n , R. T . 1 9 6 2 . E f f e c t s o f p a r e n t a l a g e on t h e l i f e c y c l e o f t h e h o u s e f l y Musca d o m e s t i c a L i n n a e u s ( D i p t e r a : M u s c i d a e ) . J . N. Y . E n t o m o l . S o c . 7 0 : 1 5 0 - 1 5 8 . C a m p b e l l , I. M. 1 9 6 2 . R e p r o d u c t i v e c a p a c i t y i n t h e g e n u s C h p r i s t o n e u r a L e d . ( L e p i d o p t e r a : T o r t r i c i d a e ) . I. Q u a n t i t a t i v e i n h e r i t a n c e a n d g e n e s a s c o n t r o l l e r s o f r a t e s . C a n . J . G e n e t . 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I n f l u e n c e de l ' a g e de l a mere s u r l e s d i m e n s i o n s d e s o e u f s d a n s une s o u c h e v e s t i g i a l de D r p s p h i l a I 6 l l B 2 i i l l § i ® £ M e i g . E t u d e e x p e r i m e n t a l du d e t e r m i n i s m e p h y s i o l o g i g u e de c e s v a r i a t i o n s . B u l l . B i o l . F r . B e l g . 96: 5 0 5 - 5 2 8 . D e l c o u r , J and H e u t s , M. J . 1968 . C y c l i c v a r i a t i o n s i n wing s i z e r e l a t e d t o p a r e n t a l a g i n g i n D r p s o p h i l a m e l a n o g a s t e r . E x p . G e r o n t o l . 3 : 4 5 - 5 3 . D e m p s t e r , J . P . 1 9 6 3 . T h e p o p u l a t i o n d y n a m i c s o f g r a s s h o p p e r s and l o c u s t s . B i o l . R e v . (Camb.) 3 8 : 4 9 0 - 5 2 9 . D i x o n , A . T . G. 1971 . 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Am. 51 : 4 2 9 - 4 3 2 . D v a r o v , B. P. 1961 . Q u a n t i t y and q u a l i t y i n i n s e c t p o p u l a t i o n s . P r o c . R. E n t o m o l . S o c . L o n d . , C . 2 5 : 5 2 - 5 8 . Van H o r n , S . N . 1966 . S t u d i e s on t h e e m b r y o g e n e s i s o f A u l o c a r a e l l i o t t i (Thomas) ( O r t h p t e r a , A c r i d i d a e ) . I I . D e v e l o p m e n t a l v a r i a b i l i t y and t h e e f f e c t s o f m a t e r n a l age and e n v i r o n m e n t . J . M o r p h o l . 120 : 1 1 5 - 1 3 4 . V i s s c h e r , S . M. 1 9 7 1 . S t u d i e s on t h e e m b r y o g e n e s i s o f A u l o c a r a e l l i o t t i ( O r t h o p t e r a : A c r i d i d a e ) . I I I . I n f l u e n c e o f 1 5 7 m a t e r n a l e n v i r o n m e n t a n d a g i n g on d e v e l o p m e n t o f t h e p r o g e n y . A n n . E n t o m o l . S o c . Am. 6 4 : 1 0 5 7 - 1 0 7 4 . W a t t i a u x , J . M. 1 9 6 8 a . C u m u l a t i v e p a r e n t a l age e f f e c t s i n D r o s p h i l a s u b o b s c u r a . E v o l u t i o n 2 2 : 4 0 6 - 4 2 1 . W a t t i a u x , J . H. 1 9 6 8 b . P a r e n t a l age e f f e c t s i n D r o s p h i l a £ s e u d o o b s c u r a . E x p . G e r o n t o l . 3 : 5 5 - 6 1 . Way, M. J . and B a n k s , C . 3. 1 9 6 7 . I n t r a - s p e c i f i c m e c h a n i s m s i n r e l a t i o n t o t h e n a t u r a l r e g u l a t i o n o f n u m b e r s o f A p h i s f a b a e S c o p . A n n . A p p l . B i o l . 5 9 : 1 8 9 - 2 0 5 . Way, M. J . amd C a m m e l l , M. 1970 . A g g r e g a t i o n b e h a v i o u r i n r e l a t i o n t o f o o d u t i l i z a t i o n by a p h i d s . I n : W a t s o n , A . ( ed . ) ' A n i m a l P o p u l a t i o n s i n R e l a t i o n t o t h e i r Food R e s o u r c e s ' , pp 2 2 9 - 2 4 7 . B l a c k w e l l s , O x f o r d . Way, M,. J . and C a m m e l l , M. E . 1972 . S e l f r e g u l a t i o n i n a p h i d p o p u l a t i o n s . P r o c . A d v . S t u d y I n s t . D y n a m i c s Numbers P o p u l . ( O o s t e r b e e k , 1 9 7 0 ) pp 2 3 2 - 2 4 2 . W e l l i n g t o n , W. G . 1957 . I n d i v i d u a l d i f f e r e n c e s a s a f a c t o r i n p o p u l a t i o n d y n a m i c s : t h e d e v e l o p m e n t o f a p r o b l e m . C a n . J . Z o o l . 3 5 : 2 9 3 - 3 2 3 . W e l l i n g t o n , W. G . 1960 . Q u a l i t a t i v e c h a n g e s i n n a t u r a l p o p u l a t i o n s d u r i n g c h a n g e s i n a b u n d a n c e . C a n . J . Z o o l , 3 8 : 2 8 9 - 3 1 4 . W e l l i n g t o n , W. G . 1964 . Q u a l i t a t i v e c h a n g e s i n p o p u l a t i o n s i n u n s t a b l e e n v i r o n m e n t s . C a n . E n t o m o l . 9 6 : 4 3 6 - 4 5 1 . W e l l i n g t o n , W. G . 1 9 6 5 . Some m a t e r n a l i n f l u e n c e s on p r o g e n y q u a l i t y i n w e s t e r n t e n t c a t e r p i l l a r M a l o c o s o m a p l u y i a l e ( D y a r ) . C a n . E n t o m o l . 9 7 : 1 - 1 4 . 158 W e l l i n g t o n , W. G . and M a e l z e r , D. A . 1 9 6 7 . E f f e c t s o f f a r n e s y l m e t h y l e t h e r on t h e r e p r o d u c t i o n o f t h e w e s t e r n t e n t c a t e r p i l l a r , ' M a l a c o s o m a p l u y i a i e : some p h y s i o l o g i c a l and p r a c t i c a l a p p l i c a t i o n s . C a n . E n t o m o l . 9 9 : 2 4 9 - 2 6 3 . W h i t e , D. F . 1968 . P o s t - n a t a l t r e a t m e n t o f t h e c a b b a g e a p h i d w i t h a s y n t h e t i c j u v e n i l e h o r m o n e . J . I n s e c t P h y s i o l . 14: 9 0 1 - 9 1 2 . W h i t e , D. F . 1971 . C o r p u s a l l a t u m a c t i v i t y a s s o c i a t e d w i t h d e v e l o p m e n t o f w ing b u d s i n c a b b a g e a p h i d e m b r y o s and l a r v a e . J . I n s e c t P h y s i o l . 1 7 : 7 6 1 - 7 7 3 . W h i t e , D. F . a n d G r e g o r y , J . M. 1972 . J u v e n i l e hormone and wing d e v e l o p m e n t d u r i n g t h e l a s t l a r v a l s t a g e i n a p h i d s . J . I n s e c t P h y s i o l . 1 8 : 1 5 9 9 - 1 6 1 9 . W i l s o n , E . 0 . 1 9 7 1 . T h e I n s e c t S o c i e t i e s . B e l k n a p P r e s s , C a m b r i d g e , M a s s . W y a t t , I. J . 1965 . T h e d i s t r i b u t i o n o f Mjrzus p e r s i c a e ( S u l z ) on y e a r - r o u n d c h r y s a n t h e m u m s . A n n . A p p l . B i o l . 5 6 : 4 3 9 - 4 5 9 . W y n n e - E d w a r d s , V . C . 1962. A n i m a l D i s p e r s i o n i n R e l a t i o n t o S o c i a l B e h a v i o u r . O l i v e r a n d B o y d , E d i n b u r g h . 159 A p p e n d i x I A p p e n d i x l a . Number o f a l a t e o f f s p r i n g p r o d u c e d (A.) and number o f p a r e n t s p r o d u c i n g them ( A . P . ) , f o r 8 a p t e r o u s g r a n d p a r e n t s . B a t c h I I I E a t c h IV A . A . P . A . A . P . 1 36 4 12 2 0 0 11 4 2 25 3 8 2 45 5 7 1 3 0 0 0 0 40 5 0 0 4 41 6 31 7 1 1 18 4 5 46 3 1 1 2 0 0 22 6 6 1 1 53 7 21 5 0 0 7 3 1 0 0 4 1 18 2 8 141 7 47 3 2 1 16 3 B a t c h I B a t c h I I A . A . P. A . A . P. 160 A p p e n d i x l b . Number o f a l a t e o f f s p r i n g p r o d u c e d (A. ) and number o f p a r e n t s p r o d u c i n g them ( A . P . ) , f o r 6 a l a t e g r a n d p a r e n t s , o B a t c h I B a t c h I I B a t c h I I I E a t c h IV 1 95 7 4 3 7 3 16 4 2 0 0 3 2 47 4 24 2 3 0 0 27 4 42 5 3 2 4 0 0 14 4 0 0 20 5 5 0 0 42 5 24 3 16 6 6 0 0 39 6 10 3 0 0 161 A p p e n d i x I c . Number o f a l a t e o f f s p r i n g p r o d u c e d (A.) and number o f p a r e n t s p r o d u c i n g them ( A . P . ) , f o r 6 a p t e r o u s g r a n d p a r e n t s ( a l a t e g r e a t - g r a n d p a r e n t s ) . B a t c h I I I B a t c h IV A* A • 3? • A* A * P » 1 73 8 5 2 2 1 20 5 2 14 3 6 2 10 3 22 3 3 82 4 38 6 12 3 2 2 4 84 8 6 2 18 5 24 3 5 36 4 12 3 64 7 1 1 6 9 2 16 2 33 3 97 4 B a t c h I B a t c h I I A* A • P» A * A • P • 162 A p p e n d i x I I V a r i a t i o n i n r e s p o n s e t o a s t a n d a r d c r o w d i n g s t i m u l u s by k£XLlh.2siphon p i s u m T h e d a t a f r o m e x p e r i m e n t s i n v e s t i g a t i n g t h e e f f e c t o f m a t e r n a l age on a l a t a - p r o d u c t i o n showed c o n s i d e r a b l e v a r i a t i o n ( A p p e n d i x I ) . T h e r e were t h r e e m a j o r e x p e r i m e n t s w h i c h w i l l be d i s c u s s e d h e r e . One u s e d a p t e r o u s g r a n d p a r e n t s . Two were r u n s i m u l t a n e o u s l y , one w i t h a l a t e g r a n d p a r e n t s , a n d one w i t h a p t e r o u s g r a n d p a r e n t s w h i c h were e a r l y - b o r n p r o g e n y o f t h e a l a t e g r a n d p a r e n t s . T h e e x p e r i m e n t s were d e s i g n e d t o r e v e a l a n y v a r i a t i o n due t o g r a d i e n t s i n t h e e x p e r i m e n t a l s y s t e m o v e r t h e p e r i o d o f t h e e x p e r i m e n t . F i g u r e I shows t h e mean number o f a l a t e o f f s p r i n g p r o d u c e d p e r b a t c h o f 10 p a r e n t s , i g n o r i n g b a t c h n u m b e r , F i g u r e l a , f o r a p t e r o u s g r a n d p a r e n t s o n l y , and F i g u r e l b , f o r a l a t e g r a n d p a r e n t s a n d a l a t e g r e a t - g r a n d p a r e n t s / a p t e r o u s g r a n d p a r e n t s c o m b i n e d . T h e r e was no t e n d e n c y f o r t h e r e s p o n s e t o i n c r e a s e o r d e c r e a s e w i t h t i m e . The c e n t r a l f i v e p o i n t s i n F i g u r e l a a r e means o f a l l f o u r t r e a t m e n t b a t c h e s w h i c h were r u n s i m u l t a n e o u s l y . T h e s e i l l u s t r a t e v e r y w e l l t h e d e g r e e o f v a r i a t i o n p r e s e n t i n t h e s y s t e m . The s o u r c e o f t h i s v a r i a t i o n i s u n c l e a r . T h e e x p e r i m e n t s were r u n i n c o n t r o l l e d t e m p e r a t u r e c h a m b e r s . H u m i d i t y and b a r o m e t r i c p r e s s u r e were n o t c o n t r o l l e d . The r e s u l t s were t e s t e d a g a i n s t b o t h o f t h e s e f a c t o r s and showed 163a F i g u r e I. Mean number o f a l a t a e p r o d u c e d p e r b a t c h o f 10 p a r e n t s , i g n o r i n g t r e a t m e n t b a t c h n u m b e r , u s i n g a) a p t e r o u s g r a n d p a r e n t s , and b) a l a t e g r a n d p a r e n t s and a l a t e g r e a t - g r a n d p a r e n t s / a p t e r o u s g r a n d p a r e n t s c o m b i n e d , f o r d i f f e r e n t c r o w d i n g d a y s . 163 Day of crowding 164 no c o r r e l a t i o n . T h e e x p e r i m e n t s were c a r r i e d o u t i n f o u r c h a m b e r s . The s t o c k c u l t u r e , a l l g r a n d p a r e n t s a n d j u v e n i l e p a r e n t s , and a l l p a r e n t s d u r i n g t h e a c t u a l c r o w d i n g p r o c e d u r e , were k e p t i n one c h a m b e r . F o r t h e e x p e r i m e n t u s i n g a p t e r o u s g r a n d p a r e n t s , e a c h t r e a t m e n t was a s s i g n e d r a n d o m l y t o a p o s i t i o n i n a n y one o f t h e r e m a i n i n g t h r e e . A l l t r e a t m e n t s f o r t h e o t h e r two e x p e r i m e n t s were k e p t i n a s i n g l e c h a m b e r . T a b l e I shows t h e d i s t r i b u t i o n o f t h e t r e a t m e n t b a t c h e s and t h e c u m u l a t i v e r e s p o n s e s f o r t h o s e b a t c h e s i n t h e t h r e e c h a m b e r s f o r t h e e x p e r i m e n t u s i n g a p t e r o u s g r a n d p a r e n t s . F e w e r a l a t e o f f s p r i n g were o b t a i n e d f r o m t h e t r e a t m e n t s i n c h a m b e r 2 t h a n f r o m t h e t r e a t m e n t s i n t h e o t h e r two c h a m b e r s . In f a c t , o f t h e s e v e n b a t c h e s o f p a r e n t s t h a t p r o d u c e d no a l a t e o f f s p r i n g , f i v e were b a t c h e s t h a t had been r a n d o m l y a s s i g n e d t o c h a m b e r 2 . T h e s e s e v e n b a t c h e s p r o d u c i n g no a l a t a e were s p r e a d o u t o v e r s i x d i f f e r e n t t r e a t m e n t d a y s . T h e s e r e s u l t s s u g g e s t t h e p r e s e n c e o f a c h a m b e r e f f e c t c o n f o u n d i n g t h e d a t a o f t h e f i r s t e x p e r i m e n t . A d d i t i o n a l v a r i a t i o n c o u l d h a v e r e s u l t e d f r o m i n h e r i t e d d i f f e r e n c e s among t h e d i f f e r e n t s t o c k c u l t u r e l i n e s . F o r t h e e x p e r i m e n t w i t h a p t e r o u s g r a n d p a r e n t s , f e m a l e s 1 , 4 , and 7 were f r o m t h e same l i n e , a s were f e m a l e s 2 , 5 , and 8 , a n d f e m a l e s 3 a n d 6 . F e m a l e 1 f r o m t h e e x p e r i m e n t w i t h a l a t e g r a n d p a r e n t s was f r o m t h e same l i n e a s f e m a l e 1 f r o m t h e e x p e r i m e n t w i t h a l a t e g r e a t - g r a n d p a r e n t s / a p t e r o u s g r a n d p a r e n t s , and s o o n . I n a d d i t i o n , i n t h e s e e x p e r i m e n t s , f e m a l e s 2 and 3 were f r o m t h e same l i n e , a s were f e m a l e s 4 and 5 . A c o m p a r i s o n o f t h e 165 Table I. T o t a l numbers of a l a t a e obtained from each r e a r i n g chamber, and the numbers (in brackets) of r e p l i c a t e s c o n t r i b u t i n g to t h a t t o t a l . Chamber Batch I Batch I I Batch I I I Batch IV T o t a l s 1 183 (3) 108 (4) 0 (1) 25 (2) 316 (10) 2 25 (2) 53 (4) 5 (3) 16 (3) 100 (12) 3 85 (3) - (0) 108 (4) 51 (3) 244 (10) 166 r e s p o n s e s o f t h e t r e a t m e n t b a t c h e s w i t h i n t h e s e d i f f e r e n t s e t s o f g r a n d p a r e n t s r e v e a l s no t r e n d s (see A p p e n d i x I ) . A n o t h e r s o u r c e o f v a r i a t i o n i n t h e d a t a may h a v e been t h e c r o w d i n g p r o c e d u r e i t s e l f . A c c o r d i n g t o S u t h e r l a n d ( 1 9 6 9 a ) , t h e number o f a l a t a e an a p h i d p r o d u c e s i s r e l a t e d t o t h e number o f a p h i d / a p h i d i n t e r a c t i o n s t o w h i c h i t i s s u b j e c t e d . Any f a c t o r w h i c h a f f e c t s t h e a c t i v i t y l e v e l o f t h e a p h i d s i n t h e v i a l d u r i n g c r o w d i n g c o u l d h a v e a n e f f e c t on t h e number o f a l a t a e p r o d u c e d . I f t h e r e s p o n s e o f a p h i d s t o c r o w d i n g i s a t h r e s h o l d r e s p o n s e r a t h e r t h a n a c o n t i n u o u s r e s p o n s e , t h e e f f e c t on t h e d a t a o f e v e n s m a l l d i f f e r e n c e s m i g h t be q u i t e l a r g e when t h e y a r e n e a r a t h r e s h o l d . Many f a c t o r s , s u c h a s c h a m b e r v i b r a t i o n s , m i g h t a f f e c t a c t i v i t y l e v e l d u r i n g t h e c r o w d i n g p e r i o d . 

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