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The influence of temperature and salinity on the cuticular permeability of some Corixidae Cannings, Sydney Graham 1977

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THE INFLUENCE OF TEMPERATURE AND SALINITY ON THE CUTICULAR PERMEABILITY OF SOME CORIXIDAE by SYDNEY GRAHAM CANNINGS B . S c , U n i v e r s i t y of B r i t i s h Columbia, 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA November, 19 77 © Sydney Graham Cannings, 1977 In presenting th i s thes is in p a r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L ibrary shal l make it f ree l y ava i lab le for reference and study. I fur ther agree that permission for extensive copying of th is thes is for scho lar ly purposes may be granted by the Head of my Department or by his representat ives . It is understood that copying or pub l i ca t ion of th is thes is for f i n a n c i a l gain sha l l not be allowed without my wri t ten permission. Department of ZOOLOGY The Univers i ty of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date November 14, 1977 ABSTRACT Most t e r r e s t r i a l , and many a q u a t i c i n s e c t s are made waterproof by a l a y e r of l i p i d i n or on the e p i c u t i c l e . At a s p e c i f i c temperature, which i s determined by t h e i r composition, these l i p i d s undergo a phase . t r a n s i t i o n which markedly i n c r e a s e s the p e r m e a b i l i t y of the integument. The major purpose of t h i s study was to assess the p o s s i b i l i t y t h a t e p i c u t i c . u l a r wax t r a n s i t i o n c o u l d d i f f e r -e n t i a l l y a f f e c t the d i s t r i b u t i o n of four s p e c i e s of water boatmen: Cenocorixa b i f i d a h u n g e r f o r d i Lansbury, Ceno-c o r i x a e x p l e t a ( U h l e r ) , Cenocorixa b l a i s d e l l i . (Hunger-ford) , and C a l l i c o r i x a v u l n e r a t a ( U h l e r ) . The r a t e s of water l o s s and c u t i c l e temperatures of a d u l t c o r i x i d s were measured i n a stream of dry carbon d i o x i d e i n steps of i n c r e a s i n g temperature. The temperatures a t which t r a n s i t i o n o c c u r r e d i n these s p e c i e s were a l l approximately the same. Although they ranged from 30.3 to 32.6 C there were no major d i f f e r ences between the two genera, among congeners with d i f f e r -ent d i s t r i b u t i o n s , or between two c o e x i s t i n g congeners. T h i s was t r u e , however, only f o r c o r i x i d s which had been a c c l i m a t e d to the same temperature: a p o s i t i v e c o r r e l a -t i o n between t r a n s i t i o n temperature and a c c l i m a t i o n temper ature was demonstrated i n C. b e f i d a . Both the short-term and long-term e f f e c t s of t r a n s i -t i o n on these i n s e c t s were examined. Immersion of l i v e C. b i f i d a a d u l t s i n water warmer than t h e i r t r a n s i t i o n temperature d i d not appear to cause any i r r e v e r s i b l e changes i n c u t i c u l a r p e r m e a b i l i t y . S u r v i v a l t e s t s a t v a r i o u s temperatures showed t h a t the s u r v i v a l time of C. b i f i d a a d u l t s decreased with i n c r e a s e d temperature. However, i n s e c t s placed i n warm water d i d not show any outward s i g n s of osmoregulatory f a i l u r e or l o s s of su r -face wax as a r e s u l t of t r a n s i t i o n . In a d d i t i o n , C.  b i f i d a p l a c e d i n water as warm as or s e v e r a l degrees warmer than t h e i r t r a n s i t i o n temperature s u r v i v e d much longer than the l e n g t h of time t h a t these i n s e c t s would .be exposed to these temperatures i n the f i e l d . The p r e s e n t study on t r a n s i t i o n e f f e c t s suggests t h a t the t r a n s i t i o n of e p i c u t i c u l a r l i p i d s does not a f f e c t the d i s t r i b u t i o n of these c o r i x i d s i n the f i e l d . However, i t appears t h a t s a l i n i t y has a pronounced e f f e c t on the p e r m e a b i l i t y of C. b i f i d a a d u l t s . I n d i v i -duals from f r e s h water h a b i t a t s and h i g h l y s a l i n e ponds e x h i b i t e d roughly equal c u t i c u l a r p e r m e a b i l i t y , but those from lakes of i n t e r m e d i a t e s a l i n i t i e s were up to twice as permeable. T h i s phenomenon was shown to be one of ph y s i o -l o g i c a l a c c l i m a t i o n , s i n c e i n d i v i d u a l s placed i n t o d i s t i l l e d water showed a s i g n i f i c a n t decrease i n p e r m e a b i l i t y a f t e r f i v e days. I t i s p o s s i b l e t h a t t h i s i n f l u e n c e of s a l i n i t y on c u t i c u l a r p e r m e a b i l i t y may a f f e c t the r e l a t i v e d i s p e r s a l success of the c o r i x i d s which i n h a b i t i n l a n d s a l i n e l a k e s . i v TABLE OF CONTENTS ABSTRACT. i i TABLE OF CONTENTS i v LIST OF TABLES v i LIST OF FIGURES v i i ACKNOWLEDGEMENTS i x I. INTRODUCTION 1 A. The Problem 1 B. . E p i c u t i c u l a r L i p i d s and The T r a n s i t i o n Phenomenon.... 6 C. The C o r i x i d s 8 I I . MATERIALS AND METHODS 10 A. T r a n s i t i o n P o i n t Determinations 10 1. The c o r i x i d s and t h e i r care 10 2. T r a n s i t i o n p o i n t d e t e r m i n a t i o n s 16 a) Apparatus 17 b) Procedure 21 c) A n a l y s i s 23 B. Pretreatment Tests 27 C. Temperature S u r v i v a l Tests 27 D. S a l i n i t y E f f e c t s 29 E. Response t o S a l i n i t y Change 30 I I I . RESULTS 31 A. T r a n s i t i o n P o i n t Determinations 31 1. General r e s u l t s . . . 31 2. Males vs. Females 36 3. D i f f e r e n c e s among s p e c i e s 41 V 4. D i f f e r e n c e s among temperature c l a s s e s 41 B. Pretreatment T e s t s 60 C. Temperature S u r v i v a l Tests 60 D. S a l i n i t y E f f e c t s 60 E. Response to S a l i n i t y Change 67 IV. DISCUSSION 68 A. T r a n s i t i o n P o i n t Determinations 68 1. Techniques 6 8 2. A n a l y s i s — sources of e r r o r 73 a) K, the area constant 73 b) The decrease i n r a t e o f t r a n s p i r a t i o n with d e s i c c a t i o n c) The r e g r e s s i o n a n a l y s i s . . . . 75 3. The e x i s t e n c e of t r a n s i t i o n . 76 4. The t r a n s i t i o n temperatures of v a r i o u s c o r i x i d s p e c i e s 77 5. The i n f l u e n c e o f a c c l i m a t i o n temperature on t r a n s i t i o n temperature 80 6. T r a n s i t i o n underwater 82 7. The e f f e c t of temperature on s u r v i v a l 83 B. S a l i n i t y and P e r m e a b i l i t y 84 C. General D i s c u s s i o n 86 LITERATURE CITED 9 0 v i LIST OF TABLES Page TABLE I The mean weights and approximate s u r f a c e areas of the c o r i x i d s used i n the t r a n s i t i o n temperature determinations TABLE- I I • The r a t e o f water l o s s a t 20 C of c o r i x i d s i n t h i s study compared with values r e p o r t e d i n the l i t e r a t u r e v i i LIST OF FIGURES Page FIGURE 1 The Springhouse and Green Timber P l a t e a u c o l l e c t i o n s i t e s . 12 FIGURE 2 The Becher's P r a i r i e c o l l e c t i o n s i t e s . . 14 FIGURE 3 The experimental apparatus used i n the d e t e r m i n a t i o n of t r a n s i t i o n temperatures 18 FIGURE 4 D e t a i l of flow-through tube 2 0 FIGURE 5 Reproduction of a r e c o r d e r t r a c e showing the l o s s of weight of a female Cenocorixa b l a i s d e l l i i n steps of i n c r e a s i n g temperature 25 FIGURE 6 The r a t e o f water l o s s (with the e f f e c t of i n c r e a s i n g s a t u r a t i o n d e f i c i t removed) of i n d i v i d u a l Cenocorixa b i f i d a as a f u n c t i o n of c u t i c l e temperature 35 FIGURE 7 The r a t e of water l o s s a t 30 C of two male C. b i f i d a as a f u n c t i o n of the percentage of t h e i r o r i g i n a l wet weight remaining 3 8 FIGURE 8 The r a t e of water l o s s o f male and female C. b i f i d a as a f u n c t i o n of c u t i c l e temperature 40 FIGURE 9 Rate of water l o s s as a f u n c t i o n of c u t i c l e temperature f o r C. b i f i d a r e a red a t 20 C and a c c l i m a t e d to 20 C 43 FIGURE 10 Rate of water l o s s as a f u n c t i o n of c u t i c l e temperature f o r C. b l a i s d e l l i a c c l i m a t e d to 20 C 45 FIGURE 11 Rate of water l o s s as a f u n c t i o n of c u t i c l e temperature f o r C. e x p l e t a a c c l i m a t e d to 20 C 47 v i i i Page FIGURE 12 Rate of water l o s s as a f u n c t i o n o f c u t i c l e temperature f o r C a l l i c o r i x a  v u l n e r a t a reared a t 20 C and a c c l i -mated to 20 C 4 9 FIGURE 13 Rate of water l o s s as a f u n c t i o n of c u t i c l e temperature of C. b i f i d a r e a r e d a t 20 C and a c c l i m a t e d to 10 C 51 FIGURE 14 Rate of water l o s s as a f u n c t i o n of c u t i c l e temperature of C. b i f i d a r e a r e d a t 25 C and a c c l i m a t e d to 25 C 53 FIGURE 15 Rate of water l o s s as a f u n c t i o n of c u t i c l e temperature f o r C. b i f i d a r e a r e d a t 25 C and a c c l i m a t e d t o 20 C 55 FIGURE 16 The t r a n s i t i o n temperatures (as d e f i n e d by r e g r e s s i o n i n t e r s e c t i o n s ) of the v a r i o u s c o r i x i d s p e c i e s s t u d i e d . . 57 FIGURE 17 The t r a n s i t i o n temperatures of the v a r i o u s temperature c l a s s e s o f C. b i f i d a 59 FIGURE 18 Rate of water l o s s a t 20 C of C. b i f i d a p r e t r e a t e d i n water of v a r i o u s temperatures 62 FIGURE 19 Temperature t o l e r a n c e of C. b i f i d a 64 FIGURE 20 Rate of water l o s s of C. b i f i d a as a f u n c t i o n of the s a l i n i t y o f t h e i r n a t i v e l a k e s 66 FIGURE 21 The r a t e o f water l o s s of C. b i f i d a p l o t t e d on a l o g a r i t h m i c o r d i n a t e as a f u n c t i o n o f c u t i c l e temperature 79 ACKNOWLEDGEMENTS I would l i k e to thank my r e s e a r c h s u p e r v i s o r , Pro-f e s s o r G. G. E. Scudder, f o r h i s encouragement, guidance, and i n v a l u a b l e c r i t i c i s m d u r i n g the course of t h i s study. Drs. J . G o s l i n e and D. McPhail provided much h e l p f u l c r i t i c i s m d u r i n g the w r i t i n g of the t h e s i s , and Dr. J . E. P h i l l i p s k i n d l y aided me i n my attempt to understand per-m e a b i l i t y . F i n a l l y , I would l i k e to thank a l l my f e l l o w i n h a b i -t a n t s of the Bug Lab f o r h e l p i n g to make the f r u s t r a t i o n s of r e s e a r c h bearable. In p a r t i c u l a r , s t i m u l a t i n g d i s c u s -sions with John Spence, Murray Isman, and Bruce Smith were much a p p r e c i a t e d , and Bruce Smith's advice on t e c h n i c a l matters was i n v a l u a b l e . T h i s r e s e a r c h was c a r r i e d out while i n r e c e i p t of a N a t i o n a l Research C o u n c i l of Canada Postgraduate S c h o l a r -shi p and was f u r t h e r aided through an NRC o p e r a t i n g grant to Dr. Scudder. 1 I . INTRODUCTION A. THE PROBLEM The l a r g e s u r f a c e / v o l u m e r a t i o o f i n s e c t s a n d o t h e r a r t h r o p o d s n e c e s s i t a t e s c o n t r o l o f i n t e g u m e n t a r y w a t e r l o s s , a n d m o s t t e r r e s t r i a l f o r m s p o s s e s s a r e m a r k a b l y i m p e r m e a b l e c u t i c l e i n o r d e r t o p r e v e n t d e s i c c a t i o n ( N e v i l l e , 1 9 7 5 ) . Many a q u a t i c i n s e c t s a l s o r e q u i r e a r e l a t i v e l y i m p e r m e a b l e c u t i c l e t o m i n i m i z e o s m o t i c f l u x e s (Beament, 1961b; F o s t e r and Treherne-, • 1976; P h i l l i p s a n d B r a d l e y , 1977) o r t o p r e v e n t d e s i c c a t i o n d u r i n g a e r i a l d i s p e r s a l . ii K u h n e l t (1928', a s c i t e d by W i g g l e s w o r t h , 1945) was t h e f i r s t t o show t h a t t h e r e l a t i v e i m p e r m e a b i l i t y o f i n -s e c t c u t i c l e was a p r o p e r t y o f i t s o u t e r m o s t p a r t , t h e e p i c u t i c l e . He f o u n d t h a t r e m o v a l o r d i s r u p t i o n o f t h i s l a y e r r e s u l t e d i n r a p i d w a t e r l o s s b y t h e i n s e c t . I t i s now known t h a t a n e p i c u t i c u l a r l i p i d l a y e r i s r e s p o n s i b l e f o r t h i s w a t e r p r o o f i n g (Beament, 1 9 4 5 , 1 9 6 1 a ; W i g g l e s -w o r t h , 1945; J a c k s o n a nd B a k e r , 1970; E b e l i n g , 197 4; J a c k s o n a nd B l o m q u i s t , 1 9 7 6 ) . The f a c t t h a t r i s i n g t e m p e r a t u r e c a n c a u s e a d r a s t i c i n c r e a s e i n t h e p e r m e a b i l i t y o f t h e e p i c u t i c l e h a s b e e n known f o r some t i m e . When Gunn (1933) o b s e r v e d a s u d d e n i n c r e a s e i n t h e r a t e o f w a t e r l o s s f r o m a c o c k r o a c h ( B l a t t a r i a ) b e t w e e n 30 a n d 35 C, he e x p l a i n e d i t i n t e r m s 2 of an i n c r e a s e i n r e s p i r a t i o n . Ramsay (1935), however, found t h a t t h i s i n c r e a s e i n e v a p o r a t i o n r a t e a l s o o c c u r r e d with the s p i r a c l e s blocked, and suggested t h a t i t was the r e s u l t of the "melting of f a t " i n the cockroach c u t i c l e . Ramsay's work l e d to s t u d i e s by V.B. Wigglesworth and J.W.L. Beament i n the 1940's: Wigglesworth (1945) s t u d i e d the e f f e c t of temperature on the p e r m e a b i l i t y of many i n -s e c t s , and h i s r e s u l t s seemed to c o n f i r m the e x i s t e n c e of a sudden i n c r e a s e i n c u t i c u l a r p e r m e a b i l i t y a t what he termed the " c r i t i c a l temperature". Beament's (1945) exper-iments with i n s e c t c u t i c u l a r waxes spread on b u t t e r f l y wing membranes showed t h a t these waxes underwent a phase t r a n s i -t i o n a t a temperature c l o s e l y corresponding to the i n s e c t ' s c r i t i c a l temperature. He concluded, t h e r e f o r e , t h a t t h i s t r a n s i t i o n of e p i c u t i c u l a r l i p i d s was the cause of the ob-served abrupt i n c r e a s e i n p e r m e a b i l i t y a t the c r i t i c a l temperature. A c o n t r o v e r s y soon arose over the e x i s t e n c e of t r a n s -i t i o n . Edney (1951) s t a t e d t h a t the seemingly sudden i n -crease i n the r a t e of water l o s s was only the r e s u l t of one's s u b j e c t i v e i n t e r p r e t a t i o n of the evaporation/tempera-t u r e p l o t . He p o i n t e d out t h a t t h i s i n c r e a s e c o u l d be accounted f o r simply by the e x p o n e n t i a l i n c r e a s e i n the s a t u r a t i o n d e f i c i t of a i r with i n c r e a s i n g temperature, and when Holdgate and S e a l (1956) e l i m i n a t e d the e f f e c t s of i n c r e a s i n g s a t u r a t i o n d e f i c i t from t h e i r r e s u l t s , they found no i n d i c a t i o n of a sudden change i n the r a t e of water l o s s . 3 Beament (1958) defended the t r a n s i t i o n theory by p o i n t i n g out a number of inadequacies i n p r e v i o u s techniques and analyses, and a f t e r performing experiments u s i n g s o p h i s -t i c a t e d equipment, he found t h a t a t r a n s i t i o n p o i n t was d i s t i n c t , even a f t e r the s a t u r a t i o n d e f i c i t was taken i n t o account. Beament (1961b) s t u d i e d the permeability/temperature r e l a t i o n s h i p s of a number of a q u a t i c i n s e c t s , and found t h a t the more impermeable forms have wax t r a n s i t i o n s s i m i l a r t o those e x h i b i t e d by t e r r e s t r i a l forms, but i n comparison with them, the t r a n s i t i o n temperatures were very low. The q u e s t i o n a r i s e s as to whether t h i s phenomenon c o u l d exclude c e r t a i n i n s e c t s from warmer waters. Beament (1961b) g i v e s evidence t h a t t h i s c o u l d be the case f o r the a q u a t i c b e e t l e s D y t i s c u s and Gyrinus. The a d u l t s of these b e e t l e s have t r a n s i t i o n temperatures of about 24 C and d i e a t temperatures o n l y a degree or two warmer than t h i s , seeming-l y from the e f f e c t s of i n t e r n a l w a t e r l o g g i n g . Beament (1961b) notes f u r t h e r t h a t the r e l a t e d b e e t l e s Agabus and I l y b i u s have much high e r t r a n s i t i o n temperatures and can withstand h i g h e r water temperatures than D y t i s c u s or Gyrinus can. In A u s t r a l -i a , Agabus occurs i n h a b i t a t s much warmer than those where D y t i s c u s i s found (Beament (1961b)). I t i s p o s s i b l e , then, t h a t r e l a t e d s p e c i e s may e x h i b i t d i f f e r e n c e s i n t r a n s i t i o n temperature t h a t c o u l d d i f f e r e n -t i a l l y a f f e c t t h e i r d i s t r i b u t i o n . An i n v e s t i g a t i o n of t r a n s i t i o n and i t s e f f e c t s would t h e r e f o r e be v a l u a b l e . 4 I n t h i s s t u d y , I d e c i d e d t o i n v e s t i g a t e t h e t r a n s i -t i o n phenomenon i n s e v e r a l c l o s e l y - r e l a t e d s p e c i e s o f w a t e r boatmen ( C o r i x i d a e ) . F i r s t , t h e p o s s i b i l i t y o f i n t e r g e n e r i c d i f f e r e n c e s i n t r a n s i t i o n s i m i l a r t o t h o s e o b s e r v e d by Beament, (1961b) i n A gabus a n d D y t i s c u s w e r e s t u d i e d b y c o m p a r i n g t h e t r a n s i t i o n t e m p e r a t u r e s o f C e n o c o r i x a b l a i s -d e l l i ( H u n g e r f o r d ) and C a l l i c o r i x a v u l n e r a t a ( U h l e r ) , two c o e x i s t i n g s p e c i e s on t h e s o u t h w e s t c o a s t o f B r i t i s h C o l -u m b i a . S e c o n d , t o d e t e r m i n e i f t r a n s i t i o n c o u l d d i f f e r e n -t i a l l y a f f e c t t h e d i s t r i b u t i o n o f c o n g e n e r s , t h e t r a n s i t i o n t e m p e r a t u r e o f C. b l a i s d e l l i was c o m p a r e d w i t h t h o s e o f two i n t e r i o r s p e c i e s : C e n o c o r i x a b i f i d a h u n g e r f o r d i L a n s b u r y a n d C e n o c o r i x a e x p l e t a ( U h l e r ) . T h i r d , t h e p o s s i b l e e f f e c t s o f t r a n s i t i o n o n t h e c o e x i s t e n c e o f two c l o s e l y - r e l a t e d s p e c i e s w e r e d e t e r m i n e d by c o m p a r i n g t h e t r a n s i t i o n t e m p e r a -t u r e s o f C. b i f i d a a n d C. e x p l e t a . O l o f f s a n d S c u d d e r (1966) s t u d i e d t h e t r a n s i t i o n p h e n o -menon i n C. e x p l e t a and f o u n d t h a t t h e t r a n s i t i o n p o i n t l a y b e t w e e n 28.5 and 33.8 C c u t i c l e t e m p e r a t u r e . S i n c e w a t e r t e m p e r a t u r e s i n some o f t h e l a k e s t h a t C. e x p l e t a i n h a b i t s c a n r e a c h 30 C s e v e r a l t i m e s i n t h e c o u r s e o f a warm summer ( J a n s s o n and S c u d d e r , 1974; C a n n i n g s , 1 9 7 5 ) , i t a p p e a r e d p o s s i b l e t h a t t h e s u c c e s s o f t h e s e bugs may be a f f e c t e d by t h e t r a n s i t i o n phenomenon. O l o f f s a n d S c u d d e r (1966) a l s o f o u n d t h a t when d e a d C. e x p l e t a w e r e p l a c e d i n w a t e r warmer t h a n t h e i r t r a n s i -t i o n t e m p e r a t u r e , t h e y showed a p e r m a n e n t i n c r e a s e i n 5 p e r m e a b i l i t y when retu r n e d to a i r a t 20C. T h i s was not the same phenomenon d e s c r i b e d by Beament (1959), s i n c e pretreatment i n warm a i r d i d not cause a permanent i n -crease i n p e r m e a b i l i t y . They suggested t h a t , at tempera-t u r e s above t r a n s i t i o n , some of the wax i s removed by water. I f t h i s i s the case, t r a n s i t i o n underwater may have more f a r - r e a c h i n g consequences f o r these i n s e c t s than merely an immediate i n c r e a s e i n p e r m e a b i l i t y . I t c o u l d c r e a t e osmoregulatory problems a f t e r p e r i o d s of high water temperature as w e l l as d u r i n g them, and the c o r i -x i d s would be s u b j e c t to d e s i c c a t i o n i f they undertook d i s p e r s a l f l i g h t s i n a i r of only moderate temperature. C o r i x i d s c o l o n i z e lakes randomly each s p r i n g , as they seem unable to s e l e c t a p p r o p r i a t e water bodies b e f o r e l a n d i n g (Popham, 1964). The r e f o r e , they may breed, or attempt to breed, i n lakes which become too warm l a t e r . While attempting to determine the t r a n s i t i o n tempera-tu r e of some f i e l d - c a u g h t bugs, I found t h a t these i n s e c t s were f a r more permeable than those t h a t I had r a i s e d i n the l a b o r a t o r y . Since the f i e l d - c a u g h t bugs had come from a s a l i n e pond and the l a b o r a t o r y ones had been r e a r e d i n d e c h o r i n a t e d tap water, I hypothesized t h a t the observed d i f f e r e n c e i n the p e r m e a b i l i t y of these i n s e c t s was the r e s u l t of the d i f f e r e n c e i n s a l i n i t y of t h e i r r e s p e c t i v e h a b i t a t s . With t h i s i n mind, f u r t h e r s t u d i e s were under-taken to determine i f s a l i n i t y and p e r m e a b i l i t y were i n -deed c o r r e l a t e d i n some way. 6 B. EPICUTICULAR L I P I D S AND THE TRANSITION PHENOMENON E p i c u t i c u l a r l i p i d s a r e g e n e r a l l y t h o u g h t o f a s a t h i n l a y e r s p r e a d o v e r t h e s u r f a c e o f t h e c u t i c l e (Beament, 1 9 6 1 a ) , b u t t h e r e h a s b e e n a g r e a t d e a l o f c o n t r o v e r s y o v e r t h e e x a c t s t r u c t u r e a n d c o m p o s i t i o n o f t h i s l a y e r . Beament (1945) s u g g e s t e d t h a t t h e l i p i d was a n o r i e n t a t e d m o n o l a y e r b o u n d t o t h e c u t i c u l i n s u b s t r a t e , and l a t e r h y p o t h e s i z e d t h a t i t c o n s i s t e d m a i n l y o f l o n g - c h a i n a l c o h o l s (Beament, 1955, 1 9 6 1 a ) . G i l b y and Cox ( 1 9 6 3 ) , h o w e v e r , f o u n d t h a t p a r a f f i n s , n o t a l c o h o l s , made up t h e b u l k o f t h e l i p i d l a y e r o f c o c k r o a c h e s ( B e a m e n t 1 s e x p e r i m e n t a l a n i m a l ) . F a t t y a c i d s , a l d e h y d e s , and e s t e r s made up m o s t o f t h e r e m a i n d e r o f t h e l i p i d s . S i n c e t h e n , work on numerous o t h e r i n s e c t s h a s shown t h a t t h e c o m p o s i t i o n o f e p i c u t i c u l a r l i p i d s i s q u i t e d i v e r s e ( r e v i e w e d i n Hackman, 1974; N e v i l l e , 1975, pp. 98-104; J a c k s o n and B l o m q u i s t , 1 9 7 6 ) . A l c o h o l s a r e r a r e l y f o u n d , h o w e v e r , e x c e p t i n some s p e c i a l i z e d waxes ( C h i b n a l l e t a l . , 1934; Hackman, 19 5 1 ; B o w e r s a n d Thompson, 1 9 6 5 ) . G i l b y and Cox (196 3) d o u b t e d t h a t t h e l i p i d m i x t u r e p r e s e n t i n c o c k r o a c h c u t i c l e c o u l d f o r m a t i g h t l y - p a c k e d m o n o l a y e r , and r e c e n t w o r k by L o c k e y (1976) h a s shown t h a t a f i l m o f c o c k r o a c h l i p i d i s u n s t a b l e d u r i n g c o m p r e s s i o n and d o e s n o t f o r m a t i g h t l y - p a c k e d m o n o l a y e r . An e f f e c t i v e l i p i d m o n o l a y e r c o u l d o n l y e x i s t , t h e r e f o r e , i f t h e r e w e r e some u n u s u a l p r o p e r t y o f t h e l i p i d s o f t h e c u t i c u l i n s u b -s t r a t e . L o c k e y (1976) s u g g e s t s t h a t t h e c r u c i a l l a y e r o f l i p i d i s l o c a t e d i n t h e o u t e r r e g i o n o f t h e c u t i c u l i n . 7 In h i s e l e c t r o n microscope s t u d i e s of l i p i d s i n the e p i c u t i c l e of Rhodnius, Wigglesworth (1975) has found t h a t the l i p i d does not occur as a d i s c r e t e l a y e r , but i s bound i n a f r a g i l e n o n - l i p i d s i l v e r - s t a i n i n g membrane; D e t a i l e d s t u d i e s such as t h i s have not been made i n other i n s e c t s , however, so we cannot g e n e r a l i z e from t h i s o b s e r v a t i o n . S i n c e the exact s t r u c t u r e of the e p i c u t i c u l a r l i p i d l a y e r i s not known, the mechanism of t r a n s i t i o n i s s t i l l the s u b j e c t of s p e c u l a t i o n . Beament (1964) suggested t h a t l o n g - c h a i n p o l a r l i p i d s are o p t i m a l l y packed a t an angle of 24.5° t o the v e r t i c a l . At the c r i t i c a l temperature, thermal a g i t a t i o n breaks the Van der Waals f o r c e s b i n d i n g the chains together and causes them to assume a mean v e r t i c a l p o s i t i o n , r e s u l t i n g i n an i n c r e a s e i n p e r m e a b i l i t y to water. Locke (1965).- proposed an e x p l a n a t i o n f o r t r a n s i t i o n based on the presence of wax f i l a m e n t s i n the e p i c u t i c l e . His e l e c t r o n micrographs of the e p i c u t i c l e show these f i l a -ments p a s s i n g through the c u t i c u l i n l a y e r and connecting with the s u r f a c e wax l a y e r s . Locke suggests t h a t these f i l a m e n t s c o n s i s t of l o n g - c h a i n l i p i d s i n the middle phase of a l i p i d w a t e r l i q u i d c r y s t a l . At the t r a n s i t i o n p o i n t , t h i s c r y s t a l s t r u c t u r e c o u l d change e i t h e r to a complex hexagonal phase or to a re v e r s e d middle phase, and water would be allowed to escape. Locke accepts the i d e a of a l i p i d monolayer p r e v e n t i n g water l o s s , but suggests t h a t the monolayer c o n s i s t s of l i p i d s i n a l i q u i d - c r y s t a l . 8 F i l s h i e (1970) , however, found t h a t these f i l a m e n t s c o u l d not be removed by lengthy e x t r a c t i o n with l i p i d s o l v e n t s , and s t a t e s " i t i s u n l i k e l y t h a t the f i l a m e n t s themselves are composed e n t i r e l y of wax precursors"'. Wigglesworth 1 s (1975) e l e c t r o n microscope s t u d i e s confirm, however, t h a t e p i c u t i c u l a r l i p i d s are t r a n s p o r t e d through these c a n a l s . Davis (1974b) p o s t u l a t e d t h a t the low p e r m e a b i l i t y of the c u t i c l e i s the r e s u l t of l i p i d s i n a s p e c i a l arrange-ment "probably d i c t a t e d by the molecular arrangement of the l i p o p r o t e i n s i n the c u t i c u l i n l a y e r " . Davis suggests t h a t at t r a n s i t i o n the l i p i d s undergo a phase change from the s o l i d c r y s t a l l i n e s t a t e to the l i q u i d c r y s t a l l i n e s t a t e . N e v i l l e (1975) favours t h i s l a s t h y p o t h e s i s . Regardless of i t s mechanism, however, e p i c u t i c u l a r l i p i d t r a n s i t i o n appears to be a r e a l p h y s i c o - c h e m i c a l phenomenon which may have a pronounced e f f e c t on some i n s e c t s . C. THE CORIXIDS Four s p e c i e s of c o r i x i d s were i n v e s t i g a t e d i n t h i s study, with the g r e a t e s t emphasis on Cenocorixa b i f i d a  h u n g e r f o r d i Lansbury. C. b i f i d a i s an i n h a b i t a n t of temp-ora r y and permanent ponds throughout much of temperate western North America, (Jansson, 1971). Although i t i s found elsewhere, i t s d i s t r i b u t i o n i s c l o s e l y l i n k e d with s e m i - a r i d r e g i o n s , where i t can be found i n freshwater or moderately s a l i n e lakes and ponds (Jansson, 1971). 9 Scudder (1969a,b) found t h a t C. b i f i d a cannot t o l e r a t e s a l i n i t i e s g r e a t e r than 20,000 umhos/cm s u r f a c e c o n d u c t i -v i t y . Cenocorixa e x p l e t a (Uhler) c o e x i s t s with C. b i f i d a i n ponds of i n t e r m e d i a t e s a l i n i t y (5990-20,000 umhos/cm), but can t o l e r a t e s a l i n i t i e s up to 33,000 umhos/cm (Scudder, 1969b). I t i s absent, however, from f r e s h water h a b i t a t s (Scudder, 1969a,b), and i s t h e r e f o r e more r e s t r i c t e d i n g e n e r a l d i s t r i b u t i o n than C. b i f i d a (Scudder, 1969a,b). Cenocorixa b l a i s d e l l i (Hungerford) i s a c o a s t a l spec-i e s , f a v o u r i n g temporary or semi-permanent ponds w i t h i n 2km of the P a c i f i c Ocean (Jansson, 1971). Although Jansson (1971) never found t h i s c o r i x i d i n waters more s a l i n e than 215 umhos/cm, Reynolds (pers. comm.) has taken i t i n the b r a c k i s h waters adjacent to W i t t y ' s Lagoon near V i c t o r i a , B.C. The C a l l i c o r i x a s p e c i e s s t u d i e d keyed to C a l l i c o r i x a  v u l n e r a t a (Uhler) i n Hungerford (1948), and furthermore, i t was t r u e C. v u l n e r a t a a c c o r d i n g to key c h a r a c t e r s s u p p l i e d by Jansson (pers. comm.). T h i s s p e c i e s ranges along the west c o a s t of North America from C a l i f o r n i a to the Alaskan P e n i n s u l a (Hungerford, 1948). In B r i t i s h Columbia i t i s found along the P a c i f i c c o a s t and through-out the southern i n t e r i o r (Scudder, 1977). I t i s a s p e c i e s c h a r a c t e r i s t i c of temporary pools and ponds, and as such, i s a frequent f l y e r (Scudder, pers. comm.). I I . MATERIALS AND METHODS A. TRANSITION POINT DETERMINATIONS 1. The c o r i x i d s and t h e i r c a r e . C o r i x i d s were c o l l e c t e d i n the f i e l d u s i n g a q u a t i c sweep nets and were t r a n s p o r t e d to the l a b o r a t o r y i n Ther-mos jugs h a l f - f i l l e d w ith water. Cenocorixa b l a i s d e l l i and C a l l i c o r i x a v u l n e r a t a were c o l l e c t e d from a sm a l l but permanent pond a t McCleery G o l f Course i n Vancouver, B.C. C. b l a i s d e l l i were a l s o taken from a sma l l temporary pond on the U n i v e r s i t y of B r i t i s h Columbia campus. Cenocorixa  b i f i d a was obtained mainly from Long L. on Becher's P r a i r i e near Riske Creek, B.C. However, some samples were taken from Round-up and Barnes Lakes (Phalarope and Box 4 Lakes r e s p e c t i v e l y i n Scudder (1969a,b)) on Becher's P r a i r i e , and from LE3 and LE5 on the Green Timber P l a t e a u west of C l i n -ton, B.C. F i g u r e s 1 and 2 g i v e the l o c a t i o n s of these l a k e s . Cenocorixa e x p l e t a was taken from Barnes and Round-up Lakes. In the l a b o r a t o r y , the c o r i x i d s were maintained i n round p l a s t i c t r a y s (8cm deep X 24cm i n diameter) h a l f -f i l l e d w ith d e c h l o r i n a t e d water. The bugs were f e d f r o z e n b r i n e shrimp every other day and the water i n the t r a y s was changed i f and when i t became p u t r e f i e d . To prevent the r o t t i n g of excess food, the water i n the c u l t u r e t r a y s was c o n t i n u o u s l y a e r a t e d . With the exceptions noted below, a l l bugs used i n t r a n s i t i o n p o i n t t e s t s were kept i n constant F I G U R E 1 The Springhouse and Green Timber Plateau c o l l e c t i o n s i t e s : A. A portion of the Cariboo and C h i l c o t i n Plateaus of B r i t i s h Columbia. The locations of Figures IB and IC are outlined. B. The Springhouse c o l l e c t i o n s i t e s . C. The Green Timber Plateau c o l l e c t i o n s i t e s . D. The location of Figure IA i n B r i t i s h Columbia. 12 FIGURE 2 The B e c h e r ' s P r a i r i e c o l l e c t i o n s i t e s . 15 t e m p e r a t u r e c h a m b e r s a t 20 C u n d e r a 1 6 h r l i g h t : 8 h r d a r k l i g h t r e g i m e . I n t h e c a s e o f C. b l a i s d e l l i and C. e x p l e t a , t h e bugs w e r e a l l o w e d a t l e a s t one week t o a c c l i m a t e t o t h e s e l a b o r -a t o r y c o n d i t i o n s b e f o r e t h e y w e re t e s t e d . O n l y f l y i n g f o r m C. e x p l e t a w e r e u s e d . A l l t h e i n d i v i d u a l s o f C a l l i c o r i x a v u l n e r a t a a n d Ceno-c o r i x a b i f i d a u s e d i n t r a n s i t i o n e x p e r i m e n t s h a d h a t c h e d f r o m eggs l a i d i n t h e c u l t u r e t r a y s . The l a r v a e w e r e r a i s e d u n d e r t h e same c o n d i t i o n s a s t h e a d u l t s w e r e m a i n t a i n e d . A f t e r t h e y h a d emer g e d , t h e a d u l t s w e r e h e l d f o r a t l e a s t one week b e f o r e t e s t i n g , s o t h a t t h e i r c u t i c l e s c o u l d h a r d e n . T h r e e g r o u p s o f C. b i f i d a w e r e t r e a t e d i n a d i f f e r e n t manner t o t h e r e s t o f t h e c o r i x i d s . Some w e r e t r a n s f e r r e d t o 25 C w h i l e s t i l l f o u r t h o r e a r l y f i f t h i n s t a r l a r v a e . A f t e r t h e s e l a r v a e emerged a s a d u l t s , a p o r t i o n o f them w e r e k e p t a t 25 C u n t i l t h e y h a d m a t u r e d , w h i l e t h e r e s t w e r e r e t u r n e d t o 20 C a f t e r t h e i r c u t i c l e s h a d h a r d e n e d . A t h i r d g r o u p , w h i c h h a d emerged as a d u l t s a t 20 C, w e r e moved down t o 10 C a f t e r t h e y h a d m a t u r e d . The c o r i x i d s i n t h e s e l a s t two g r o u p s w e r e a l l o w e d a t l e a s t o n e week t o a c c l i m a t e t o t h e new t e m p e r a t u r e s b e f o r e b e i n g t e s t e d . 2. T r a n s i t i o n p o i n t d e t e r m i n a t i o n s The t e m p e r a t u r e o f c u t i c u l a r l i p i d t r a n s i t i o n i n t h e s e i n s e c t s was d e t e r m i n e d u s i n g t h e b a s i c t e c h n i q u e s d e s c r i b e d by Beament (1958, 1 9 6 1 a ) . G e n e r a l l y , t h e m e t h o d was t o suspend an i n s e c t i n a dry CC>2 atmosphere and measure i t s r a t e of water l o s s over time. F i g u r e 3 shows the e x p e r i -mental apparatus used, a. Apparatus Carbon d i o x i d e from a p r e s s u r i z e d c y l i n d e r was r e -lea s e d s l o w l y through a two-stage r e g u l a t o r . Flowing through tygon t u b i n g , i t entered a copper heat-exchanging c o i l 1.6m long submerged i n a water bath. I t then flowed through a g l a s s d e s i c c a t o r : t u b e (16.5 cm long X 3.2 cm i n diameter) packed with 8-mesh c a l c i u m sulphate ( D r i e r i t e ) . From here, the C0 2 passed through a Gilmont RG F1500 flow meter. The volume of gas fl o w i n g through the system was too s m a l l to be a c c u r a t e l y measured by t h i s flow meter, but every e f f o r t was made to keep the flow r e l a t i v e l y con-s t a n t . The CC>2 then entered the g l a s s flow-through tube i n which the i n s e c t was suspended, and escaped through the bottom of t h i s tube. F i g u r e 4 g i v e s the d e t a i l s of t h i s p o r t i o n of the apparatus. The experimental temperature was c o n t r o l l e d by the water bath. This c o n s i s t e d of two g a l l o n s of water i n a c y l i n d r i c a l PYREX brand j a r (22.2cm o u t s i d e diameter X 25.4cm h i g h ) . The water bath temperature was c o n t r o l l e d by a Haake E12 constant temperature c i r c u l a t o r . Water was pumped through s u r g i c a l t u b i n g i n t o the j a c k e t of the do u b l w a l l e d flow-through tube. A f t e r f l o w i n g up and around the water j a c k e t , the water l e f t by the upper p o r t and retu r n e d FIGURE 3 The experimental apparatus used i n the d e t e r m i n a t i o n of t r a n s i t i o n temperatures A. E l e c t r o b a l a n c e B. Flow-through tube (see F i g u r e 4 ) C. Micromanipulator with termocouple leads D. Ice bath: c o l d j u n c t i o n f o r thermo-couple E. E l e c t r o b a l a n c e c o n t r o l u n i t F. Time d e r i v a t i v e computer G. Two-pen r e c o r d e r H. Thermocouple potentiometer J . CC>2 c y l i n d e r K. Water bath L. Copper heat-exchanging c o i l M. Constant temperature c i r c u l a t o r N. D e s i c c a t o r tube 0 . Flow meter FIGURE 4 D e t a i l of flow-through tube (B i n F i g u r e 3 ) . Diagram i s to f u l l s c a l e . hangdown wore to the bath (Figure 4). The water bath i t s e l f s a t i n a r e c t a n g u l a r p l a s t i c t r a y 30 X 36 cm X 15 cm deep, which served to c a t c h overflow water from the bath. T h i s t r a y i s not shown i n F i g u r e 4. Both CC>2 and c u t i c l e temperature were measured u s i n g a chromel/alumel thermocouple made from 29 s.w.g. w i r e s . This was mounted on a micromanipulator so t h a t the thermo-couple c o u l d be e a s i l y moved w i t h i n the weighing tube. The chromel and alumel leads were connected to copper leads i n ; an i c e bucket, which served as a 0 C r e f e r e n c e . A Doran M i n i Thermocouple Potentiometer was used to determine the output of the thermocouple. The weighing apparatus c o n s i s t e d of a Cahn RG E l e c t r o -balance and a m o d i f i e d Cahn " L i t t l e Gem" Thermogravimetric A n a l y s i s K i t . As shown i n F i g u r e 4, the experimental bug was hooked under the head and suspended from the balance arm by a 0.10mm nichrome w i r e . Output from the balance's c o n t r o l u n i t f o l l o w e d two r o u t e s : f i r s t l y , i t went d i r e c t l y to a lmv two-pen r e c o r d e r (Omniscribe, Houston Instrument). Secondly, i t went to a Cahn Time D e r i v a t i v e Computer (Mark I I ) . T h i s u n i t t r a n s l a t e d the changing weight s i g n a l of the balance i n t o a r a t e of change s i g n a l . I t s output a l s o went to the r e c o r d e r . b. Procedure C o r i x i d s to be used i n t r a n s i t i o n p o i n t d e t e r m i n a t i o n were handled as l i t t l e and as c a r e f u l l y as p o s s i b l e i n order to minimize damage to the e x t e r n a l l i p i d l a y e r . They 22 were removed from the c u l t u r e t r a y s with a p l a s t i c mesh scoop and t r a n s f e r r e d to smal l s c i n t i l l a t i o n v i a l s p a r t l y f i l l e d w ith d i s t i l l e d water. The a i r i n the v i a l was then r e p l a c e d with CC>2 and the v i a l was capped. The c o r i x i d u s u a l l y came to the s u r f a c e a f t e r a few minutes, began to j e r k , and then f l o a t e d motionless on the s u r f a c e of the water. I t was l e f t i n the v i a l from two to f i v e hours be-f o r e the t e s t began. A f t e r being removed from the v i a l , i t was l i g h t l y d r i e d on t i s s u e paper and then suspended on the hook of the hangdown wire. The lower flow-through tube was then r a i s e d over the bug and was connected to an upper tube by means of a r e t a i n i n g c o l l a r . The c o r i x i d was now i n a dry C0 2 atmosphere approximately the temperature of the water bath. A f t e r w a i t i n g 15 minutes f o r the s u r f a c e water of the c o r i x i d to evaporate, the temperature of the CC>2 and of the c o r i x i d ' s c u t i c l e was recorded. The c u t i c l e tempera-t u r e was taken by g e n t l y moving the thermocouple t i p forward u n t i l i t touched some p o i n t on the v e n t r a l s u r f a c e of the i n s e c t ' s abdomen. F o l l o w i n g the temperature r e a d i n g , the temperature was r a i s e d as q u i c k l y as p o s s i b l e to the next l e v e l . T h i s was accomplished by pouring b o i l i n g water i n t o the water bath. I t was not p o s s i b l e to r a i s e the temperature p r e c i s e l y to a predetermined l e v e l with t h i s procedure, but an approximate l e v e l c ould be e a s i l y reached. The next read-in g was taken as soon as the r a t e of weight change t r a c e had l e v e l l e d o f f . T h i s procedure was repeated u n t i l readings h a d b e e n t a k e n a t a l l t h e d e s i r e d t e m p e r a t u r e s . F i g u r e 5 shows a t r a c i n g f r o m t h e r e c o r d e r o f a t e s t w i t h a f e m a l e C. b l a i s d e l l i . c. A n a l y s i s The o r i g i n a l w e t w e i g h t s o f t h e c o r i x i d s w e r e d e t e r -m i n e d by e x t e n d i n g t h e w e i g h t l o s s t r a c e b a c k t o t h e t i m e t h a t t h e b u g was p l a c e d o n t h e b a l a n c e . T h i s p r o c e d u r e e l i m i n a t e d t h e w e i g h t o f e x t e r n a l w a t e r . The t i m e d e r i v a t i v e c o m p u t e r , when c a l i b r a t e d , g a v e a v a l u e o f t h e r a t e o f w a t e r l o s s i n mg/hr. To e l i m i n a t e t h e e f f e c t o f t h e b u g ' s s i z e on i t s r a t e o f w a t e r l o s s , t h i s v a l u e was d i v i d e d b y a n a p p r o x i m a t i o n o f t h e b u g ' s s u r f a c e 2 a r e a t o g i v e a v a l u e i n mg/cm / h r . The s u r f a c e a r e a a p p r o x -i m a t i o n was d e r i v e d u s i n g t h e g e n e r a l f o r m u l a 2 A = M /3 k ; w h e r e : A = s u r f a c e a r e a M = mass k = a c o n s t a n t . I n o r d e r t o d e r i v e k f o r a d u l t C. e x p l e t a , O l o f f s and S c u d d e r (1966) d i s m e m b e r e d s e v e r a l b ugs and m e a s u r e d t h e s u r f a c e a r e a o f t h e s e p a r a t e d p a r t s u n d e r a m i c r o s c o p e w i t h an e y e p i e c e g r a t i c u l e . They r e p o r t e d a mean v a l u e o f k f o r t h i s s p e c i e s o f 10.8 (when mass i s m e a s u r e d i n grams and s u r f a c e a r e a i n s q u a r e c e n t i m e t e r s ) . T h i s v a l u e o f k was u s e d i n t h i s s t u d y f o r a l l e s t i m a t i o n s o f s u r f a c e a r e a , s i n c e a l l s p e c i e s s t u d i e d w e r e a b o u t t h e same s h a p e . The e f f e c t o f i n c r e a s e d t e m p e r a t u r e u p o n e v a p o r a t i o n r a t e h a d t o be d e a l t w i t h as w e l l . T h i s e f f e c t was e l i m i -FIGURE 5 Reproduction of a r e c o r d e r t r a c e showing the l o s s of weight of a female Cenocorixa  b l a i s d e l l i i n steps of i n c r e a s i n g temper-ature ( l e f t o r d i n a t e ) . The t r a c e i n c r e a s -i n g i n steps with time i s the r a t e of weight l o s s t r a c e from the time d e r i -v a t i v e computer ( r i g h t o r d i n a t e ) . The values above each of these steps i n d i c a t e the c u t i c l e temperature a t t h a t p o i n t , while the v a l u e s beneath i n d i c a t e the C0 9 temperature. MASS of INSECT (mg) nated by d i v i d i n g the water l o s s per u n i t area by the s a t u r a t i o n d e f i c i t of water a t the experimental tempera-t u r e . In a dry atmosphere, such as the one used i n t h i s study, the s a t u r a t i o n d e f i c i t of water i s equal to i t s vapour pressure a t t h a t temperature. We thus a r r i v e a t an index of c u t i c l e p e r m e a b i l i t y which i s the i n s e c t ' s e v a p o r a t i o n r a t e (mg/hr) d i v i d e d 2 by i t s s u r f a c e area (cm ) and by the s a t u r a t i o n d e f i c i t of water (mm Hg). Regression a n a l y s i s was used to determine a mean t r a n s i t i o n temperature with c o n f i d e n c e l i m i t s . The data s e t (e.g. F i g . 6) was d i v i d e d i n two a t a c e r t a i n tempera-t u r e ( s e v e r a l degrees below what appeared to be the i n f l e c -t i o n point) and a p a i r of r e g r e s s i o n s were p l o t t e d . A sequence of r e g r e s s i o n p a i r s were then c a l c u l a t e d with the data d i v i d e d between s e q u e n t i a l data p o i n t s along the temp-e r a t u r e a x i s u n t i l a p a i r of best f i t was found. The i n t e r -s e c t i o n of t h i s p a i r d e f i n e d the t r a n s i t i o n p o i n t . Ninety-s i x percent confidence l i m i t s were p l a c e d on the t r a n s i t i o n temperature by p l o t t i n g the 80% con f i d e n c e l i m i t s of each of the two r e g r e s s i o n l i n e s and f i n d i n g the two extreme i n t e r -s e c t i o n s of the confidence l i m i t s (the p r o b a b i l i t y of these l i n e s i n t e r s e c t i n g a t or beyond these c o n f i d e n c e l i m i t s i s the product of the p r o b a b i l i t i e s of the i n d i v i d u a l r e g r e s -s i o n s being o u t s i d e them; i . e . 0.20 x 0.20 = 0.04). B. PRETREATMENT TESTS The b a s i c procedure of these t e s t s was to " p r e t r e a t " c o r i x i d s i n water of v a r y i n g temperatures, then measure t h e i r p e r m e a b i l i t y a t a constant low temperature. The i n d i v i d u a l C. b i f i d a used i n these t e s t s were members of the f i r s t summer g e n e r a t i o n . They were c o l l e c -ted from Round-up and Barnes Lakes (Figure 2), and t r a n s -p o r ted to the l a b o r a t o r y and maintained there a t 20 C as d e s c r i b e d p r e v i o u s l y . Four d i f f e r e n t pretreatment temperatures were used: 25, 30, 32, and 35 C. Three to f i v e c o r i x i d s were t e s t e d a t each of these temperatures. The bugs were taken from t h e i r 20 C c u l t u r e t r a y and p l a c e d d i r e c t l y i n t o water h e l d constant at the treatment temperature. The aquarium used i n t h i s t e s t i s d e s c r i b e d i n the next s e c t i o n . A f t e r t hree minutes, the c o r i x i d s were taken out of the aquarium and put i n t o i n d i v i d u a l v i a l s , and were k i l l e d as d e s c r i b e d p r e v i o u s l y . T h e i r e v a p o r a t i v e water l o s s at 20 + 1 C was then determined u s i n g the apparatus and proced ure as d e s c r i b e d f o r the t r a n s i t i o n temperature determina-t i o n s , but o n l y one rea d i n g was taken. C. TEMPERATURE SURVIVAL TESTS In t h i s experiment, the l e n g t h of time r e q u i r e d f o r ea c o r i x i d to d i e a t a g i v e n temperature was measured. C. b i f i d a from Long Lake (Becher's P r a i r i e ) were used, and these bugs had been d i v i d e d i n t o two groups: one group a c c l i m a t e d a t 10 C f o r a t l e a s t two weeks be f o r e t e s t i n g , 28 the other group h e l d a t 20C. At each experimental tempera-t u r e , both groups were t e s t e d at the same time. Ten c o r i x i d s made up each group, but o c c a s i o n a l l y one managed to escape and the t e s t f i n i s h e d with o n l y nine bugs accounted f o r . The sex r a t i o was kept as c l o s e to 1:1 as p o s s i b l e . Four experimental temperatures were used: 27, 30, 33, and 36 C. At 27 C, no c o r i x i d s a c c l i m a t e d to 10 C were t e s t e d . The t e s t s took p l a c e i n a 5 g a l l o n aquarium which was d i v i d e d i n t o three s e c t i o n s : one end was separated from the r e s t of the aquarium by f i n e p l a s t i c s c r e e n i n g and the other s e c t i o n was d i v i d e d lengthwise i n t o two s e c t i o n s by c o a r s e r p l a s t i c s c r e e n i n g . A Haake E12 constant temperature c i r c u l -a t o r was p o s i t i o n e d at the end enclosed by the f i n e s c r e e n -i n g . T h i s s c r e e n i n g reduced the turbu l e n c e caused by the heater pump while a l l o w i n g water to c i r c u l a t e f r e e l y . Another sheet of coarse screen covered the aquarium to prevent c o r i x i d s from f l y i n g out. The c o r i x i d s were taken from t h e i r c u l t u r e t r a y s and put i n t o the aquarium a t 20 C. The temperature was then r a i s e d by approximately 0.7 5 C/min u n t i l the experimental temperature was reached. A c o r i x i d was c o n s i d e r e d "dead" when i t c o u l d no longer swim i n a c o o r d i n a t e d manner. For'the t e s t a t 3 6 C, the bugs were watched c o n s t a n t l y , while a t 33 C, the c o r i x i d s were observed c o n t i n u o u s l y f o r the f i r s t hour, and h o u r l y t h e r e a f t e r . During the t e s t s a t 27 and 30 C, the bugs were checked a t l e a s t twice d a i l y , and deaths were recorded as having o c c u r r e d i n the middle of the 12 hour p e r i o d s . When the c o r i x i d s were checked more o f t e n than every 12 hr., deaths were recorded as having o c c u r r e d i n the middle of the previous p e r i o d of absence. To f i n d out i f p r e v i o u s a c c l i m a t i o n temperature has a s i g n i f i c a n t e f f e c t on the s u r v i v a l of these c o r i x i d s , the r e s u l t s of bugs a c c l i m a t e d to 10 and 20 C were compared us i n g o n e - t a i l e d t - t e s t s . The hypothesis t e s t e d was t h a t bugs a c c l i m a t e d to 10 C would not s u r v i v e as long as those a c c l i m a t e d to 20 C. D. SALINITY EFFECTS The C. b i f i d a used i n these t e s t s were c o l l e c t e d from SP8, Westwick, and Boitano Lakes near Springhouse, B. C. (Figure l b ) ; from Sapper (Box 22), B a r k l e y (Opposite Box. 4) ;\ Long, and Barnes (Box 4) Lakes on Becher'.s . P r a i r i e . (Figure 2) ; and from LE3 and Long Lake on the Green Timber P l a t e a u (Figure l c ) (names i n parentheses are those used by Scudder (1969 a, b They were t r a n s p o r t e d to the l a b o r a t o r y i n r e f r i g e r a t e d s t y r o -foam tubs (6cm high, 10cm o u t s i d e diameter a t the top, 7,5cm o u t s i d e diameter a t the base) h a l f - f i l l e d w ith lake water. In the l a b o r a t o r y the c o r i x i d s were h e l d i n these tubs a t 5 C u n t i l they were t e s t e d . T h e i r r a t e of. water l o s s a t 20 + 1 C was determined by the procedure used i n the pretreatment t e s t s 30 One d i f f e r e n c e , however, was t h a t they were k i l l e d i n v i a l s c o n t a i n i n g lake water, r a t h e r than d i s t i l l e d water. The c o n d u c t i v i t y of the lake water was determined a t the time of t e s t i n g by use of a Radiometer CDM2 c o n d u c t i v i t y meter, and c o r r e c t e d to 25 C. E. RESPONSE TO SALINITY CHANGE To i n v e s t i g a t e the p o s s i b i l i t y t h a t i n d i v i d u a l C. b i f i d a c o u l d change t h e i r c u t i c u l a r p e r m e a b i l i t y i n response to a change i n s a l i n i t y the f o l l o w i n g experiment was undertaken. Twenty C. b i f i d a c o l l e c t e d from Long Lake (Becher's P r a i r i e ) i n the f i r s t week of September 1977 were used f o r t h i s t e s t . Ten of these bugs were pl a c e d i n t o d i s t i l l e d water and ten remained i n t h e i r own lake water. A l l the bugs were kept i n styrofoam tubs (5 c o r i x i d s per tub) a t 20 C f o r 5 days. No food was given to the bugs d u r i n g t h i s p e r i o d . T h e i r r a t e s of water l o s s a t 20 + 1 C were then determined f o l l o w i n g the procedures o u t l i n e d p r e v i o u s l y . Since i t was hypothesized t h a t the c o r i x i d s i n d i s -t i l l e d water would be l e s s permeable than those i n lake water, a o n e - t a i l e d t - t e s t was used to determine the s i g n i -f i c a n c e of the r e s u l t s . 31 I I I . RESULTS A. TRANSITION POINT DETERMINATIONS i) General r e s u l t s . Table 1 g i v e s the mean masses and approximate s u r f a c e areas of the v a r i o u s groups of c o r i x i d s t e s t e d . The r a t e s of water l o s s of the c o r i x i d s s t u d i e d here are comparable to values obtained i n other i n v e s t i g a t i o n s . Table 2 compares some of the values found i n the l i t e r a t u r e to the data obtained i n t h i s study. In g e n e r a l , i n d i v i d u a l bugs showed a g r e a t i n c r e a s e i n p e r m e a b i l i t y above a c e r t a i n temperature. A sharp t r a n s -i t i o n p o i n t was not e v i d e n t i n many cases, however: i n s t e a d , the water l o s s t r a c e s curve upwards over a range of 2 or 3 C. F i g u r e 6 i l l u s t r a t e s the p e r m e a b i l i t y / t e m p e r a t u r e r e l a t i o n s h i p f o r i n d i v i d u a l C. b i f i d a . No i n d i v i d u a l curves f o r the other sets of c o r i x i d s t e s t e d are i l l u s t r a t e d , but with regard to the p r o p e r t i e s of i n d i v i d u a l t r a c e s , those shown i n F i g u r e 6 are r e p r e s e n t a t i v e . One can see t h a t there was c o n s i d e r a b l e v a r i a t i o n i n the r a t e s of water l o s s f o r i n d i v i d u a l bugs: at 20 C some c o r i x i d s showed water l o s s r a t e s twice those of o t h e r bugs. The o v e r a l l p e r m e a b i l i t y / t e m p e r a t u r e r e l a t i o n s h i p was f a i r l y c o n s i s t e n t , however. Below t r a n s i t i o n most bugs showed a constant g e n t l e i n c r e a s e i n water l o s s r a t e (about 0.0017 2 mg/hr/cm /mm - Hg/°C. . TABLE I The mean weights and approximate s u r f a c e areas of the c o r i x i d s used i n the t r a n s i -t i o n temperature d e t e r m i n a t i o n s . TABLE I I The r a t e o f water l o s s a t 20C o f c o r i x i d s i n t h i s study compared with v a l u e s r e p o r t e d i n the l i t e r a t u r e . 33 mean sur-Species temperature conditions sex N mean mass (mg) S.E.. face area ,-..2. (cm ) S.E. Cenocorixa bifida hungerfordi emerged at 20 C and acclimated to 20C o* 5 1 6 . 7 6 0 . 4 7 0 . 7 0 7 0 . 0 1 3 ? 5 2 2 . 6 5 0 . 4 0 0 . 8 6 5 0 . 0 1 0 emerged at 20C and acclimated to 10 C cf 7 1 5 . 7 4 0 . 2 5 0 . 6 7 8 0 . 0 1 4 ? 5 2 0 . 8 0 0 . 5 0 0 . 8 1 7 0 . 0 2 6 emerged at 25 C and acclimated to 20 C cf 5 1 4 . 8 4 0 . 1 8 0 . 6 5 2 0 . 0 1 1 ? 3 1 9 . 2 0 0 . 0 8 0 . 7 7 5 0 . 0 0 3 emerged at 25 C and acclimated to 25 C c* 5 1 4 . 9 6 0 . 4 4 0 . 6 5 6 0 . 0 1 3 ? 1 1 8 . 9 2 — 0 . 7 6 7 — C. b l a i s d e l l i acclimated to 20 C cf 3 1 1 . 1 6 0 . 1 7 0 . 5 3 9 0 . 0 0 5 ? 7 1 3 . 2 1 0 . 6 2 0 . 6 0 3 0 . 0 1 9 C. expleta acclimated to 20 C cf 1 1 6 . 7 1 0 . 7 0 6 ? 4 2 4 . 5 2 1 . 1 8 0 . 9 1 1 0 . 0 2 9 Callicorixa vulnerata emerged at 20 C and cf 9 1 2 . 5 1 0 . 2 6 0 . 520 0 . 008 acclimated to 20 C 1 J 1 5 . 3 5 — 0 . 5 4 5 — Species Corixa punctata Corixa sp. Cenocorixa expleta C. expleta C. bifida C. b l a i s d e l l i Callicorixa vulnerata rate of water loss (mg/cm /hr) 2.1 1.2-1.8 3.5 3.8 1 .4 1.6 2.2 Source Holdgate (1956) Beament (1961b) Oloffs & Scudder (1966) this study this study this study this study 34 FIGURE 6 The r a t e of water l o s s (with the e f f e c t of i n c r e a s i n g s a t u r a t i o n d e f i c i t removed) of i n d i v i d u a l Cenocorixa b i f i d a as a f u n c t i o n of c u t i c l e temperature. 35 X . E 0.30 CUTICLE TEMPERATURE (°C) 36 The r e s u l t s shown here are a c t u a l l y c o n s e r v a t i v e e s t i -mates of the i n c r e a s e i n r a t e of water l o s s with temperature. As a bug l o s e s water a t a constant temperature, i t s r a t e of water l o s s decreases. Two male C. b i f i d a were t e s t e d a t 30 C: t h e i r water l o s s r a t e s decreased by 0.092 and 0.066 mg/ 2 cm /hr/mm Hg/per cent o r i g i n a l weight l o s t (Figure 7). Since c o r i x i d s u s u a l l y l o s t about 20 per cent of t h e i r o r i g i n a l weight i n the t r a n s i t i o n p o i n t t e s t s , the l a s t r a t e of water l o s s v alue o b t a i n e d would be about 20°/o times 0.08mg/cm^/hr/ 2 per cent o r i g i n a l weight l o s t , or 1.6 mg/cm /hr too low. At 40 C, t h i s would correspond to an underestimate of about 0.03 2 mg/cm /hr/mm Hg. Although a l l the p e r m e a b i l i t y data given here have been analysed and presented with r e f e r e n c e to c u t i c l e temperature, the temperature of the carbon d i o x i d e surrounding the c o r i x i d was a l s o measured. I f the water bath was below room tempera-t u r e , the c o r i x i d ' s c u t i c l e temperature was u s u a l l y equal to or s l i g h t l y (up to 1 C) high e r than the CC>2 temperature. As the experimental temperature i n c r e a s e d , the c u t i c l e tempera-t u r e became i n c r e a s i n g l y c o o l e r r e l a t i v e to the CC^ temperature. At 43 C C0 2 temperature, the c u t i c l e was approximately 40-41 C. i i ) Males vs. Females. F i g u r e 8 shows t h a t there was no apparent d i f f e r e n c e be-tween male and female C. b i f i d a with regard to t h e i r c u t i c u l a r FIGURE 7 The r a t e of water l o s s a t 30 C of two male C. b i f i d a as a f u n c t i o n of the percentage of t h e i r o r i g i n a l wet weight remaining. 39 FIGURE 8 The r a t e of water l o s s of male ( c l o s e d c i r c l e s ) and female (open c i r c l e s ) C. b i f i d a as a f u n c t i o n of c u t i c l e temper-ature . 4 0 0.30 U) E £ £ O i . 0.20 </> co O o e • o o «» o o o L U I— I o LU < 0.10 0.00 o o 10 o o o o o 20 30 40 CUTICLE TEMPERATURE (°C) 50 permeability/temperature r e l a t i o n s h i p . T h e r e f o r e , no attempt was made to separate males and females i n the a n a l y s i s of the t r a n s i t i o n p o i n t data. i i i ) D i f f e r e n c e s among s p e c i e s . The r e s u l t s o f the t r a n s i t i o n p o i n t t e s t s are summar-i z e d i n F i g u r e s 9 to 15. The t r a n s i t i o n temperatures of the three Cenocorixa s p e c i e s and C a l l i c o r i x a v u l n e r a t a were q u i t e s i m i l a r to each o t h e r . F i g u r e 16 shows t h a t a l -though t h e i r t r a n s i t i o n temperatures ranged from 30.25 to 32.60 C, they a l l l a y w i t h i n the confidence l i m i t s of each o t h e r . C. b i f i d a and C. v u l n e r a t a , which were r a i s e d under the same l a b o r a t o r y c o n d i t i o n s , were e s p e c i a l l y s i m i l a r . The r e s u l t s f o r C. e x p l e t a i n t h i s study are comparable with those d e r i v e d from the data of O l o f f s and Scudder (1966) iv) D i f f e r e n c e s among temperature c l a s s e s . A d i f f e r e n t s t o r y appears, however, when we compare the t r a n s i t i o n temperatures of C. b i f i d a i n d i v i d u a l s kept under d i f f e r e n t l a b o r a t o r y c o n d i t i o n s (Figure 17). C o r i x i d s which emerged a t 20 C and then a c c l i m a t e d to 10 C had much lower t r a n s i t i o n temp'eratures than those a c c l i m a t e d to 20 C. S i m i l a r l y , C. b i f i d a a d u l t s which emerged a t 25C and a c c l i m a t e d to 25 C had s i g n i f i c a n t l y h igher t r a n s i t i o n temperatures than bugs which were a c c l i m a t e d to 20 C. The data from c o r i x i d s which emerged a t 25 C and were a c c l i m a t e d to 20 C show high v a r i a t i o n , so nothi n g c o n c r e t e can be concluded from them. I t appears, though, t h a t these bugs had a lower t r a n s i t i o n FIGURE 9 Rate o f water l o s s as a f u n c t i o n o c u t i c l e t e mperature f o r C. b i f i d a r e a r e d a t 20 C and a c c l i m a t e d t o 20 C. P o i n t s used i n t h e c a l c u l a -t i o n o f t h e lower r e g r e s s i o n a r e r e p r e s e n t e d by c i r c l e s : p o i n t s used i n t h e c a l c u l a t i o n o f the upper r e g r e s s i o n a r e r e p r e s e n t e d by s q u a r e s . '43 0) X I 0.30 CUTICLE TEMPERATURE (°C) FIGURE 10 Rate of water l o s s as a f u n c t i o n of c u t i c l e temperature f o r C. b l a i s d e l l i a c c l i m a t e d to 20C. Symbols as i n F i g . 4 5 CUTICLE TEMPERATURE (°C) FIGURE 11 Rate of water l o s s as a f u n c t i o n of c u t i c l e temperature f o r C. e x p l e t a a c c l i m a t e d t o 20 C. Symbols as i n F i g . 9. CUTICLE TEMPERATURE °C FIGURE 12 Rate of water l o s s as a f u n c t i o n of c u t i c l e temperature f o r C a l l i c o r i x a  v u l n e r a t a r e a r e d a t 20 C and a c c l i m ated to 20 C. Symbols as i n F i g . 9 4 9 FIGURE 13 Rate of water l o s s as a f u n c t i o n of c u t i c l e temperature f o r C. b i f i d a r e a r e d a t 20 C and a c c l i m a t e d to 10 C. Symbols as i n F i g . 9. 51 CUTICLE TEMPERATURE (°C) FIGURE 14 Rate of water l o s s as a f u n c t i o n o c u t i c l e temperature f o r C. b i f i d a r e a red a t 25 C and a c c l i m a t e d to 2 C. Symbols as i n F i g . 9. 53 FIGURE 15 Rate of water l o s s as a f u n c t i o n of c u t i c l e temperature f o r C. b i f i d a r e a r e d a t 25 C and a c c l i m a t e d to 20 C. Symbols as i n F i g . 9. 55 FIGURE 16 The t r a n s i t i o n temperatures (as de f i n e d by r e g r e s s i o n i n t e r s e c t i o n s ) of the v a r i o u s c o r i x i d s p e c i e s s t u d i e d . V e r t i c a l bars i n d i c a t e the 96°/o co n f i d e n c e i n t e r v a l s of the i n t e r s e c t i o n s . a. C. b i f i d a b. C. b l a i s d e l l i c. C. e x p l e t a d. C. e x p l e t a ( d e r i v e d from O l o f f & Scudder, 1966) e. C. v u l n e r a t a CUTICLE TEMPERATURE °C W W (A> O Ui O O Ui VJ1 58 FIGURE 17 The t r a n s i t i o n temperatures of the v a r -ious temperature c l a s s e s of C. b i f i d a . V e r t i c a l bars i n d i c a t e the 96°/o c o n f i -dence i n t e r v a l s of the r e g r e s s i o n s . a. Reared a t 20 C and a c c l i m a t e d to 10 C. b. Reared a t 20 C and a c c l i m a t e d to 20 C. c. Reared a t 25 C and a c c l i m a t e d to 20 C. d. Reared a t 25 C and a c c l i m a t e d to 25 C. CUTICLE TEMPERATURE °C K£ k ) W W ^ O u i o m © u i VO temperature than those t h a t were a c c l i m a t e d to 2 5 C. B. PRETREATMENT TESTS There were no 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 r a t e s of water l o s s of c o r i x i d s p r e t r e a t e d i n water a t v a r i o u s temp-e r a t u r e s (Figure 18). C. TEMPERATURE SURVIVAL TESTS The r e s u l t s of t h i s experiment are summarized i n F i g u r e 19. The s u r v i v a l time of the c o r i x i d s decreased with i n -creased temperature, and appeared to decrease more q u i c k l y above 30 C. The bugs at both 3 3 and 36 C showed no outward si g n s of osmotic i n f l u x of water, and there were no i n d i c a t i o n s of wax removal ( i . e . there was a high c o n t a c t angle with water, and the p l a s t r o n s remained f u n c t i o n a l ) . At 36 C, the s u r v i v a l time of c o r i x i d s a c c l i m a t e d to 10 C was s i g n i f i c a n t l y lower than t h a t of c o r i x i d s a c c l i -mated to 20 C (t = 2.457, p l e s s than 0.05). At 30 and 33 C, however, the s u r v i v a l times of the two groups were not s i g n i f i c a n t l y d i f f e r e n t (t = 0.52, p g r e a t e r than 0.05; t = 0.17, p g r e a t e r than 0.05 r e s p e c t i v e l y ) . D. SALINITY EFFECTS The t r a n s p i r a t i o n r a t e s of c o r i x i d s from l a k e s of a wide range of s a l i n i t i e s i n d i c a t e t h a t p e r m e a b i l i t y and s a l i n i t y are indeed c o r r e l a t e d (Figure 20). Cenocorixa  b i f i d a from SP8, Sapper, B a r k l e y , and Westwick Lakes ( a l l l e s s than 2000 umhos/cm s u r f a c e c o n d u c t i v i t y a t 25 C) l o s t 61 FIGURE 18 Rate of water l o s s a t 20 C of C. b i f i d a p r e t r e a t e d i n water of v a r i o u s tempera-t u r e s . V e r t i c a l bars i n d i c a t e 1 SE above and below the mean. Numbers below these bars are the sample s i z e s . 6 2 1.0 25 30 PRETREATMENT TEMPERATURE 35 FIGURE 19 Temperature t o l e r a n c e of C. b i f i d a . Mean s u r v i v a l times of i n s e c t s a c c l i m a t e d to 20 C are represented by squares, and those of i n s e c t s a c c l i m a t e d to 10 C are represented by c i r c l e s . V e r t i c a l bars i n d i c a t e 1 SE above and below the mean. Numbers below these bars are the sample s i z e s . The o r d i n a t e i s a l o g a r i t h -mic s c a l e . 6h FIGURE 20 Rate o f water l o s s o f C. b i f i d a as a f u n c t i o n o f the s a l i n i t y o f t h e i r n a t i v e l a k e s . V e r t i c a l b a r s r e p r e s e n t 1 SE above and below the mean. Numbers above the b a r s g i v e the sample s i z e . 6.0 5.0 4.0 9 3.0- 10 10 2.0 3 1.0 0.0 10 1 5 JJ mhos/cm x 10 3 at 25°C water a t r a t e s l e s s than 3.5 mg/cm /hr, while those from Long Lake on Becher's P r a i r i e (8000 umhos/cm) l o s t water 2 a t approximately 5 mg/cm /hr. C o r i x i d s from Boitano Lake (5500 umhos/cm) showed i n t e r m e d i a t e r a t e s of water l o s s . As s a l i n i t y i n c r e a s e s f u r t h e r , the tre n d of i n c r e a s i n g p e r m e a b i l i t y r e v e r s e s : C. b i f i d a from Long Lake on the Green Timber P l a t e a u (17,000 umhos/cm) were as impermeable as those from Barkley and Westwick Lakes. LE3 (10,300 umhos) and Barnes Lake (13,500 umhos) c o r i x i d s showed i n -termediate r a t e s of water l o s s . E. RESPONSE TO SALINITY CHANGE A f t e r remaining i n t h e i r Long Lake (Becher's P r a i r i e ) water (84 00 umhos/cm) f o r f i v e days, the e i g h t c o n t r o l bugs 2 t e s t e d had a mean r a t e of water l o s s a t 20 C of 3.56 mg/cm hr. The seven bugs which had spent f i v e days i n d i s t i l l e d 2 water showed a mean r a t e of 2.72 mg/cm /hr, which was s i g n i f i c a n t l y lower (p l e s s than 0.05) a c c o r d i n g to a o n e - t a i l e d t - t e s t . 68 IV. DISCUSSION A. TRANSITION POINT DETERMINATIONS 1. Techniques. The general technique f o l l o w e d i n t h i s study was t h a t d e s c r i b e d by Beament (1958, 1961a). In h i s d i s c u s s i o n of t h i s method, Beament (1958) mentions s e v e r a l c r i t i c a l f a c -t o r s i n the d e t e c t i o n of the t r a n s i t i o n temperature of an i n s e c t . F i r s t , the p e r m e a b i l i t y / t e m p e r a t u r e curves of i n d i v i -d ual i n s e c t s should be used: i n d i v i d u a l v a r i a b i l i t y can obscure a sharp t r a n s i t i o n i f composite curves are p l o t t e d . O l o f f s and Scudder (1966) demonstrate w e l l t h i s phenomenon i n t h e i r work with Cenocorixa e x p l e t a . Second, Beament (1958) claims t h a t c u t i c l e temperature, not a i r temperature should be used i n the a n a l y s i s of the r a t e of water l o s s . Since e v a p o r a t i o n from a s u r f a c e causes a decrease i n temperature at t h a t s u r f a c e , the c u t i c l e temperature w i l l be lower than the temperature of the surrounding a i r . I f an abrupt i n -crease i n e v a p o r a t i o n occurs a t t r a n s i t i o n , the c u t i c l e temperature would l a g behind the a i r temperature even more. Thus, the use of a i r temperature i n c a l c u l a t i n g the r e l a t i v e p e r m e a b i l i t y of the i n s e c t c o u l d obscure a t r a n s i t i o n p o i n t . In o p p o s i t i o n to t h i s , O l o f f s and Scudder (1966) showed t h a t t r a n s i t i o n p o i n t s i n C. e x p l e t a were d e t e c t a b l e u s i n g e i t h e r a i r or c u t i c l e temperatures. However, Beament (1958) 69 p o i n t s out t h a t , although a knowledge of a i r temperature at t r a n s i t i o n may be e c o l o g i c a l l y important, i t i s the temperature of the c u t i c l e t h a t i s e s s e n t i a l i n any p h y s i c o -chemical i n t e r p r e t a t i o n . In a d d i t i o n , these c o r i x i d s pro-bably never encounter a i r temperatures warm enough to cause e p i c u t i c u l a r l i p i d t r a n s i t i o n . They on l y f l y d u r i n g the day i n the s p r i n g (Scudder, pers. comm.; Smith, pers. comm.), a time when d i u r n a l temperatures are not extreme (Cannings, 1973; Jansson and Scudder, 1974; Smith, 1977). In the summer and e a r l y autumn they f l y d u r i n g the evening (Scudder pe r s . comm.), again a v o i d i n g extreme temperatures (Jansson and Scudder, 1974, Cannings, 1975). Probably the o n l y s i t u a -t i o n i n which t r a n s i t i o n c o u l d occur i n these i n s e c t s , then, would be as they swim through shallow water d u r i n g hot summer days. Since t h e i r c u t i c l e temperature i s equal to t h a t of the surrounding water, I decided to measure c u t i c l e , r a t h e r than a i r temperature, i n the d e t e r m i n a t i o n of the t r a n s i t i o n p o i n t s of these i n s e c t s . Beament (1959) a l s o s t r e s s e s t h a t h a n d l i n g o f e x p e r i -mental i n s e c t s must be minimized, f o r any d i s t u r b a n c e of the e x t e r n a l wax l a y e r can a l s o obscure a sharp t r a n s i t i o n p o i n t . Most other r e s e a r c h e r s i n t h i s f i e l d have measured the r a t e of water l o s s from i n s e c t s a t a known temperature by determining t h e i r weight b e f o r e and a f t e r exposure f o r a given p e r i o d of time. T h i s i n v o l v e s h a n d l i n g the i n s e c t 70 to some extent between re a d i n g s , and the p e r i o d d u r i n g t r a n s f e r to the balance c o u l d i n t r o d u c e e r r o r s as w e l l . A l s o , one wants to minimize the d u r a t i o n of the experiment so t h a t the d r y i n g of the i n s e c t changes i t s r a t e o f water l o s s as l i t t l e as p o s s i b l e . Beament (1958) s o l v e d these problems by b u i l d i n g a balance i n t o the experimental system, while Edney and McFarlane (1974) suspended cockroaches i n the experimental chamber from a balance above. Since i t appeared t o be the si m p l e s t s o l u t i o n to the problems mentioned above, I chose to use a below-the-balance continuous weighing procedure s i m i l a r to t h a t o f Edney and McFarlane (1974). Instead of hanging the i n s e c t i n a r e l a t i v e l y l a r g e chamber suspended i n a water bath, however, I decided to use a much more compact system. With the apparatus shown i n F i g u r e s 3 and 4, access to the i n s e c t i s much improved, and thermocouple p o s i t i o n i n g i s a simple o p e r a t i o n . The major reason f o r usi n g a narrow tube f o r an experimental chamber, however, i s t h a t i t would be imp o s s i b l e to a c c u r a t e l y weigh an i n s e c t the s i z e of a c o r i x i d i n a chamber i n which a i r was being s t i r r e d by a fan. Cahn and Peterson (1967) found t h a t with f l o w i n g gas systems, t u r b u l e n c e was much g r e a t e r i n tubes with diameters g r e a t e r than 16 mm. In f a c t , I found t h a t with a tube 16 mm i n diameter, the s i g n a l n o i s e (caused by t u r b u l e n c e b u f f e t i n g the i n s e c t ) was SS gre a t t h a t the t r a c e c o u l d not be analysed. In a chamber one would expect 7L t h i s n o i s e t o be g r e a t e r s t i l l . W i t h t h e i r a p p a r a t u s , Edney and McFarlane (1974) used c o n t i n u o u s l y r i s i n g t e m p e r a t u r e s t o examine changes i n the r a t e o f water l o s s w i t h t e m p e r a t u r e . A l t h o u g h t h i s method reduces e x p e r i m e n t a l time c o n s i d e r a b l y (and t h u s m i n i m i z e s the i n s e c t ' s water l o s s ) , i t has s e v e r a l d i s a d v a n t a g e s , namely: i ) C u t i c l e t e mperature cannot be measured a t the same time w e i g h t l o s s i s b e i n g r e c o r d e d . Edney and M c F a r l a n e ( 1 9 7 4 ) e s t a b l i s h e d a r e l a t i o n s h i p between a i r and c u t i c l e t e m perature d u r i n g t r i a l r u n s , b u t c o u l d p r e d i c t c u t i c l e t e mperature t o w i t h i n o n l y 1°C. i i ) The r i s e i n temperature must be k e p t c o n s t a n t among d i f f e r e n t r u n s , because the c u t i c u l a r r e s ponse may d i f f e r a t v a r i o u s r a t e s o f t e m p e r a t u r e i n c r e a s e . T h i s c o n s t a n c y i s a l s o n e c e s s a r y t o p r e d i c t c u t i c l e t e mperature from a known a i r t e m p e r a t u r e . i i i ) Perhaps t h e g r e a t e s t d i s a d v a n t a g e o f t h i s p r o c e -dure i s t h a t the r a t e o f water l o s s o f the i n s e c t may l a g b e h i n d the r i s i n g t e m p e r a t u r e . T h i s l a g would cause an u n d e r e s t i m a t e o f the r e l a t i v e p e r m e a b i l i t y o f t h e c u t i c l e , e s p e c i a l l y a t the h i g h e r t e m p e r a t u r e s . I n f a c t , t h i s phenomenon c o u l d be the r e a s o n why Edney and M c F a r l a n e (1974) f a i l e d t o d e t e c t a sharp t r a n s i t i o n i n the c o c k r o a c h e s t h a t t hey were s t u d y i n g . I n s h o r t , t h e n , I d e c i d e d t o use a s t e p - b y - s t e p p r o c e d -ure ( r a t h e r than h a v i n g the temperature r i s e c o n t i n u o u s l y ) f o r the f o l l o w i n g r e a s o n s : 1) C u t i c l e t e mperature c o u l d be e a s i l y measured a t the end o f each r a t e o f w e i g h t change r e a d i n g . 2) Any l a g phenomena were e l i m i n a t e d . 3) The l e n g t h o f time t h a t the i n s e c t was exposed t o a d e s i c c a t i n g atmosphere c o u l d be k e p t t o a r e a s o n a b l y s h o r t p e r i o d o f time (about l h r ) by r a i s i n g t he water b a t h tempe-r a t u r e as q u i c k l y as p o s s i b l e between te m p e r a t u r e l e v e l s . A f r u s t r a t i n g problem t h a t sometimes o c c u r r e d w i t h t h i s p r o c e d u r e r e g a r d e d the f l o w o f C 0 2 . When a f r e s h tank o f CO,, was i n use, the extreme p r e s s u r e d i f f e r e n c e a c r o s s the r e g u l a t o r made i t v e r y d i f f i c u l t t o produce a smooth, even f l o w o f gas. A " f l u t t e r y " f l o w o f C 0 2 caused the i n -s e c t t o sway i n a j e r k y f a s h i o n on t h e hangdown w i r e , and the r e c o r d e r t r a c e s were rough. A t h i g h e r t e m p e r a t u r e s , t h e measurement o f C 0 2 tempera-t u r e was a l s o a problem. The temperature n e x t t o the water j a c k e t c o u l d be 1 C o r so g r e a t e r than t h a t o f the CO,, i n the c e n t e r o f the tube. Thus, depending on the e x a c t p o s i -t i o n o f the thermocouple t i p w i t h r e s p e c t t o the tube w a l l , the C 0 2 temperature c o u l d v a r y . S i n c e I was n o t r e a l l y c oncerned w i t h a t m o s p h e r i c t e m p e r a t u r e , I made no e f f o r t t o c o r r e c t t h i s problem. I n t h e f u t u r e , however, i t might be a d v i s a b l e t o measure a t m o s p h e r i c temperature w i t h t h e t i p o f the thermocouple p o s i t i o n e d d i r e c t l y above the i n s e c t . T h i s problem c o u l d a l s o be e l i m i n a t e d by k e e p i n g the C 0 2 warm so t h a t i s not a p p r e c i a b l y cooler-.than t h e water j a c -k e t as' i t e n t e r s the f l o w - t h r o u g h t u b e . 2. A n a l y s i s -- sources of e r r o r , a) k, the area constant. Holdgate (1956) s t a t e s t h a t the measurement of s u r f a c e area (using t h i s c a l c u l a t i o n ) was one of the c h i e f causes of i n a c c u r a c y i n h i s work. O l o f f s (1964) g i v e s the range of k's he c a l c u l a t e d f o r an unknown number of C. e x p l e t a as 9.3 to 12.3. I f t h i s range were used as the c o n f i d e n c e l i m i t s of k f o r c o r i x i d s , the s u r f a c e area c a l c u l a t i o n would c o n t a i n an e r r o r of roughly + 14°/o. T h i s c o u l d account f o r a good p o r t i o n of the v a r i a b i l i t y i n the t r a n s i t i o n data. I t i s obvious t h a t t h i s method of s u r f a c e area e s t i -mation does not take i n t o account any c o n v o l u t i o n s of the c u t i c u l a r s u r f a c e . Glynne-Jones (1955) found t h a t the e p i c u t i c l e of the honey bee may have an area 10 times i t s p r o j e c t e d s u r f a c e area, and u s i n g krypton a d s o r p t i o n , Lockey (1960) demonstrated t h a t the e l y t r a of s e v e r a l i n s e c t s had t r u e areas 6.7 to 8.2 times t h e i r p r o j e c t e d s u r f a c e areas. Thus, the p e r m e a b i l i t y index t h a t i s d e r i v e d u s i n g an e s t i -mate of p r o j e c t e d area c o u l d be an o v e r e s t i m a t e . On the other hand, e x t e n s i v e h a i r p i l e s may t r a p a i r and thus r e -duce the s a t u r a t i o n d e f i c i e n c y of the a i r adjacent to the c u t i c l e s u r f a c e . T h i s would cause the p e r m e a b i l i t y index to be underestimated. However, d e s p i t e the problems i n v o l v e d i n o b t a i n i n g an index of p e r m e a b i l i t y accurate enough to be u s e f u l com-p a r a t i v e l y , the index d e r i v e d u s i n g the area constant (k) can c e r t a i n l y be used to compare the c u t i c u l a r p e r m e a b i l i t -i e s of i n s e c t s as m o r p h o l o g i c a l l y s i m i l a r as c o r i x i d s . b) The decrease i n r a t e of t r a n s p i r a t i o n with d e s i c c a t i o n . S e v e r a l workers (Wigglesworth, 194 5; Edney, 1951; Loveridge, 1968) have found t h a t the r a t e of water l o s s from some arthropods g r a d u a l l y decreases as the animal d r i e s out. B u r s e l l (1957) p o i n t s out t h a t t h i s decrease a l s o occurs i n the t s e t s e f l y ( G l o s s i n a m o r s i t a n s ) , but here i t i s not caused by a change i n the c u t i c l e , but i n s t e a d by s p i r a c u l a r r e g u l a t i o n . However the A g r i o t e s l a r v a e t h a t Wigglesworth (1945) s t u d i e d were dead and had t h e i r s p i r a c l e s blocked before t r a n s p i r a t i o n measurements were taken. A l s o , s i n c e the major s p i r a c l e s i n c o r i x i d s are s i t u a t e d i n enclosed spaces (Parsons, 1970, 1976), and s i n c e the c o r i x i d s used were dead, i t i s u n l i k e l y t h a t s p i r a c u l a r r e g u l a t i o n c o u l d account f o r t h i s phenomenon i n the present study. O l o f f s (1964) d i d not f i n d any evidence of t h i s decrease i n the r a t e of t r a n s p i r a t i o n i n h i s t e s t s with C. e x p l e t a , but h i s procedure d i d not a l l o w him to t e s t t h i s with i n d i v i d u a l bugs. B u r s e l l (1955) suggests t h a t t h i s phenomenon might be because of the i n c r e a s e i n c o n c e n t r a t i o n of body f l u i d s , which lowers the a c t i v i t y of water on the i n s i d e of the membrane. Wigglesworth (1945), on the other hand, a t t r i -butes i t to the d r y i n g of the c u t i c l e i t s e l f which c o u l d decrease the p e r m e a b i l i t y of the endo- and e x o c u t i c l e . King (1944) shows t h i s to be the case i n k e r a t i n membranes. I t seems to me, however, t h a t t h i s l a t t e r mechanism would g i v e a n o t i c e a b l e decrease i n p e r m e a b i l i t y o n l y at tempera-t u r e s above t r a n s i t i o n , when the wax' l a y e r may not be the l i m i t i n g f a c t o r i n the d i f f u s i o n of water. In cases of extreme d e s i c c a t i o n , the a c t u a l a v a i l a b i l i t y of water f o r d i f f u s i o n through the c u t i c l e would cause a decrease i n the r a t e of water l o s s (Wigglesworth, 1945). c) The r e g r e s s i o n a n a l y s i s . The r e g r e s s i o n a n a l y s i s c o u l d have c o n t r i b u t e d to the e r r o r (as seen i n the c o n f i d e n c e l i m i t s of the t r a n s i t i o n temperatures) i n two ways: i ) I f the c o r i x i d s showed t r a n s i t i o n p o i n t s a t approximately the same temperature, but v a r i e d w idely i n t h e i r p e r m e a b i l i t y below t r a n s i t i o n , the c onfidence l i m i t s of the r e g r e s s i o n overestimate the v a r i a b i l i t y of the t r a n s -i t i o n temperature i t s e l f . A good example of t h i s can be seen i n the s e t of C. b i f i d a which emerged at 25 C and were a c c l i m a t e d to 20C. Most of the c o r i x i d s i n t h i s group showed a t r a n s i t i o n a t about 32-35 C, but the c o n f i d e n c e l i m i t s c a l c u l a t e d from the r e g r e s s i o n s encompass 27.7 C to 41 C. i i ) These r e g r e s s i o n s are used with the assumption t h a t the p e r m eability/temperature r e l a t i o n s h i p i s l i n e a r below and :above t r a n s i t i o n . Beament (1961a) s t a t e s t h a t below t r a n s i t i o n hard wax has a l i n e a r temperature c o e f f i c i e n t of p e r m e a b i l i t y , whereas above t r a n s i t i o n t here i s a " q u a s i -76 e x p o n e n t i a l " r e l a t i o n s h i p between temperature and permea-b i l i t y . N e v e r t h e l e s s , he i n t e r p r e t s experimental data as two s t r a i g h t l i n e s meeting at the t r a n s i t i o n p o i n t . With my data, e x p o n e n t i a l f u n c t i o n s d i d not e x p l a i n more of the v a r i a b i l i t y observed, so I continued u s i n g l i n e a r r e g r e s s i o n i n the a n a l y s i s of the data. . 3. The e x i s t e n c e of t r a n s i t i o n . A few years a f t e r Beament (1945) and Wigglesworth (1945) had proposed e p i c u t i c u l a r wax t r a n s i t i o n as an ex-p l a n a t i o n f o r t h e i r p e r m eability/temperature data, a con-t r o v e r s y arose over i t s e x i s t e n c e . Edney (1951), Mead-Br i g g s (1956), and Holdgate and Seal (1956) a l l argued t h a t the i n c r e a s e i n the r a t e of water l o s s with r i s i n g tempera-t u r e was e x p o n e n t i a l . Beament (1958) defended the t r a n s i t i o n theory by demonstrating the importance of u s i n g r e s u l t s from i n d i v i d u a l i n s e c t s and u s i n g c u t i c l e , r a t h e r than a i r tempera-t u r e s i n the a n a l y s i s of these r e s u l t s . Recently, however, Hackman (1974) c i t e s unpublished data of A. R. G i l b y and says t h a t "More re c e n t work on s e v e r a l s p e c i e s of i n s e c t confirms the f a c t t h a t temperature and water l o s s are r e l a t e d e x p o n e n t i a l l y " . A l s o , Edney and McFarlane (1974) f a i l e d to f i n d a sharp i n c r e a s e i n the r a t e of water l o s s of P e r i p l a n e t a americana with r i s i n g temperatures. T h i s f a i l u r e , however, i s probably a r e s u l t of t h e i r techniques, as was d i s c u s s e d p r e v i o u s l y . To ensure t h a t ray r e s u l t s showed two separate per-m e a b i l i t y s t a t e s of the c u t i c l e , r a t h e r than one r e l a t e d e x p o n e n t i a l l y to temperature, I p l o t t e d the r a t e of water l o s s (per u n i t s a t u r a t i o n d e f i c i t ) on a l o g a r i t h m i c s c a l e versus temperature (Figure 21). T h i s c l e a r l y shows t h a t there i s a d i s c o n t i n u i t y i n the o r i g i n a l d ata. I f the r e l a t i o n s h i p was a simple e x p o n e n t i a l , the p o i n t s i n F i g -ure 21 would l i e i n a s t r a i g h t l i n e . 4) The t r a n s i t i o n temperatures of the v a r i o u s c o r i x i d s p e c i e s . The t r a n s i t i o n temperatures of a group of c l o s e l y -r e l a t e d s p e c i e s , e s p e c i a l l y those of c o e x i s t i n g s p e c i e s , have never r e a l l y been s t u d i e d b e f o r e . In the past, workers have e i t h e r concentrated on one s p e c i e s (e.g. Davis, 1974a,; Hadley, 1970; Loveridge, 1968; O l o f f s and Scudder, 1966), s t u d i e d two s p e c i e s from d i f f e r e n t h a b i t a t s (e.g. Edney and McFarlane, 1974) or surveyed a broad spectrum of i n s e c t s (Wigglesworth, 1945; Beament, 1945, 1959, 1961b). Beament (1961b) found a g r e a t v a r i e t y of t e m p e r a t u r e / p e r m e a b i l i t y r e l a t i o n s h i p s i n a q u a t i c i n s e c t s . The two c l o s e l y - r e l a t e d d y t i s c i d genera Agabus and I l y b i u s , however, both had t r a n s i -t i o n temperatures of about 34 or 35 C, whereas t h e i r more d i s t a n t r e l a t i v e D y t i s c u s had a t r a n s i t i o n temperature of o n l y 24 C (Beament, 1961b). Th e r e f o r e , the f a c t t h a t the t r a n s i t i o n temperatures of the four s p e c i e s i n v e s t i g a g e d i n t h i s study a l l f e l l 78 FIGURE 21 The r a t e of water l o s s of C. b i f i d a (from F i g . 13) p l o t t e d on a l o g a r i t h m i c o r d i n a t e as a f u n c t i o n of c u t i c l e temperature. X 0.35 E £ 0.30 CUTICLE TEMPERATURE °C w i t h i n 2 or 3 C of each other i s not an unexpected r e s u l t . These c o r i x i d s are a l l congeners, with the e x c e p t i o n of C a l l i c o r i x a v u l n e r a t a , which i s a co-member of the T r i b e C o r i x i n i with the three Cenocorixa s p e c i e s . (Hungerford, 1948). In a d d i t i o n , they are i n h a b i t a n t s of ponds with approximately the same summer temperature regimes (Jansson & Scudder, 1974) . I t appears, then,that these s p e c i e s do not d i f f e r i n t h e i r s u s c e p t i b i l i t y to the t r a n s i t i o n phenomenon. Whether i t s e f f e c t s on each s p e c i e s d i f f e r i s another q u e s t i o n , and one which was not i n v e s t i g a t e d i n t h i s study. The f a c t o r s i n f l u e n c i n g p e r m e a b i l i t y , and t r a n s i t i o n i n p a r t i c u l a r , as w e l l as the e f f e c t s of t r a n s i t i o n on the w e l l - b e i n g of c o r i x i d s were s t u d i e d i n Cenocorixa b i f i d a alone. v) The i n f l u e n c e of a c c l i m a t i o n temperature on t r a n s i -t i o n temperature. There was a d e f i n i t e p o s i t i v e c o r r e l a t i o n between a c c l i mation temperature and the t r a n s i t i o n temperatures of the c o r i x i d s (Figure 17). Although t h i s has not been noted i n p r e v i o u s s t u d i e s with i n s e c t c u t i c u l a r l i p i d s , F r a e n k e l & Hopf (1940) and Munson (1953) showed t h a t a t h i g h e r a c c l i -mation temperatures, the body l i p i d s o f i n s e c t s were more s a t u r a t e d and had higher m e l t i n g p o i n t s . In a d d i t i o n , work on the c u t i c u l a r l i p i d s of p l a n t s has shown t h a t there tends to be a s h i f t to g r e a t e r c h a i n l e n g t h of c u t i c u l a r alkanes at higher temperatures (Wilkinson & Kasperbauer, 1972; Giese, 1975; Hass, 1977). T h i s i n c r e a s e i n c h a i n l e n g t h would r e s u l t i n a higher t r a n s i t i o n temperature (Chapman and L e s l i e , 1970). Temperature c o u l d a l s o i n t e r a c t with d i e t to produce changes i n c u t i c u l a r l i p i d s (although t h i s would not be the case i n t h i s s t u d y ) . Blomquist and Jackson (1973) r e p o r t t h a t a c o n s i d e r a b l e p o r t i o n of the n-alkane c o n s t i t u e n t s of the e p i c u t i c l e of the grasshopper Melanoplus sanguinipes are d e r i v e d d i r e c t l y from the d i e t . Branched hydrocarbons-, secondary a l c o h o l s , and ketones, however, are s y n t h e s i z e d . I t i s p o s s i b l e , then, t h a t the prey of c o r i x i d s produce l i p i d s which are more s a t u r a t e d and are l o n g e r - c h a i n e d a t higher a c c l i m a t i o n temperatures. I f a p o r t i o n of these d i e t -ary l i p i d s are i n c o r p o r a t e d d i r e c t l y i n t o the e p i c u t i c u l a r l i p i d l a y e r , the a c c l i m a t i o n temperature would cause an i n -crease i n the i n s e c t ' s t r a n s i t i o n temperature v i a i t s d i e t . There appears to be d i f f e r e n c e s i n the e f f e c t s of the temperature a t a d u l t emergence and the temperature at which the c o r i x i d was a c c l i m a t e d . For example, the t r a n s i t i o n temperatures of C. b i f i d a which had developed and emerged at 25 C and had then been a c c l i m a t e d to 20 C appears to be s t i l l 3 C or so higher than those of C. b i f i d a which had emerged a t 20 C and had been h e l d at 20 C. Whether or not t h i s i s a permanent d i f f e r e n c e cannot be determined from 82 these data alone. I t might be t h a t the change i n the l i p i d s takes a longer p e r i o d of time than was allowed i n t h i s e x p e r i -ment. On the other hand c e r t a i n l i p i d s may not change at a l l , g i v i n g the r e s u l t i n g l i p i d composition an i n t e r m e d i a t e t r a n s i -t i o n p o i n t . Few l i p i d s may have to change to produce a n o t i c e -able change i n the t r a n s i t i o n temperature, f o r c e r t a i n l i p i d s have s y n e r g i s t i c e f f e c t s on t r a n s i t i o n (Chapman and L e s l i e , 1970). The q u e s t i o n of why there i s a change i n the c o m p i s i t i o n of t h e i r e p i c u t i c u l a r l i p i d s i s s t i l l a matter f o r s p e c u l a t i o n . Does t h i s hedge a g a i n s t the p o s s i b i l i t y of t r a n s i t i o n o c c u r r i n g ? Or, as Beament (1962, 1976) suggests, does t h i s merely maintain the proper m o b i l i t y o f the wax, so t h a t damage to the c u t i c u l a r s u r f a c e can be e a s i l y r e p a i r e d ? Whether or not the former i s the case w i l l probably de-pend on what e f f e c t t r a n s i t i o n has on these i n s e c t s . T h i s i s d i s c u s s e d i n the f o l l o w i n g s e c t i o n s , v i ) T r a n s i t i o n underwater. The r e s u l t s of the pretreatment experiment i n d i c a t e t h a t there was no permanent i n c r e a s e i n c u t i c u l a r p e r m e a b i l i t y f o l l o w i n g immersion i n water warmer than the c o r i x i d ' s t r a n s i -t i o n temperature. T h i s i s i n d i r e c t o p p o s i t i o n to the r e s u l t s of O l o f f s and Scudder (1966), which show a 25°/o i n c r e a s e i n the r a t e of water l o s s from a c o r i x i d p r e t r e a t e d i n warm water. T h i s d i f f e r e n c e c o u l d have r e s u l t e d from one of the f o l l o w i n g : a) The v a r i a b i l i t y o f my data i s r a t h e r g r e a t , and c o u l d have obscured a t r e n d . On the other hand, the v a r i a -b i l i t y i n O l o f f s and Scudder's data i s not g i v e n : i f i t i s l a r g e , the trend seen i n t h e i r data may be f a l s e . b) The c o r i x i d s used i n t h i s study were p l a c e d i n the pretreatment water while a l i v e , whereas those used by O l o f f s and Scudder were k i l l e d before they were immersed. L i v i n g i n s e c t s may somehow be able to prevent the l o s s of wax or q u i c k l y r e p l a c e i t as i t i s l o s t . c) The e p i c u t i c u l a r wax l a y e r s of C. b i f i d a and C. e x p l e t a may d i f f e r enough to g i v e opposing r e s u l t s i n t h i s experiment. One can say, however, t h a t i n the experiment done i n t h i s study, nothing o c c u r r e d which permanently a l t e r e d the p e r m e a b i l i t y of the c o r i x i d s to a g r e a t e r degree than could be a t t r i b u t e d to normal v a r i a b i l i t y . T h i s means, of course, t h a t t r a n s i t i o n can o n l y a f f e c t a l i v i n g c o r i x i d while the temperature i s g r e a t e r than t h a t of t r a n s i t i o n , v i i ) The e f f e c t of temperature on s u r v i v a l . F i g u r e 19 shows t h a t there i s a c o n t i n u a l decrease i n s u r v i v a l time w i t h i n c r e a s i n g temperature. The apparent change i n the slope of the l o g a r i t h m i c f u n c t i o n between 30 and 33 C may or may not be s i g n i f i c a n t . I t i s p o s s i b l e t h a t t h i s change r e s u l t s from a d e t r i m e n t a l e f f e c t of t r a n s i t i o n . However, i f t r a n s i t i o n was the f a c t o r c a using death, I would expect the e f f e c t to be much more immediate: the bugs kept 84 at 33 C s u r v i v e d f o r a mean p e r i o d of 17h, and many of them s u r v i v e d a day or even longer. T h i s i s not the speedy death d e s c r i b e d by Beament (1962) f o r D y t i s c u s a d u l t s , which he claims to be the r e s u l t of c u t i c u l a r wax t r a n s i t i o n . In t h e i r n a t u r a l h a b i t a t , these c o r i x i d s would be exposed to temperatures above t r a n s i t i o n f o r o n l y an hour or two a t the most (Jansson and Scudder, 1974; Cannings, 1975). T h i s f a c t , coupled with the o b s e r v a t i o n t h a t most of the c o r i x i d s removed from the t e s t s a t 33 and 36 C recovered q u i c k l y a f t e r being p l a c e d i n c o o l e r water, makes i t appear t h a t these bugs do not s u f f e r any immediate d r a s t i c e f f e c t s as a r e s u l t of t r a n s i t i o n . The f a c t t h a t these c o r i x i d s u s u a l l y d i e s h o r t l y a f t e r breeding (Scudder, 1975) might e x p l a i n why t h e i r s u r v i v a l time a t 27 C i s s h o r t e r than might be expected from the other data. The bugs at 27 C, as w e l l as one or two a t 30 C, bred and l a i d eggs d u r i n g the course of the t e s t s . The comparison of the s u r v i v a l of bugs a c c l i m a t e d to 10 and 20 C i s an i n t e r e s t i n g one. I t seems t h a t a c c l i m a t i o n temperature makes l i t t l e d i f f e r e n c e i n s u r v i v a l time at lower temperatures (30 and 33C), where s u r v i v a l i s lengthy and the shock of the i n i t i a l i n c r e a s e i n temperature i s l e a s t . At 36 C, however, the 10 C bugs were c l e a r l y more q u i c k l y a f f e c t e d . B. SALINITY AND PERMEABILITY The c u t i c u l a r p e r m e a b i l i t y of C. b i f i d a was g r e a t e s t i n those bugs which i n h a b i t e d lakes of i n t e r m e d i a t e s a l i n i t y (Figure 20) : C. b i f i d a a d u l t s which l i v e d i n higher s a l i n i t y -water or f r e s h water had more impermeable c u t i c l e s . I t was demonstrated t h a t these d i f f e r e n c e s i n p e r m e a b i l i t y r e p r e -sented a process of p h y s i o l o g i c a l a c c l i m a t i o n , as the c u t i -c u l a r p e r m e a b i l i t y of Long Lake (Becher's P r a i r i e ) c o r i x i d s decreased s i g n i f i c a n t l y while they were i n d i s t i l l e d water f o r f i v e days. The data i n d i c a t e s t h a t C. b i f i d a " r e l a x e s " i t s c u t i c u l a r p e r m e a b i l i t y as the osmotic g r a d i e n t between i t s e l f and i t s environment decreases. Scudder e t a_l. (1972) found t h a t the haemolymph osmotic pressure of C. b i f i d a i s f a i r l y constant at lower s a l i n i t i e s , whereas at higher s a l i n i t i e s the haemo-lymph osmotic pressure of C. b i f i d a i n c r e a s e s with i n c r e a s i n g s a l i n i t y . T h i s i n c r e a s e begins at s a l i n i t i e s of about 10,000 umhos/cm at 25 C. Thus, the c o r i x i d s begin to reduce c u t i c u l a p e r m e a b i l i t y at approximately the p o i n t where i s becomes d i f f i c u l t to m a intain a constant haemolymph osmotic p r e s s u r e . S u b j e c t i v e o b s e r v a t i o n s i n d i c a t e t h a t the peak i n c u t i -c u l a r p e r m e a b i l i t y occurs i n the s a l i n i t y range i n which C. b i f i d a does b e s t . T h i s s p e c i e s can reach extremely high d e n s i t i e s i n lakes such as Long (Becher's P r a i r i e ) (8110 umhos/cm at 25 C), Round-up (10,700 umhos/cm*), and LE5 (7133 umhos/cm* but i s u s u a l l y sparse i n lakes l i k e B a r k l e y (1225 umhos/cm) ( C o n d u c t i v i t i e s marked by an a s t e r i s k are from the September 1976 data of Smith (1977); unmarked valu e s are from the sam-p l e s used i n the p e r m e a b i l i t y t e s t s of t h i s s t u d y ) . 86 The r e l a t i o n s h i p between s a l i n i t y and the c u t i c u l a r p e r m e a b i l i t y of a q u a t i c i n s e c t s has been s t u d i e d very l i t t l e . N i c o l s o n and Leader (1974) and P h i l l i p s and Bradley (1977) found t h a t s a l t water mosquito l a r v a e have c u t i c l e s which are 3 to 10 times l e s s permeable than those of f r e s h water i n s e c t l a r v a e . Osmotic r e g u l a t i o n i n Ephydra c i n e r e a i s known to be aided by a very impermeable c u t i c l e (Nemenz, 1970). However, I know of no r e p o r t s of changes i n c u t i c u l a r permeabil-i t y i n response to s a l i n i t y changes. P h i l l i p s and Bradley (1977) s t a t e t h a t the c u t i c u l a r p e r m e a b i l i t y of Aedes campestris l a r v a e does not change d u r i n g a d a p t a t i o n to extreme hyposmotic and hyperosmotic c o n d i t i o n s . The e f f e c t of s a l i n i t y on p e r m e a b i l i t y found i n t h i s study c o u l d have a pronounced impact on the d i s p e r s a l of these c o r i x i d s . Unless t h e i r c u t i c u l a r p e r m e a b i l i t y i s reduced be-f o r e f l i g h t takes p l a c e , c o r i x i d s f l y i n g out of i n t e r m e d i a t e s a l i n i t y lakes ( i n which they are so abundant) would d e s i c c a t e at twice the r a t e as those l e a v i n g f r e s h water l a k e s . In a d d i t i o n , i f these more permeable bugs land i n a r e l a t i v e l y f r e s h pond (which are much more common than s a l i n e ponds on Becher's P r a i r i e (Smith, 1977)), they would experience a sudden osmotic i n f l u x of water. C. GENERAL DISCUSSION Beament (1960, 1961b, 1962, 1976) has suggested t h a t low t r a n s i t i o n temperatures may l i m i t the d i s t r i b u t i o n s of some aqu a t i c i n s e c t s . In t h i s study I have attempted to determine whether or not e p i c u t i c u l a r l i p i d t r a n s i t i o n c o u l d l i m i t the d i s t r i b u t i o n or d i s p e r s a l of some water boatmen. These bugs a v o i d the p o s s i b i l i t y of t r a n s i t i o n o c c u r r i n g d u r i n g d i s p e r -s a l by f l y i n g i n the evening d u r i n g the l a t e summer (Scudder, pers. comm.). S p r i n g d i s p e r s a l takes p l a c e d u r i n g the day (Scudder, pers. comm.; Smith, pers. comm.), but maximum a i r temperatures at t h i s time do not g e n e r a l l y exceed 25 C (Cannings, 1973; Jansson and Scudder, 1974). What, then, i s the p r o b a b i l i t y o f t r a n s i t i o n o c c u r r i n g underwater? The r e s u l t s i n d i c a t e t h a t the t r a n s i t i o n p o i n t s of these c o r i x i d s are a l l at the extreme upper range of water temperatures they are l i k e l y t o experience i n t h i s p a r t of t h e i r range. The h i g h e s t temperature recorded by Jansson and Scudder (1974) f o r the l i t t o r a l r e g i o n s of Becher's P r a i r i e and Springhouse lakes was about 30 C, and Cannings (1975) r e -corded shallow water temperatures i n the range o f 30 and 30.5 C i n White Lake, a shallow s a l i n e pond near Okanagan F a l l s , B. C. In a d d i t i o n , C. b i f i d a i n d i v i d u a l s are ab l e to i n c r e a s e t h e i r t r a n s i t i o n temperature a t high water temperatures, making i t even more u n l i k e l y t h a t t r a n s i t i o n of t h e i r c u t i -c u l a r l i p i d s w i l l occur n a t u r a l l y . There i s a l s o the p o s s i -b i l i t y t h a t these c o r i x i d s c o u l d a v o i d shallow water warmer than t h e i r t r a n s i t i o n p o i n t by seeking c o o l e r m i c r o h a b i t a t s . Cannings (1975) found experimental evidence f o r t h i s i n C. e x p l e t a , but not i n C. b i f i d a . I f t r a n s i t i o n o f the c u t i c u l a r wax of these i n s e c t s does occur i n t h e i r n a t u r a l h a b i t a t there are i n d i c a t i o n s 88 that i t would not have any obvious adverse e f f e c t s on them. F i r s t , the pretreatment t e s t s i n d i c a t e d t h a t no n o t i c e a b l e permanent i n c r e a s e i n p e r m e a b i l i t y o c c u r r e d as a r e s u l t of t r a n s i t i o n underwater. Second, these c o r i x i d s would probably on l y be exposed to temperatures above t r a n s i t i o n f o r a very s h o r t p e r i o d of time, a t l e a s t i n t h i s p a r t of t h e i r range (Jansson and Scudder, 1974; Cannings, 1975). The s u r v i v a l t e s t a t 33 C showed t h a t C. b i f i d a i n d i v i d u a l s l i v e d many times longer than the n a t u r a l exposure would be. In the f i e l d , a c c l i m a t i o n responses would probably g r e a t l y i n c r e a s e t h i s s u r v i v a l time. T h i r d , the c o r i x i d s t e s t e d at 33 and 36 C showed no outward sig n s of decreased osmoregulatory a b i l i t y or l o s s of e x t e r n a l wax. Since i t seems u n l i k e l y t h a t t r a n s i t i o n would occur i n the f i e l d , and s i n c e i t appears t h a t these bugs s u f f e r no short-term adverse e f f e c t s from l i p i d t r a n s i t i o n , I must conclude t h a t e p i c u t i c u l a r l i p i d t r a n s i t i o n i s not r e l e v a n t to the d i f f e r e n t i a l d i s t r i b u t i o n of these s p e c i e s or to the c o e x i s t e n c e of C. e x p l e t a and C. b i f i d a . The e f f e c t of h a b i t a t s a l i n i t y on the c u t i c u l a r perme-a b i l i t y of C. b i f i d a was only s u p e r f i c i a l l y s t u d i e d here, but the r e s u l t s of t h i s study t i e d i n very w e l l w i t h the p ermeability/temperature r e s u l t s . The important general f e a t u r e t h a t emerged from both of these s t u d i e s was the very dynamic nature of the c o n t r o l of c u t i c u l a r p e r m e a b i l i t y . Wigglesworth (1945) s t a t e d t h a t a constant r a t e of water l o s s 89 was t y p i c a l o f e a c h s p e c i e s and o f e a c h d e v e l o p m e n t a l s t a g e o f t h e s p e c i e s , b u t H o l d g a t e and S e a l (1956) a n d Beament (1959) showed t h a t t h e p e r m e a b i l i t y and p e r m e a b i l i t y / t e m p e r a -t u r e r e l a t i o n s h i p o f p u p a l c u t i c l e s c h a n g e d m a r k e d l y w i t h t h e age o f t h e p u p a e . A l t h o u g h S i l h a c e k e t a l . (1972) and H a r r i s e t a l . (1976) d i d n o t s t u d y p e r m e a b i l i t y , t h e y showed t h a t t h e c u t i c u l a r h y d r o c a r b o n p a t t e r n s o f t h e s t a b l e f l y and h o u s e f l y a l s o v a r i e d c o n s i d e r a b l y w i t h t h e age o f t h e i n d i v i -d u a l . I n t h e s e c a s e s , h o w e v e r , t h e c o m p o s i t i o n a l c h a n g e s a p p e a r e d t o be r e l a t e d t o t h e p h e r o m o n a l r o l e o f a p o r t i o n o f t h e c u t i c u l a r h y d r o c a r b o n s . S t i l l , c h a n g e s s u c h as t h e s e c o u l d a l t e r t h e p e r m e a b i l i t y and p e r m e a b i l i t y / t e m p e r a t u r e r e l a t i o n s h i p o f t h e c u t i c l e . D a v i s (1974a) showed t h a t t h e c u t i c u l a r p e r m e a b i l i t y and t r a n s i t i o n t e m p e r a t u r e o f t h e r a b b i t t i c k H a e m a p h y s a l i s l e p o r i s p a l u s t r i s ( P a c k a r d ) c h a n g e m a r k e d l y d u r i n g t h e l i f e o f t h e a n i m a l , b o t h b e t w e e n a n d w i t h i n d e v e l o p m e n t a l s t a g e s . More i m p o r t a n t l y , he was a b l e t o c o r r e l a t e t h e s e c h a n g e s w i t h c h a n g e s i n c o m p o s i t i o n and amount o f e p i c u t i c u l a r l i p i d s ( D a v i d , 1 9 7 4 b ) . U n t i l now, h o w e v e r , t h e r e h a s b e e n no e v i d e n c e t h a t i n s e c t s o r o t h e r a r t h r o p o d s h a v e t h e a b i l i t y t o a l t e r t h e p e r m e a b i l i t y o f t h e i r e p i c u t i c u l a r wax l a y e r 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 . 90 LITERATURE CITED Beament, J.W.L. 1945. The c u t i c u l a r l i p o i d s of i n s e c t s . J . exp. B i o l . 2_1: 115-131. Beament, J.W.L. 1955. Wax s e c r e t i o n i n the cockroach. J . exp. B i o l . 3_2: 514-538. Beament, J.W.L. 1958. The e f f e c t of temperature on the waterp r o o f i n g mechanism of an i n s e c t . J . exp. B i o l . 35: 494-519. Beament, J.W.L. 1959. The wa t e r p r o o f i n g mechanism of arthropods. I. 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