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Responses of some hoverflies to oviposition sites Henderson, Deborah Elizabeth Hood 1981

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RESPONSES OF SOME HOVERFLIES TO OVIPOSITION SITES by Deborah Elizabeth|Hood Henderson B . S c , Trent U n i v e r s i t y , 1974 M.Sc, U n i v e r s i t y o f Manitoba, 1977 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES (Department of Pla n t Science & I n s t i t u t e of Animal Resource Ecology) We accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December, 1981 © Deborah E l i z a b e t h Hood Henderson, 1981 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It i s understood that copying or p u b l i c a t i o n of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of P I C I P C V Y Ar-rvncp The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date fof^Ufw- \\  D F - 6 ( 2/79) i -ABSTRACT Antennal s e n s i l l a of Metasyrphus venablesi (Cn.) and Eupeodes  v o l u c r i s 0. S. ( D i p t e r a : Syrphidae) were studied by scanning and transmission e l e c t r o n microscopy. Males and females both had four types of s e n s i l l a . Three of these, two multiporous perforated (MPP) s e n s i l l a (one round-tipped and one po i n t e d ) , and a grooved peg, multiporous sen-s i l l u m , were also confirmed by SEM on the f o l l o w i n g species: Syrphus  torvus (0*, Q) Scaeva p y r a s t r i (Cf, Q), Dasysyrphus amalopsis (0,), Xantho- grarnma f l a v i p e s (Q), Brachyopa perplexa (d*), P i p i z a sp. (Q), X y l o t a sp. ((f). The f o u r t h MPP s e n s i l l u m had t h i c k e r w a l l s and fewer pores. A l l four types were lo c a t e d among dense non-innervated setae on the antennal bulb and appeared to be o l f a c t o r y . EAG study of the antennae o f female M. venablesi and E_. v o l u c r i s showed th a t both species responded t o : common green plant v o l a t i l e s , t r a n s - and cis-2-hexen-1-ol, t r a n s - and cis-3-hexen-1-ol, c i s - 3 -hexenylacetate, and hexanol; other v o l a t i l e plant substances, m e t h y l s a l -i c y l a t e and amylacetate; crushed c a r n a t i o n p e t a l s and crushed aphids. There was no response to honeydew or some o f i t s components (e.g. t r y p -tophan, indolealdehyde o r indoleacetaldehyde) nor was there a response to water vapour. A gustatory s e n s i l l u m on the o v i p o s i t o r of these two species was studied by scanning and transmission e l e c t r o n microscopy and by neuro-p h y s i o l o g i c a l methods. One mechanosensitive and four chemosensitive neurons innervate each h a i r . The chemosensitive neurons are exposed to the e x t e r i o r by a t e r m i n a l pore, and respond to honeydew, tryptophan, indoleacetaldehyde, a l a n i n e , sucrose, and water. L a b e l l a r h a i r s are also s e n s i t i v e to sucrose. Olfactometer study of M. venablesi and E. v o l u c r i s showed that o l f a c t o r y stimulation by flowers would induce searching by both sexes. A mixture of tryptophan and indoleacetaldehyde induced mated females to search f o r and locate the stimulus. Other components of the o v i p o s i t i o n stimulus - crushed plant, un infested plant, and aphids did not induce searching. Physiological condition of the insect affected response to aphid-infested plants. Mated females, previously exposed to the stimulus, were more responsive than mated, inexperienced females or unmated, previously exposed females. Unmated, inexperienced females were least responsive. Mated and previously exposed males were more responsive than unmated, inexperienced males. Elements of the o v i p o s i t i o n stimulus were presented on green glass rods to mated females. The a t t r a c t i v e elements included fresh dead aphids, honeydew, crushed bean, tryptophan and indoleacetaldehyde, and c l u s t e r s of black spots s i m i l a r in size to aphids. Males were attracted only to honeydew and crushed bean. Both sexes responded to p o t e n t i a l food sources, such as honeydew, but only females responded to aphids and attractants that characterized oviposition s i t e s . A stimulus-response sequence i s proposed f o r these aphidophagous syrphids that involves dual and/or multiple stimulus combinations. - i i i -TABLE OF CONTENTS ABSTRACT i LIST OF TABLES v i i LIST OF FIGURES . . v i i i LIST OF PLATES x GENERAL INTRODUCTION 1 CHAPTER 1. ULTRASTRUCTURE OF CHEMOSENSILLA ON THE SYRPHID ANTENNA AND OVIPOSITOR 6 I n t r o d u c t i o n 7 M a t e r i a l s and Methods 13 Rearing of Syrphids 13 Scanning E l e c t r o n Microscopy 14 Transmission E l e c t r o n Microscopy 15 Re s u l t s 16 E l e c t r o n Microscopy of Antennae 16 E l e c t r o n Microscopy of O v i p o s i t o r S e n s i l l a 28 Scanning E l e c t r o n Microscopy of Larvae 37 Disc u s s i o n 42 Antennal S e n s i l l a 42 1. P i t t e d - S u r f a c e d , Multiporous S e n s i l l a 42 2. Grooved-Surfaced, Multiporous Sensillum 44 3. Thick-Walled, Multiporous S e n s i l l a 45 Uniporous O v i p o s i t o r S e n s i l l u m 47 L a r v a l S e n s i l l a 48 CHAPTER 2. OVIPOSITIONAL BEHAVIOUR OF APHIDOPHAGOUS SYRPHIDS 49 I n t r o d u c t i o n 50 - iv -Materials and Methods 54 Olfactometer Experiments 54 Coloured Glass Rod Experiments 57 Crushed-bean Rods 59 Tryptophan and Indoleacetaldehyde Rods 59 Honeydew Rods 59 Dead Aphids Glued to Rods 59 Black Spots 60 Aphids i n Glass V i a l s 60 Results and Discussion 61 Olfactometer Experiments 61 1. A i r Flow Controls 61 2. Colour Over Mixing Zone 61 3. Real Flower Olfactory Stimuli 62 4. Uninfested Bean Plants; Whole and Crushed 64 5. Aphids as a Stimulus 65 6. Tryptophan and Indoleacetaldehyde as Olfactory Stimuli 65 7. Infested Host Plants as Olfactory Stimuli 66 7.1 Mated Naive Insects: Never Exposed to Aphids or Plants 67 7 . 2 Unmated Experienced Insects: Previously Exposed to Plants and Honeydew, but not Aphids 67 7.3 Mated Naive Females: Never Exposed t o Aphids or Plants 68 7.4 Mated Experienced Insects: Oviposition Plant in Cage Immediately P r i o r to Experiment 68 - V -7.5 Experienced I n s e c t s : O v i p o s i t i o n P l a n t Deprived 24 or 48 Hours 69 O v i p o s i t i o n Rod Experiments 72 P r e l i m i n a r y Experiments 72 1. A c t i v i t i e s Unrelated t o the Stimulus 76 2. V i s u a l Colour Preference 76 2.1 Green vs Yellow Rods (both untreated) 77 2.2 Green vs Yellow Rods (both honeydew t r e a t e d ) . . . . 77 3. Gustatory and O l f a c t o r y S t i m u l i without V i s u a l Cues 80 3.1 Crushed Broad Bean vs D i s t i l l e d Water C o n t r o l . . . 80 3.2 Tryptophan and Indoleacetaldehyde vs Untreated Control 83 3.3 Honeydew vs Untreated Control 86 3.4 Honeydew vs Tryptophan and Indoleacetaldehyde Treated 89 4. V i s u a l With Limited or No Gustatory or O l f a c t o r y Cues 91 4.1 Black Marker Spot "Colonies" vs Untreated Control 91 4.2 L i v e Aphids i n a Glass V i a l vs Empty V i a l C o n t r o l 94 5. V i s u a l With Gustatory and O l f a c t o r y Cues 96 5.1 Dead Aphids Glued t o a Rod vs Control ( g l u e ) . . . . 96 5.2 Dead Aphids Glued t o a Rod vs Honeydew Treatment 99 Disc u s s i o n and Conclusions 106 CHAPTER 3. NEUROPHYSIOLOGY OF SYRPHID ANTENNAL AND OVIPOSITOR SENSILLA 110 - v i -I n t r o d u c t i o n 111 M a t e r i a l s and Methods 115 Re s u l t s 120 Electroantennogramme Recordings 120 S i n g l e O v i p o s i t o r Hair Recordings 128 L a b e l l a r and T a r s a l Hair Recordings 129 Di s c u s s i o n 133 DISCUSSION AND SUMMARY 138 LITERATURE CITED 146 APPENDIX 1 155 - v i i -LIST OF TABLES Table I. O l f a c t o r y Response of Syrphids Exposed to Real Flowers 63 Table I I . A t t r a c t i v e n e s s I n d i c e s f o r a l l S t i m u l i Presented to Syrphids i n the Olfactometer 71 Table I I I . Percentage o f V i s i t s to Glass O v i p o s i t i o n Rods that Included S p e c i f i c Landing A c t i v i t i e s 103 Table IV. Percentage o f V i s i t s to Glass O v i p o s i t i o n Rods that Included S p e c i f i c Stimulus-Related A c t i v i t i e s . . . . 104 Table V. Percentage o f V i s i t s to Glass O v i p o s i t i o n Rods that Included A c t i v i t i e s Unrelated to the Stimulus.... 105 - v i i i -LIST OF FIGURES Fig u r e 1. Syrphid A c t i v i t y on Untreated Green and Yellow Glass Rods 78 F i g u r e 2. Syrphid A c t i v i t y on Honeydew-Treated Green and Yellow Rods 79 F i g u r e 3- Syrphid A c t i v i t y on Green G l a s s Rods; Crushed Broad-bean Treated vs Untreated Control 82 F i g u r e 4. Female Syrphid A c t i v i t y on Green Glass Rods; Tryptophan and Indoleacetaldehyde Treated vs Water Control 85 F i g u r e 5. Syrphid A c t i v i t y on Green Glass Rods; Honeydew Treated vs Untreated Control 88 Fi g u r e 6. Syrphid A c t i v i t y on Green Glass Rods; Honeydew Treated vs Tryptophan and Indoleacetaldehyde Treated 90 F i g u r e 7. Syrphid A c t i v i t y on Green Glass Rods; Black Marker Spot-Treated vs Untreated Control 93 Fi g u r e 8. Syrphid A c t i v i t y on Green Glass Rods; L i v e Aphids i n an Attached V i a l vs Empty V i a l 95 Fi g u r e 9. Syrphid A c t i v i t y on Green Glass Rods; Dead Aphids Glued t o a Rod vs Glue Control 98 Fi g u r e 10. Syrphid A c t i v i t y on Green Glass Rods; Dead Aphids Glued t o a Rod vs a Honeydew Treated Rod 101 Fi g u r e 11. Electroantennogramme Apparatus 116 Fi g u r e 12. S i n g l e Hair Recording Apparatus 119 Figure 13. Electroantennogramme Responses 1 123 Fi g u r e 14. Electroantennogramme Responses 2 124 F i g u r e 15. Electroantennogramme Responses 3 125 - i x -Figure 16. Green P l a n t V o l a t i l e Substance Dose Response Curves 1 126 F i g u r e 17. Green P l a n t V o l a t i l e Substance Dose Response Curves 2 127 Fi g u r e 18. O v i p o s i t o r Hair E l e c t r i c a l Responses 1 131 F i g u r e 19. O v i p o s i t o r Hair E l e c t r i c a l Responses 2 132 F i g u r e 20. Spectroradiometer P l o t s o f R e f l e c t e d L i g h t Energy From A r t i f i c i a l Coloured Flowers 158 LIST OF PLATES P l a t e 1. Syrphid Antennal O l f a c t o r y S e n s i l l a 1 21 P l a t e 2. Syrphid Antennal O l f a c t o r y S e n s i l l a Types 22 P l a t e 3. Syrphid Antennal O l f a c t o r y S e n s i l l a 2 23 P l a t e 4. Syrphid Antennal O l f a c t o r y S e n s i l l a 3 24 P l a t e 5. L o n g i t u d i n a l S e c t i o n of a Multiporous, Thin-walled Antennal Chemosensillum 25 P l a t e 6. U l t r a s t r u c t u r e of a Multiporous Thin-walled, Grooved Peg Antennal Chemosensillum 26 P l a t e 7. U l t r a s t r u c t u r e of the Thick-walled Multiporous Antennal O l f a c t o r y Sensillum 27 P l a t e 8. Syrphid O v i p o s i t o r S e n s i l l a 31 P l a t e 9. Uniporous O v i p o s i t o r Contact Chemosensillum 32 P l a t e 10. U l t r a s t r u c t u r e of the Uniporous O v i p o s i t o r Chemosensillum 1 33 P l a t e 11. U l t r a s t r u c t u r e of the Uniporous O v i p o s i t o r Chemosensillum 2 34 P l a t e 12. U l t r a s t r u c t u r e of the Uniporous O v i p o s i t o r Chemosensillum 3 35 P l a t e 13. U l t r a s t r u c t u r e of the Uniporous O v i p o s i t o r Chemosensillum 4 36 P l a t e 14. Syrphid Larva 1 39 P l a t e 15. Syrphid Larva 2 40 P l a t e 16. Syrphid Larva 3 41 P l a t e 17. Olfactometer Apparatus 55 ACKNOWLEDGEMENTS F i r s t I would l i k e to thank my supervisor Dr. W.G. Wellington f o r encouragement and support throughout the research and writing of t h i s t h e s i s . For capable f i e l d and laboratory assistance I thank Carmen Rida and Barbara Henderson. Special thanks go to the s t a f f of the A g r i c u l -ture Canada Research Station, Vancouver not only f o r providing lab space and f a c i l i t i e s , but f o r th e i r i n t e r e s t and encouragement throughout the three years I spent at the s t a t i o n . Warm thanks go to Dr. B.K M i t c h e l l of the University of Alberta f o r providing expertise, f a c i l i t i e s , and encouragement for the neurophy-s i o l o g i c a l i n v e s t i g a t i o n s . I would e s p e c i a l l y l i k e to thank my s i s t e r L e s l i e f o r her hours of help with the figures and pl a t e s , and my daughter Lindsay f or constant companionship throughout the writing. GENERAL INTRODUCTION - 2 -Insects l i v e i n a world that c o n t i n u a l l y requires them to respond appropriately to various types of sensory information. Stimuli that are ava i l a b l e to them include l i g h t and darkness, l i g h t wavelength, sound, the g r a v i t a t i o n a l f i e l d , pressure, temperature, odours, tastes and tex-tures. Which of these they respond to depends on the i r s p e c i f i c requirements for food, s h e l t e r , mating, or ovipos i t i o n . Some insects are able to use only a l i m i t e d number of s t i m u l i , whereas others have more complex c a p a b i l i t i e s , and often u t i l i z e combi-nations of s t i m u l i to i n i t i a t e or reinforce behaviour patterns. Some s t i m u l i , however, are used almost u n i v e r s a l l y by i n s e c t s , as evidenced by the widespread occurrence of the sp e c i a l structures required to receive these s t i m u l i . Chemical s t i m u l i (be they v o l a t i l e or not) belong to this group, and one can find evidence of s p e c i a l i z e d receptors for chemical recognition on any insect studied. The antennae are the most obvious l o c a t i o n f o r chemical receptors, and they bear numerous small structures e s p e c i a l l y adapted f o r r e c e i v i n g chemical information. These s e n s i l l a are not l i m i t e d to antennae but may also be found on legs, the i n t e r n a l and external mouthparts, and the ov i p o s i t o r . The wide d i s t r i b u t i o n and d i v e r s i t y of these s e n s i l l a in other animals besides i n s e c t s have led to speculation that chemical recognition was the e a r l i e s t sensory system evolved (Zacharuk 1980). Even though insects have such a va r i e t y of receptors for d i s t i n -guishing chemical s t i m u l i , Dethier (1971) pointed out that there i s s t i l l a paucity of such receptors, considering the r i c h v a r i e t y of chemical s t i m u l i a v a i l a b l e . Insects, l i k e other animals, have evolved chemical sensing devices f o r detecting s t i m u l i that have adaptive value, - 3 -so t h a t most of t h e i r r e c e p t o r s are s e n s i t i v e to a " s p e c i f i c " s e l e c t i o n of the s t i m u l i a v a i l a b l e . Food a q u i s i t i o n r e q u i r e s f i r s t , searching, then r e c o g n i t i o n , and f i n a l l y feeding, i n response to a sequence of appropriate perceived s t i m u l i . O v i p o s i t i o n also r e q u i r e s perception of s e q u e n t i a l s t i m u l i . Egg-laying may be coupled w i t h food, so that the stimulus t o feed i s a l s o the stimulus to o v i p o s i t . Of course, i n such i n s e c t s , adult and l a r v a l food s u p p l i e s must be the same. Having a v a r i e t y of responses t o one set of s t i m u l i i s economical both i n terms of energy u t i l i z a t i o n and sensory d i v e r s i t y . Other i n s e c t s may use d i f f e r e n t sets of s t i m u l i f o r each a c t i v i t y , and have equally diverse behavioural responses. Mating i s also commonly l i n k e d w i t h food, so that feeding s t i m u l i b r i n g the sexes together. S p e c i f i c mating s t i m u l i then come i n t o play. Responses t o the presence of the opposite sex may be l i m i t e d to d e p o s i t -ing a spermatophore on the substrate near a female ( i n Collembola, S c h a l l e r 1971), or they may be more e l a b o r a t e , i n c l u d i n g r i t u a l dancing or g i f t bearing (e.g. i n the s c o r p i o n f l y B i t t a c u s a p i c a l i s , T h o r n h i l l 1976) t o enhance the female's r e c e p t i v i t y . No matter what degree of complexity may be involved i n the i n i t i a l s timulus or the r e s u l t i n g behaviour, the neural c i r c u i t r y of i n s e c t s l a c k s y n a p t i c connections between the s e n s i l l u m and the b r a i n . Whatever "turns on" a s e n s i l l u m i s transmitted d i r e c t l y to the <CNS. ' T h i s d i r e c t l i n k a g e makes i n s e c t s i d e a l s u b j e c t s f o r studying sensory responses at the organismal or the c e l l u l a r l e v e l s . - 4 -Searching f o r an o v i p o s i t i o n s i t e i n v o l v e s s e v e r a l s e n s i l l a s imul-taneously o r s e q u e n t i a l l y . The eyes r e c e i v e v i s u a l s t i m u l i , while the antennae, the t a r s i , the l a b e l l u m , and the o v i p o s i t o r r e c e i v e chemical i n f o r m a t i o n . For most i n s e c t s , chemoreception i s d i v i d e d i n t o two c l a s s e s , o l f a c t o r y and contact chemoreception, although the neural mechanism of- r e c o g n i t i o n i s thought to be the same (Zacharuk 1980). O l f a c t i o n and contact chemoreception u s u a l l y take place on d i f f e r e n t s e n s i l l a and often on d i f f e r e n t parts of the body. A major p r e r e q u i s i t e f o r i n t e r p r e t i n g i n s e c t s ' sensory responses to t h e i r world i s an understanding of the k i n d s of s e n s i l l a which are a v a i l a b l e to them, and the p o s s i b l e f u n c t i o n s these may serve. Chemore-ceptors comprise a l a r g e proportion of an i n s e c t ' s array of s e n s i l l a , d i s p l a y i n g great v a r i e t y i n form and f u n c t i o n . I n s e c t s w i t h d i f f e r e n t feeding and o v i p o s i t i o n a l requirements o f f e r a r i c h e r behavioural reper-t o i r e i n response to s p e c i f i c chemical s t i m u l i than those i n s e c t s which combine these requirements. Insects which have d i s t i n c t l y d i f f e r e n t behaviours f o r feeding and o v i p o s i t i o n provide better o p p o r t u n i t i e s f o r t e s t i n g these behaviours s e p a r a t e l y . The Syrphidae i n c l u d e e x c e l l e n t examples of t h i s l a t t e r type. The a d u l t syrphids are p r i m a r i l y p o l l e n and nectar feeders (Schneider 1969; Holloway 1976) although some may also u t i l i z e aphid honeydew. They search out flowers t o provide t h e i r own food. L a r v a l syrphids have diverse feeding p a t t e r n s , but a number are o b l i g a t e preda-t o r s of aphids. Since these predatory l a r v a e are b l i n d and move only s l o w l y , the a d u l t females must o v i p o s i t i n or very near an aphid colony to ensure s u r v i v a l o f t h e i r o f f s p r i n g . Since the females of these o b l i -- 5 -gate predators must search f o r two d i f f e r e n t food sources, flowers f o r th e i r own requirements and plants infested with aphids f o r the i r o f f s p r -ing, I chose t h i s group f o r studying the morphological differences among chemoreceptors, and the possible differences in r e s u l t i n g behaviour pat-terns . V i s u a l l y and chemically, s i t e s f o r feeding and f o r o v i p o s i t i o n may have l i t t l e in common. Flowers tend to be va r i o u s l y coloured, and many are fragrant. An aphid-infested plant, on the other hand, has a more li m i t e d range of colour ( i . e . green to yellow) but may reta i n many of the o r i g i n a l chemical c h a r a c t e r i s t i c s of the plant, as well as s i g n a l -l i n g the presence of the aphids. The purpose of t h i s thesis i s to investigate syrphid responses to these two d i f f e r e n t s i t e s by: 1. examining the u l t r a s t r u c t u r e of the chemosensilla of a highly spe-c i a l i z e d syrphid group; 2. t e s t i n g behavioural responses of i n d i v i d u a l insects to various components of each s i t e ; 3. further separating i d e n t i f i a b l e components of any a t t r a c t i v e s t i m u l i f o r additional t e s t i n g by neurophysiological techniques. - 6 -CHAPTER 1 . ULTRASTRUCTURE OF CHEMOSENSILLA ON THE SYRPHID ANTENNA AND OVIPOSITOR - 7 -Introduction Chemosensilla are primarily olfactory or gustatory, and these two categories have well defined and quite different structures. The differences are sufficiently marked to allow one to predict the function of a sensillum from i t s external and internal morphological structure. It was once thought that the olfactory and gustatory categories were mutually exclusive, but Dethier (1972) and Stadler & Hanson (1975) have shownthat this is not invariably so. In fact, the transition from exclusively gustatory to exclusively olfactory sensilla includes sen-s i l l a sensitive to both kinds of stimuli. Morphologically, these tran-sitional sensilla give no indication that they have a dual function. But even when structure may not always denote function, morphological classes are s t i l l useful for classifying the majority of sensilla and w i l l be so used here. Most olfactory receptors of insects are on the antennae, though some have been found on maxillary and labial palps [e.g. in Aedes aegypti (Kellogg 1970) and in the larval beetle, Orthosoma brunneum (White et a_l. 1974)] and even tarsi [in blackflies (Mclver et a l . 1980)]. Antennal sensilla have been investigated in many insects, including several of the Diptera: e.g. in mosquitoes (Boo 1980; Boo & Mclver 1975, 1976; Jez & Mclver 1980; Mclver 1972a, 1973, 1974, 1978; Mclver & Hudson 1972; Mclver & Siemicki 1976, 1978, 1979; Steward & Atwood 1963); blackflies (Elizarov & Chaika 1 9 7 5 ; Mercer & Mclver 1 9 7 3 ) ; ceratopogonids (Braverman & Hulley 1 9 7 9 ; Chu-Wang et a l . 1 9 7 5 ; Navai & Wirth 1 9 7 8 ) ; tabanids (Elizarov & Chaika 1 9 7 7 ) ; muse id f l i e s (Bay & Pitts 1 9 7 6 ; Lewis 1 9 7 1 ; White & Bay 1 9 8 0 ) and blowflies (Larsen & - 8 -Dethier 1963). However, they have not p r e v i o u s l y been i n v e s t i g a t e d i n the Syrphidae. The f o l l o w i n g summary gives a generalized d e s c r i p t i o n of the s t r u c t u r e o f o l f a c t o r y chemoreceptors. Those s e n s i l l a responsive to airborne chemicals t y p i c a l l y have t h i n c u t i c u l a r w a l l s ( S l i f e r 1970) w i t h many pores that allow contact between the dendrite w i t h i n and the stimulus molecules. These multiporous s e n s i l l a (MP a f t e r the t e r m i n o l -ogy used by Zacharuk 1980) come i n many shapes and s i z e s : h a i r s , domes, cones, and pegs, e i t h e r p r o j e c t i n g from the c u t i c l e or surrounded by deep or shallow c u t i c u l a r p i t s . The surfaces of these s e n s i l l a may have grooves or p i t s . P i t t e d - s u r f a c e d multiporous (MPP) s e n s i l l a are the most common. MPP s e n s i l l a are most o f t e n found i n protected l o c a t i o n s (e.g. among other h a i r s ) and i n s p e c i f i c d i s t r i b u t i o n p a t t e r n s on anten-nae, but a l s o can be found i n smaller numbers on other body p a r t s (e.g. o v i p o s i t o r , p a l p s ) . These l a t t e r r e c e p t o r s may have r e l a t i v e l y t h i c k w a l l s w i t h fewer smaller pores, o r may have r e l a t i v e l y t h i n w a l l s with l a r g e r pores and a higher pore d e n s i t y . The pore may connect with the i n t e r i o r of the s e n s i l l u m by a s t r a i g h t narrow channel or by f l a r i n g i n t o a wider channel - the "pore k e t t l e " of Ernst ( 1969 i n Bay & P i t t s 1976). D i r e c t connections w i t h the dendrites may e x i s t i n the form of 10-20 nm diameter pore tubules (Zacharuk 1980) extending through the c u t i c l e to the c e n t r a l d e n d r i t i c chamber. Within the c u t i c u l a r covering i s a d e n d r i t i c chamber extending the l e n g t h of the s e n s i l l u m . The d i s t a l p o r t i o n s of the b i p o l a r neurons extend i n t o t h i s chamber. They may extend as one strand (most common in t h i c k - w a l l e d MPP s e n s i l l a ) or r o l l , f o l d , l a m e l l a t e , or branch i n the - 9 -thin-walled MPP s e n s i l l a . In most insect MPP s e n s i l l a , the c u t i c u l a r d e n d r i t i c sheath does not reach into the porous part of the sensillum but terminates at i t s base. There need not be a d e n d r i t i c sheath in these s e n s i l l a (Zacharuk 1980). The second type of multiporous sensillum has a grooved surface (MPG). These have only been observed scattered sparsely over insect antennae. Most are small and peg-like, with l o n g i t u d i n a l grooves extending to the t i p of the sensillum. Pores extending into the peg lumen from the grooves may be st r a i g h t and narrow (giving the grooves an outwardly plain appearance) or have elaborate external o r i f a c e s that give the grooves an ornate appearance. The d e n d r i t i c sheath may l i n e the d e n d r i t i c chamber and be perforated along i t s length. There are no pore tubules in these s e n s i l l a . MPG s e n s i l l a usually have 2-5 unbranched sensory neurons (Mclver 1974; Chu-Wang et a l . 1975). In both types of MP s e n s i l l a , the d i s t a l d e n d r i t i c portion of the o neuron extends out from a c i l i a r y region and contains o n l y ' l o n g i t u d i n a l microtubules in a granular cytoplasmic ground substance. From the c i l i -ary region, with i t s t y p i c a l microtubule pattern of nine pairs of micro-tubules surrounding a center without microtubules (9X2 + 0), the dendri-t i c sheath ( i f i t i s present) arises and encloses the bundle of den-dr i t e s . The proximal d e n d r i t i c portion extends below the c i l i a r y and r o o t l e t region to the perikaryon. Within the cytoplasm of t h i s portion of the c e l l are found i n c l u s i o n s i n d i c a t i v e o f high metabolic a c t i v i t y , e.g. mitochondria, rough E.R., Golgi bodies, v e s i c l e s . The nucleus i s c h a r a c t e r i s t i c a l l y large and round with pale, f i n e l y dispersed - 10 -chromatin. The axon extends without synapse (Dethier 1971) or f u s i o n (Mclver 1978; Moeck 1968; S t e i n b r e c h t 1969) t o the c e n t r a l nervous sys-tem . There are commonly four (but the range i s 2-5) sheath c e l l s of epidermal o r i g i n that surround the neuron. Two c e l l s envelope the d i s t a l p a r t s of the neuron. The t r i c h o g e n surrounds the dendrites (and the d e n d r i t i c sheath i f i t i s present), and perhaps an inner sheath c e l l , from the perikaryon to near the base o f the c u t i c u l a r p o r t i o n . The tormogen c e l l surrounds t h i s c e l l to the base of the e x t e r n a l sen-s i l l u m . Between them i s a s e n s i l l a r sinus (Zacharuk 1980) or r e c e p t o r lymph space (Mclver 1975). Both sheath c e l l s have cytoplasmic extensions i n t o the s i n u s . These may be m i c r o v i l l i i f showing signs of a c t i v e s e c r e t i o n , ( i . e . mitochondria) or l a m e l l a e . The granular f l u i d i n the s i n u s i s b e l i e v e d to be a n u t r i e n t source f o r the dendrites (Gnatzy & Weber 1978; P h i l l i p s & Vande Berg 1976) as i t bathes them to the e x t e r -nal sensory s t r u c t u r e . The tormogen c e l l i s b e l i e v e d to sequester n u t r i e n t s from the haemolymph and secrete them i n t o t h i s s i n u s . I n t e r i o r to the trichogen c e l l there may be an inner sheath c e l l . e n c l o s i n g dendrites i n a c i l i a r y s i n u s from the perikaryon to the base of the d e n d r i t i c sheath. The f l u i d i n t h i s sinus i s b e l i e v e d to be secreted by the inner sheath c e l l and provide n u t r i e n t s f o r the den-d r i t e s (Bellamy & Zacharuk 1976). The basal sheath c e l l wraps around the perikaryon and the axon, overlapping the inner sheath c e l l ( i f p r e s e n t ) . I f a f i f t h sheath c e l l i s present, i t i s wrapped around the tormogen c e l l and may even secrete i n t o the s e n s i l l a r s i n u s . - 1 1 -Gustatory chemosensilla are s i m i l a r i n t e r n a l l y to o l f a c t o r y recep-tors, but have two notable d i f f e r e n c e s . The two kinds of receptor have in common terminal or subterminal, s i n g l e or multiple openings through which chemical communication between dendrites and stimulus can occur. But i n addition gustatory s e n s i l l a often have a mechanosensitive neuron associated with them. These s e n s i l l a generally have t h i c k walls and, because of t h e i r s i n g l e opening, are c a l l e d "uniporous" (UP) s e n s i l l a . Those with a simple pore (UPP) are further separated from those with a sculptured pore (UPS). The pores may contain plugs or exudate which may represent a c l o -sure apparatus (Zacharuk 1980). Pore tubules may be present i n some UPS s e n s i l l a ( o p . c i t ) . In UPS and UPP s e n s i l l a , dendrites usually extend through the lumen of the sensillum encased i n a d e n d r i t i c sheath that i s open at the end. In some, there are two channels in the sensillum, separated either by c u t i c l e or by the d e n d r i t i c sheath. These gustatory or contact chemoreceptors are most often located on appendages used i n contact sensing: e.g. t a r s i , labellum, terminal antennal segments, maxillary and l a b i a l palps, galea and c e r c i , and o v i -p o s i t o r s . They can have the dual function, as previously noted, of responding to air-borne as well as contact chemicals. Their mechanore-ceptive neurons are ind i s t i n g u i s h a b l e from chemoreceptive ones, except f o r t h e i r d i s t a l t i p , which d i f f e r e n t i a t e s into an e l e c t r o n - dense "tubular body" of filaments and microtubules before attaching to the wall at the base of the hair (Mclver 1975). - 1 2 -Although there i s a great deal of l i t e r a t u r e on the contact chemoreceptors, s t u d i e s s p e c i f i c to the o v i p o s i t o r are few, and those concerning D i p t e r a are fewer s t i l l (Behan & Ryan 1977; Hooper et al_. 1972; R i c e 1976). The s e n s i l l a of the syrphid o v i p o s i t o r have never been s t u d i e d . This t h e s i s t r e a t s the syrphid antenna as the primary organ of o l f a c t o r y r e c e p t i o n , and the o v i p o s i t o r as an organ of gustatory recep-t i o n used e x c l u s i v e l y f o r o v i p o s i t i o n a l s t i m u l i . Their receptors are studied i n t h i s chapter by scanning and t r a n s m i s s i o n e l e c t r o n microscopy to e l u c i d a t e the s t r u c t u r e s a v a i l a b l e to c e r t a i n aphidophagous syrphids f o r r e c e i v i n g s t i m u l i . These s t r u c t u r e s can be compared w i t h s i m i l a r s t r u c t u r e s in other i n s e c t s , so that when f u n c t i o n a l information i s a v a i l a b l e , f u n c t i o n s can be suggested f o r the s e n s i l l a described. Syrphid l a r v a e are also i n v e s t i g a t e d by S.E.M. to p r e d i c t t h e i r sensory s e n s i t i v i t y to aphids, t o add to what i s already known from one l i g h t microscope study (Bhatia 1939) and v a r i o u s behavioural studies (e.g. Chandler 1969; Ruzicka 1976). - 13 -Materials and Methods Rearing Syrphids Most species tested were not amenable to laboratory rearing, but Metasyrphus venablesi (Cn.) and Eupeodes v o l u c r i s 0.S. could be reared by Frazer's method (1972) with some minor changes. In addition to the large cages with feeding platforms used by Frazer (op.cit) I also used s l i g h t l y smaller cages (70 X 60 X 50 cm) for adult f l i e s . Fresh pollen in the form of flowers was supplied when available in addition to freeze dried Corylus sp. p o l l e n . Acyrthosyphon pisum or Aphis fabae were used to induce o v i p o s i t i o n . These aphids were reared on broad bean, V i c i a  faba major cv Broad Windsor, planted i n s t e r i l e s o i l . Upon hatching, syrphid larvae were not removed i n d i v i d u a l l y from the ov i p o s i t i o n plants. Instead, a whole plant was placed i n a cage with 9 pots con-ta i n i n g 17 cm high broad beans infested with A. pisum so that the larvae could move at random from t h e i r o r i g i n a l plant. Cannibalism was never a problem unless aphids were scarce. Rearing-room temperature ranged between 22 and 25 degrees C, with a r e l a t i v e humidity of 65%. The cages were illuminated by a combination' of warm and cool fluorescent l i g h t s with a l i g h t / d a r k regime of 16/8 h. To avoid further problems with a v i r u s that appeared during the f i r s t few months of continuous rearing, a l l cages were cleaned after each rearing session with b a c t e r i c i d a l soap (Bactrex) and 70% ethanol. An independent colony of A. pisum was kept in a cooler room, 10-15 degrees C, with 16 hours l i g h t , s p e c i f i c a l l y to i n f e s t new broad beans with "clean" aphids. The virus ceased to be a problem with t h i s t r e a t -- l a -ment, so that colonies of M. venablesi were kept for 10 to 12 months at a time, and E. v o l u c r i s could be kept for 4 to 5 months. Syrphid gen-eration time under these conditions was 5 to 6 weeks. Additional species studied here were c o l l e c t e d l o c a l l y before study or reared to the adult stage from eggs. Some were supplied by Dr. J.R. Vockeroth, Biosystematics Research I n s t i t u t e , Ottawa, and these species are noted where r e s u l t s apply to them. Scanning Electron Microscopy For scanning electron microscopy (SEM) of antennae, either whole heads or patches of c u t i c l e containing antennae were excised from f r e s h l y k i l l e d syrphids and stored i n 70% ethanol (EtOH) u n t i l required. Antennae were cleaned by sonication f i r s t in a 50% ammonium hydroxide sol u t i o n f o r 50 seconds, and then f o r three consecutive 30 s e c periods in pure acetone . The specimens were a i r dried, mounted on stubs with rubber cement d i l u t e d 50/50 with chloroform, and gold coated (Eiko Engineering IB-2 Ion Coater). Other body parts were studied i n the same way. A few excised heads were fixed on stubs in 4% osmium tetroxide vapour for 90 min before SEM study, but t h i s lengthy treatment was aban-doned, because i t produced r e s u l t s very s i m i l a r to those obtained more rapi d l y by a i r drying. Larvae were preserved in 70% EtOH, dehydrated through an ethanol series to 100% amyl acetate, and c r i t i c a l point dried (Omar SPC 1500 c r i t i c a l point d r y e r ) . To obtain larvae with p a r t i a l l y extended mouth parts, feeding larvae were k i l l e d quickly i n 70% EtOH. They were then mounted on stubs with silver-conducting paint and gold coated before - 15 -study. A l l specimens were studied with a Hi t a c h i S 500 Scanning E l e c -tron Microscope. Transmission Electron Microscopy Several methods of f i x a t i o n and embedding were undertaken in an attempt to improve antennal sections. The most successful i s presented here. Specimens were dissected i n 5% gluteraldehyde in phosphate buffer (ph 7.0-7.2) on ice, then l e f t overnight i n f i x a t i v e i n p a r t i a l vacuum. Post f i x a t i o n wa s in 2% osmium tetroxide (pH 7.0) on ice for one hour. Specimens were slowly dehydrated i n ethanol i n an ice bath to 70%, then at room temperature from 80% to propylene oxide, a l l i n p a r t i a l vacuum. Tissue was i n f i l t r a t e d overnight in 50:50 propylene oxide and an embed-ding medium (Epon 812, Spurr's, Quetol, or Epon-Araldite) in p a r t i a l vacuum. The following day, specimens were embedded i n the appropriate pure resin f o r polymerization. Sections of antennae were cut with glass knives, but a few were cut with a diamond knife on a Reichert OM U-2 Ultramicrotome. Ovipositor t i s s u e was treated in the same manner, embedded i n Epon 812, and sec-tioned with a diamond k n i f e . A l l sections were stained i n uranyl acetate and lead c i t r a t e , on c o l l o d i o n , carbon-coated, 100 mesh copper gri d s . A P h i l i p s model 200 or model 300 transmission electron microscope was used to view sections. - 1 6 -Results Electron Microscopy of Antennae Syrphid antennae consist of three segments, a scape and pedicel, and a large terminal bulb. The bulb i s oval-shaped i n cross section and covered with a dense mat of non-innervated c u t i c u l a r setae. A long slender a r i s t a a r t i c u l a t e s from the la t e r o d o r s a l margin of the bulb (Plate 1 a). Antennal sensory structures (excluding Johnston's organ) are a l l located on the antennal bulb. The most numerous structures on the bulb are the non-innervated setae (Plate 1 b). They are long (mean 13.1 mu + 0.39, n=4) curved, and s p i r a l l y grooved, tapering to a f i n e point. Interspersed among these setae are four types of chemoreceptive s e n s i l l a , a l l multiporous. Because of the dense mat of setae, t o t a l numbers of these s e n s i l l a could not be accurately counted, but searches f o r each type soon showed that they varied i n density. The two most common were t y p i c a l multiporous s e n s i l l a with e x t e r i o r s perforated by numerous pores (Plate 1 c ) . These s e n s i l l a d i f f e r e d i n size and shape. The more common type had a rounded t i p , mean length of 7.78 mu + 0.31, n=2, and diameter of 1.5 mu + 0.31, n=9 (Plate 1 c, 2 a ) . The less common type had a pointed t i p , with a mean length about h a l f that of the other type (3.66 mu + 0.34, n=3) and a diameter at widest point of 1.67 mu + 0.27, n=5. At the t i p , the diam-eter measured 0.17 mu ± 0.01, n=3. Density of pores was not estimated (Plate 1 c ). - 17 -The l e s s common was a grooved peg sensillum of mean length 2.5 mu + 0.59, n = 9, and width at the widest point 0.89 mu + 0.16, n = 11. Each peg had a co l l a r e d appearance, as i t arose from a shallow p i t and each had twelve grooves in cross.section (Plate 2 b ) . There were no pores in the d i s t a l t i p of the peg (Plate 2 c ) . The t i p s i n M. venablesi and E. v o l u c r i s were blunt (Plate 2 c) but in some other species e.g. Xylota sp. (Plate 4 b) they have a more pointed extremity. These three types of s e n s i l l a were found i n both males and females of M.venablesi, E. v o l u c r i s , Syrphus torvus and Scaeva p y r a s t r i . There was no appreciable difference in size among s e n s i l l a of these species. The fourth sensillum was also multiporous but was thinner, shorter, and had fewer pores and thi c k e r walls (Plate 2 d). These sen-s i l l a were only infrequently encountered on the antennal bulb though they occurred i n higher density around the late r o d o r s a l margin, i n the v i c i n i t y of the a r i s t a . In this area, the other kinds of MPP s e n s i l l a were l e s s frequent. This fourth type of sensillum arises out of a depression in the c u t i c l e , as do the grooved pegs. Some workers have reported apparent "sensory p i t s " on the antennal bulbs of syrphids. These p i t s are much more prominent i n some species than in others. In the species examined here, " p i t s " are v i s i b l e but not prominent (Plate 1 a ) . Upon examination by SEM, these structures proved not to be " p i t s " , but areas with a thinner population of non-innervated setae, which allowed a clearer view of the underlying sensory structures. S e n s i l l a did not appear more numerous in these " p i t s " . - 18 -A specimen o f Brachyopa perplexa was obtained from Dr. J. R. Vockeroth, Biosystematics Research I n s t i t u t e , Ottawa, f o r SEM study of i t s very prominent antennal " p i t " ( P l a t e 3 a ) . Under the scanning e l e c -t r o n microscope t h i s round area could be seen to contai n a dense popula-t i o n o f round-tipped multiporous s e n s i l l a ( P l a t e 3 b). Surface c u t i c l e was not v i s i b l e , as the s e n s i l l a were so dense. These s e n s i l l a were not confined to the " p i t " , however, but were also s c a t t e r e d over the e n t i r e bulb a t a lower density ( P l a t e 3 c ) . Multiporous round-tipped s e n s i l l a and grooved pegs were also found i n the f o l l o w i n g species supplied by Dr. J. R. Vockeroth; a female Dasysyrphus amalopsis , a female P i p i z a sp., a female Xanthogramma f l a v i p e s , a male X y l o t a sp. and Brachyopa  perplexa ( P l a t e s 3 o, d, 1 a, b). The two common MPP s e n s i l l a , types I and I I , could not be d i s -t i n g u i s h e d i n T.E.M. s e c t i o n s , as c r o s s e c t i o n a l diameters were very s i m i l a r and the t i p was r a r e l y seen. They w i l l be described together here. C u t i c l e i n the e x t e r n a l s e n s i l l u m was t h i n (0.15 um ± 0.04 um, n = 1 0) w i t h numerous pores. Narrowest pore diameter was 32.8 nm + 6.8 nm, n = 1 0. Pores f l a r e d s l i g h t l y to the e x t e r i o r and opened i n t o b e l l -shaped c a v i t i e s t o the i n t e r i o r ( P l a t e 4 d). No pore tubules were pr esent. U s u a l l y t h r e e , but sometimes two d e n d r i t e s , without a d e n d r i t i c sheath, could be found i n the c u t i c l e below these s e n s i l l a ( P l a t e 5 ). As the dendrites entered the base of a s e n s i l l u m , they branched into many s m a l l d e n d r i t i c extensions ( P l a t e 4 c, P l a t e 5 ) . The p a t t e r n i n c r o s s e c t i o n was v a r i a b l e ( P l a t e 4 c, d). Often one dendrite remained unbranched or - 1 9 -else r o l l e d upon i t s e l f i n concentric c i r c l e s (Plate 4c-d). Dendritic processes were close to but did not appear to enter the f l a r e d pore bases (Plate 1 d ) . Sheath c e l l s included a trichogen c e l l , which wrapped around the d i s t a l dendrites u n t i l they began to branch (Plate 5). The tormogen c e l l with i t s cytoplasmic lamellae l i n e d the outside of the rather l i m i t e d s e n s i l l a r sinus (Plate 5). Structures below the c u t i c l e l e v e l were not e a s i l y d i s t i n g u i s hable, due to crowding together of the c e l l u l a r portions of many s e n s i l l a . Grooved MP s e n s i l l a are innervated by three unbranched dendrites (Plate 6c) in a small d e n d r i t i c chamber. In crossection, (Plate 6 c) the grooves appear as c l e f t s between rounded c u t i c u l a r f l u t e s . The c l e f t s are rounded at t h e i r base and extend the length of the groove. Pores (Plate 6 c) are s l i t - l i k e , extending from the base of the c l e f t to the d e n d r i t i c chamber without pore tubules. There i s an outer r i n g of channels with the c u t i c u l a r f l u t e s giving the s e n s i l l a a double-walled appearance (Plate 6 a, c ) . Below the c u t i c l e , dendrites are enclosed i n a t h i c k d e n d r i t i c sheath (Plate 6 a). The trichogen c e l l does not surround the sheath, except below the c u t i c l e . The tormogen c e l l with i t s lamellae l i n e s the s e n s i l l a r sinus part way through the c u t i c l e but not to the base of the sensillum (Plate 6 b). The s e n s i l l a r sinus is continuous with the den-d r i t i c chamber and the c u t i c l e - l i n e d channels. Structures below the t h i c k antennal c u t i c l e could not be delineated because i t was d i f f i c u l t to obtain adequate s e r i a l sections of antennal material. - 20 -The fourth type of MPP sensillum, the thicker-walled s e n s i l l a , were rare and consequently only infrequently sectioned. Wall thickness was 0.18 um + 0.02 um, n = 10. There were most commonly two dendrites in a sensillum crossection (Plate 7 c, d) but occasionally 5 were seen (Plate 7 e). From the similar diameters of dendrites, i t seems l i k e l y that there were two s i m i l a r s e n s i l l a , with d i f f e r e n t numbers of den-dr i t e s rather than d e n d r i t i c branching. The pores of these s e n s i l l a lacked "pore k e t t l e s " (Plate 7 a-d) and had a narrowest diameter of 222 A + 52 n=10. The pore channel f l a r e d s l i g h t l y to the e x t e r i o r and to the i n t e r i o r . Pore tubules could not be seen. In longitudinal and crossections, there was no d e n d r i t i c sheath in the large c e n t r a l chamber (Plate 7 a-d). Neither was there a d e n d r i t i c sheath surrounding d i s t a l d e n d r i t i c extensions in and below the c u t i c l e in most of these sections. There were a few s e n s i l l a in t h i s area, however, that did have a d e n d r i t i c sheath although there were always more than two dendrites inside i t . Though evidence i s incom-plete, t h i s v a r i a t i o n supports the suggestion that the 5-dendrite sen-s i l l u m was not a branched 2-dendrite sensillum but a separate type. - 2 1 -Plate j_. Syrphid Antennal Olfactory S e n s i l l a 1 A) Scanning electron micrograph of the antenna of a female E. volucr i s showing f a i n t "sensory p i t " areas (sp) and a dense cover-ing of non-innervated setae (nis) on the bulb but not on the scape (s) or pedicel (p). The ar i s t a (a) has very few setae. B) S.E.M. of a male S. opinator antennal bulb showing the grooved non-innervated setae (nis) and thin-walled perforated multiporous (mpp) olfact o r y s e n s i l l a a r i s i n g out of shallow depressions in the c u t i c l e . C) S.E.M. of a female M. venablesi antennal bulb showing two types of thin-walled perforated multiporous s e n s i l l a ; rounded (mpp-1) and pointed (mpp-2). a b P sp nis mmp mmp-1 mpp-2 Legend a r i s t a antennal bulb pedicel sensory p i t non-innervated setae multiporous sensillum multiporous sensillum type 1 multiporous sensillum type 2 - SU i * -- 22 -Plate 2. Syrphid Antennal Olfactory S e n s i l l a Types A) S.E.M. of a female S. opinator antennal bulb showing surface pores of a multiporous thin-walled sensillum type 1 (mpp-1). B) S.E.M. of a male S. p y r a s t r i antennal bulb, grooved peg sensillum (thin-walled, multiporous grooved type - mpg). C) S.E.M. of a female S. p y r a s t r i antennal bulb; grooved peg sensillum t i p showing absence of terminal pores. D) S.E.M. of a female M. venablesi antennal bulb with a grooved peg sen-si l l u m (mpg) and a thick-walled multiporous chemosensillum (mppt) both a r i s i n g from c u t i c u l a r depressions. Legend mpg mpp-1 mppt multiporous grooved peg sensillum multiporous perforated thin-walled sensillum type 1 multiporous perforated thick-walled sensillum Plate 3. Syrphid Antennal Olfactory S e n s i l l a 2 A) S.E.M. of a whole head preparation of a male B. perplexa showing prominent antennal sensory p i t s (sp) and extended mouthparts (labellum 1). B) S.E.M. of the antennal bulb of a male B. perplexa sensory p i t area showing numerous round tipped multiporous s e n s i l l a (mpp-1). Note the absence of non-innervated setae (nis) in the area of the p i t . C) S.E.M. of the antennal bulb of a male B,. perplexa some distance from the sensory p i t , showing multiporous s e n s i l l a (mpp-1), grooved peg s e n s i l l a (mpg) and several non-innervated setae ( n i s ) . D) S.E.M. of the antennal bulb of a female P i p i z a sp.; multiporous per-forated, round tipped s e n s i l l a (mpp-1). Legend sensory p i t labellum multiporous perforated thin-walled sensillum type 1 multiporous grooved peg sensillum non-innervated setae sp 1 mpp-1 mpg nis - >3A -- 24 -Pla t e 4. Syrphid Antennal O l f a c t o r y S e n s i l l a 3 A) S.E.M. of the antennal bulb o f a male X. quadrimaculata mul-t i p o r o u s grooved peg s e n s i l l u m (mpg). Note the depression i n the cu t i c l e . B) S.E.M. of the antennal b u l b o f a female Dasysyrphus amaiopsis w i t h round tipped multiporous perforated s e n s i l l u m (mpp-1) type 1. s) Transmission e l e c t r o n micrograph of cross s e c t i o n s of several t h i n -w a l l e d multiporous s e n s i l l a (mpp type 1 or 2 or both). Note the v a r i a b l e number of d e n d r i t i c branches (db) and dendrite p a t t e r n d i f f e r e n c e s . The s e n s i l l a emerging from the c u t i c l e i s a mpp and i t s d e n d r i t i c branching s t a r t s from the point of emergence from the c u t i c l e . A non-innervated seta ( n i s ) i s also present. D) T.E.M. of a cross s e c t i o n of a t h i n - w a l l e d s e n s i l l a (mpp) w i t h a large dendrite r o l l e d upon i t s e l f , and s e v e r a l other d e n d r i t i c branches (db). The pores (p) widen i n t e r n a l l y . Pore tubules are not present but there i s a matrix of elec t r o n dense m a t e r i a l i n the s e n s i l l a r lumen. Legend mpg multiporous grooved t h i n - w a l l e d s e n s i l l u m mpp-1 multiporous p e r f o r a t e d t h i n - w a l l e d s e n s i l l u m type 1 mpp multiporous perforated t h i n - w a l l e d s e n s i l l u m n i s non-innervated setae db d e n d r i t i c branches p pore - 25 -Plate 5. Longitudinal Section of a Multiporous, Thin-walled Antennal Chemosensillum T.E.M. of two MPP s e n s i l l a a r i s i n g from the c u t i c l e surface without c u t i c u l a r c o l l a r s or depressions. D i s t a l dendrites (dd) are not surrounded by a c u t i c u l a r sheath and the s e n s i l l a r sinus (ss) i s some-what li m i t e d . The p i t t e d thin-walled c u t i c l e of the external sensillum (pc) can be seen in one sensillum. Legend dd Pd to t i pc ss trichogen c e l l p i t t e d c u t i c l e s e n s i l l a r sinus d i s t a l dendrites proximal dendrites tormogen c e l l lamellae - SLSft -- 26 -Plate 6. Ultrastructure of a Multiporous Thin-walled, Grooved peg Antennal Chemosensillum A) T.E.M. of a l o n g i t u d i n a l section through a grooved peg sensillum. Dendrites (dd) extend almost to the t i p of the peg without branch-ing, and the gooves (g) extend to the t i p also. Note the cu t i c l e - l i n e d channels (cc). B) T.E.M. of a female M. venablesi antennal bulb; l o n g i t u d i n a l oblique section through the d i s t a l dendrites (dd) of a grooved peg s e n s i l -lum. There i s a dark c u t i c u l a r d e n d r i t i c sheath (cs) surrounding the dendrites, and the s e n s i l l a r sinus (ss) and lamellae of the tormogen c e l l (to) are v i s i b l e . C) T.E.M. of a cross section of a grooved peg sensillum. Note the three unbranched dendrites (dd), c u t i c l e l i n e d spaces (cc), and the pore between the the grooves (p). Legend dd d i s t a l dendrites cs c u t i c u l a r d e n d r i t i c sheath ss s e n s i l l a r sinus to tormogen c e l l mpg multiporous, thin-walled grooved sensillum g grooves cc c u t i c l e l i n e d channel p pore - 27 -Plate 7. Ultrastructure of the Thick-walled Multiporous Antennal Olfactory Sensillum A) T.E.M. of a female E. vo l u c r i s antennal bulb, l o n g i t u d i n a l section of a thick-walled multiporous sensillum. Note the thick walls (tc) with straight pores (p) without pore k e t t l e s , and dendrites (dd) in the s e n s i l l a r lumen. B) T.E.M. of a female E. v o l u c r i s thick-walled multiporous sensillum, l o n g i t u d i n a l section showing i r r e g u l a r undulations of dendrites (dd) and pores (p) only s l i g h t l y f l a r e d t o the i n t e r i o r of the sensillum. C) T.E.M. cross section of a thick-walled multiporous sensillum with two dendrites (dd) and a single long pore (p). D) T.E.M. cross section of a thick-walled multiporous sensillum with two dendrites (dd) and several pores (p). E) T.E.M. cross section of a thick-walled multiporous sensillum with 5 dendrites (dd) and several pores (p). Note the absence of a den-d r i t i c sheath. Legend t c P dd thick c u t i c u l a r walls pore d i s t a l dendrites -3-1A -- 28 -Electron Microscopy of Ovipositor S e n s i l l a The syrphid o v i p o s i t o r i s te l e s c o p i c , with two pads that normally cover the gonopore (Plate 8 a). These pads have four types of setae and hair. There are very long, pointed s t r a i g h t h a i r s around the i n t e r i o r margins of the pads (mean length 111.2 mu ± 12.8 mu, n = 1 1 ; Plate 8 b). Numerous medium-length, slender, curved, contact chemosensilla with s l i g h t l y spatulate t i p s (Plate 8 b, 9 a) are dispersed over most of the surface of the pads. A t h i r d type, and l e a s t frequent of the four kinds of setae found on the pads, i s a short, finely-pointed h a i r or peg, i n a p i t (Plates 8 c, d). The fourth type forms a dense mat of pointed, curved, non-innervated m i c r o t r i c h i a - l i k e hairs (Plates 8 b, c ) . The medium-length curved hair has a pore or pores at i t s d i s t a l extremity (Plate 9 b, c) and has a mean length of 58.3 mu + 8.9 mu, (n=15). These medium-length hairs responded as contact chemoreceptors i n electrophy-s i o l o g i c a l tests (Chapter 3) and thus were the s e n s i l l a selected f o r sectioning and further study here. These s e n s i l l a have a t y p i c a l thick-walled uniporous structure (the " t h i c k walled h a i r s " of S l i f e r 1970). The mean pore size i s 0.55 mu ± 0.12 mu, n=6. E x t e r i o r hair walls are smooth at the base, becoming narrower and shallowly grooved as they taper to a spatulated end. There are 12 grooves. One mechanosensitive and four chemosensitive dendrites innervate the sensillum. The mechanosensitive dendrite d i f f e r e n t i a t e s into a tubular body (Plate 10) and, with i t s d e n d r i t i c sheath, separates from the others, attaching to the c u t i c l e at the l e v e l of the d i s t a l edge o f the inner c o l l a r (Plate 10). The remaining f i v e dendrites do not branch, but pass to the t i p of the hair in the small d e n d r i t i c cav-- 29 -i t y of the sensillum (Plate 11). The larger cavity i s empty, save f o r f l u i d continuous with that in the s e n s i l l a r sinus. D i s t a l dendrites contain tubules of 31 nm + 4.9 nm, n=21, diame-t e r . There are c e l l u l a r components in the proximal dendrite segments; microtubule fragments, ribosomes and mitochondria. The c i l i a r y body has a t y p i c a l 9X2 + 0 configuration of microtubules (Plate 12 b). Branches of the d e n d r i t i c sheath arise in the c i l i a r y region and fuse to form a continuous dense sheath of 0.1 mu + 0.04 mu, n = 1 0, thickness. The sheath has invaginations that sometimes separate dendrites in the c l u s -t e r (Plate 13 a) but these invaginations do not continue up into the ha i r . The tubular body, however, i s separated i n i t s own sheath soon after d i f f e r e n t i a t i o n (Plates 10, 13 b). The sheath fuses with the c u t i c l e at the base of the hair - i t s electron-dense dark "colour" separating i t as though into branches again- and eventually i t fuses with the s e n s i l l a r c u t i c l e . Examination of the inner wall of the den-d r i t e chamber shows that the d e n d r i t i c sheath l i n e s the lumen to the t i p (Plate 11). An inner sheath c e l l surrounds the dendrites up to the d e n d r i t i c sheath l e v e l , supplying a thin granular matrix medium e a s i l y v i s i b l e when the d e n d r i t i c sheath becomes continuous (Plate 10). Below the c i l i a r y region of the dendrites, t h i s c e l l becomes invaginated and displays m i c r o v i l l i - l i k e protrusions into the f l u i d - f i l l e d cavity (Plate 12 a). - 30 -The intermediate sheath c e l l (or trichogen) l i n e s the i n t e r n a l cavity of the s e n s i l l a r sinus, wrapping i t s e l f around the d e n d r i t i c sheath. Numerous long, slender lamellae characterize the walls of t h i s c e l l (Plate 10). The outer sheath, or tormogen c e l l , makes up the outer l i n i n g of the s e n s i l l a r sinus. Differences between the l i q u i d i n t h i s sinus and i n that surrounding the dendrites are s l i g h t , i f present at a l l . The s e n s i l l a r sinus l i q u i d appears to have coarser granules. Sep-tate desmosomes also occur in the proximal dendrite walls (Plate 1 2 a ) . Neurons have large, round nuclei and can be distinguished from sheath c e l l s by t h e i r denser cytoplasm, which contains more abundant rough endoplasmic reticulum (Plate 10). The large neuronal n u c l e i were situated approximately 15. 1 mu + 2.2 rmj (n=3) below the base of the c o l -l a r . - 31 -P l a t e £5. Syrphid O v i p o s i t o r S e n s i l l a A) S.E.M. of a whole o v i p o s i t o r of M. ve n a b l e s i . The gonopore (gp) i s covered d o r s a l l y by two o v i p o s i t o r pads (op). B) S.E.M. of o v i p o s i t o r pads of M. venablesi w i t h four types of h a i r ; long setae ( I s ) , medium l e n g t h s l i g h t l y recurved uniporous chemoreceptors (up), pointed h a i r s i n p i t s (hp), and non-innervated short setae. C) S.E.M. of the o v i p o s i t o r pad of M. ve n a b l e s i . Note c u t i c u l a r c o l l a r (cc) around UP s e n s i l l a , the mat of short non-innervated setae (ss) and the prominent depressions surrounding the pointed h a i r s i n p i t s (hp). D) S.E.M. of the o v i p o s i t o r pad of M. venablesi showing a pointed h a i r i n i t s p i t (hp). Note i t s wide base i n contrast t o the narrow pointed t i p . Legend go op Is gonopore o v i p o s i t o r pad long setae short setae h a i r s in p i t s uniporous chemoreceptors c u t i c u l a r c o l l a r ss hp up cc -2>l ft -- 32 -Plate 9_. Uniporous Ovipositor Contact Chemosensillum A) S.E.M. of M. venablesi^ ovipositor contact chemoreceptor. Note the curved t i p and shallow grooves. B) S.E.M. of M. venablesi ovipositor uniporous contact chemoreceptor. The pore (p) on the terminal t i p i s apparent. C) S.E.M. of M. venablesi ovipositor UP contact chemoreceptor with three pores (p) on i t s d i s t a l t i p . Legend p pores - 33 -P l a t e 1 0 . U l t r a s t r u c t u r e of the Uniporous O v i p o s i t o r Chemosensillum 1 L o n g i t u d i n a l T.E.M. through a contact chemosensillum on the ovipo-s i t o r pad of M. v e n a b l e s i . D i s t a l dendrites (dd) are enclosed by a c u t i c u l a r d e n d r i t i c sheath (cs) and the mechanoreceptor t u b u l a r body (tb) i s enclosed i n i t s own sheath. There i s a c u t i c u l a r c o l l a r (cc) surrounding the s e n s i l l u m . Note the v a r i a t i o n i n c u t i c u l a r s t r u c t u r e allowing f o r movement of the h a i r . The s e n s i l l a r sinus (ss) i s l a r g e and l i n e d on the i n s i d e by the t r i c h o g e n c e l l w i t h i t s l a m e l l a e ( t r ) and on the outside by the tormogen c e l l and i t s l a m e l l a e ( t o ) . Tormogen (ton) and trichogen ( t r n ) n u c l e i are d i s t i n g u i s h a b l e from the neuron n u c l e i (nn) i n having l i g h t e r coloured contents. Likewise, t h e i r cytoplasm i s l e s s dense than that of the neurons. Legend nn cs t r t r n ss to ton tb dd cc c u t i c u l a r c o l l a r t u b u l a r body d i s t a l d e n d r i t e s c u t i c u l a r d e n d r i t i c sheath tri c h o g e n c e l l t r i chogen c e l l nucleus s e n s i l l a r sinus tormogen c e l l tormogen c e l l nucleus neuron nucleus - 3.3A -- 34 -Plate 11. Ultrastructure of the Uniporous Ovipositor Chemosensillum 2 T.E.M. cross section of a uniporous contact chemosensillum on the ovi p o s i t o r pad of M. venablesi. Five unbranched dendrites (dd) f i l l the d e n d r i t i c chamber (dc). The large cresent-shaped chamber ( l c ) i s con-tinuous with the s e n s i l l a r sinus below the c u t i c l e . A fin e d e n d r i t i c sheath layer (cs) l i n e s the d e n d r i t i c chamber. Legend dc dd l c cs d e n d r i t i c chamber dendrites large chamber of UP h a i r c u t i c u l a r d e n d r i t i c sheath 0.37 um - 35 -P l a t e }2. U l t r a s t r u c t u r e o f t h e Un i p o r o u s O v i p o s i t o r C h e m o s e n s i l l u m 3 A) T.E.M. c r o s s s e c t i o n t h r o u g h the p r o x i m a l d e n d r i t e s (pd) i n a UP c o n -t a c t c h e m o s e n s i l l u m on t h e o v i p o s i t o r pad o f M. v e n a b l e s i . The f i v e d e n d r i t e s are sur r o u n d e d by a h i g h l y i n v a g i n a t e d i n n e r s h e a t h c e l l ( i s ) . Desmosomes (de) a r e numerous between d e n d r i t e s and t h e i n n e r s h e a t h c e l l . B) T.E.M. c r o s s s e c t i o n t h r o u g h the c i l i a r y r e g i o n o f t h e d e n d r i t e s of the UP c o n t a c t c h e m o s e n s i l l u m o n t h e o v i p o s i t o r pad o f M. v e n a -b l e s i . M i c r o t u b u l e s ( t ) a r e o r g a n i z e d i n t o t h e t y p i c a l 9X2+0 arrangement. Legend t Pd neur o t u b u l e s p r o x i m a l d e n d r i t e s i n n e r s h e a t h c e l l desmosome c o n n e c t i o n s i s de - 35ft -- 36 -Plate 13• Ultrastructure of the Uniporous Ovipositor Chemosensillum 4 A) T.E.M. cross section through u n d i f f e r e n t i a t e d d i s t a l dendrites (dd) of a UP contact chemosensillum on the ovipositor of M. venablesi. The auticular d e n d r i t i c sheath (cs) i s invaginated around i n d i v i -dual dendrites. The trichogen c e l l ( t r ) surrounds the c u t i c u l a r d e n d r i t i c sheath and forms the inner l i n i n g of the s e n s i l l a r sinus (ss). The cuter l i n i n g i s formed by the tormogen c e l l (to). Exte-r i o r to the tormogen c e l l i s a th i c k layer of material containing microtubules and desmosomes. The section i s s l i g h t l y oblique, cutting through c u t i c l e (c) on only one side. B) T.E.M. cross section through d i f f e r e n t i a t e d d i s t a l dendrites (dd) j u s t below the base of the UP contact chemosensillum on the ovipo-s i t o r pads of M. venablesi. The tubular body (tb) has d i f f e r e n -t i a t e d completely and i s separated into i t s own c u t i c u l a r sheath. I t i s fused to the c u t i c l e cn one side of the c u t i c u l a r wall (c) d i s t a l to this section but i t has almost made contact. Both t r i -chogen (tr) and tormogen (to) c e l l s are nearing the d i s t a l l i m i t of their extension. Legend dd d i s t a l dendrites cs c u t i c u l a r d e n d r i t i c sheath t r trichogen c e l l to tormogen c e l l ss s e n s i l l a r sinus c c u t i c l e tb tubular body - 3feA -- 37 -Scanning E l e c t r o n Microscopy of Larvae Second- and t h i r d - i n s t a r l a r v a e of M. venablesi were used t o study body sensory s t r u c t u r e s . M. venablesi and S. opinator l a r v a e were used to study s e n s i l l a associated w i t h the mouthparts. Larvae are covered w i t h a very t h i n wrinkled c u t i c l e which has many smooth, roundish, t u b e r c u l a r warts ( P l a t e 14 c) and rows of non-innervated c u t i c u l a r setae arranged i n rows ( P l a t e 1 4 b ) . The c u t i c l e , when stretched smooth by l a r v a l movements, shows only the l a r g e s t , most prominent h a i r s ( P l a t e 1 4 a ) . These h a i r s have a cup-shaped depression or p i t at t h e i r base ( P l a t e 14 d ) , though no e x t e r n a l sensory s t r u c t u r e was associated w i t h the p i t . These large h a i r s were found i n a crown at r e g u l a r i n t e r v a l s surrounding the pseudocephalon and t h o r a c i c r e g i o n s . The s h o r t e r setae, but not the cupped h a i r s , were found on the v e n t r a l s u r f a c e , from the f i r s t p a i r of pseudopods t o the p o s t e r i o r t i p of the l a r v a . There are what appear to be mucous gland openings on the v e n t r a l surface j u s t l a t e r a l to each of the s i x p a i r s of pseudopods on abdominal segments one through s i x ( P l a t e s 1 5 a , b). Mouthparts are normally r e t r a c t e d into the buccal c a v i t y . The pseudocephalon has i t s a n t e r i o r p o r t i o n d i v i d e d i n t o two l a t e r a l l o b e s , one on each s i d e o f a median groove ( P l a t e s 15 c ) . The two smaller lobes at the apex of each c e p h a l i c lobe are the l a r v a l antennal lobe ( l y i n g next to the groove) and the more p e r i p h e r a l m a x i l l a r y palp lobe ( P l a t e 15 d). The antennal primordium possesses two s t r u c t u r e s ; a s l e n d e r , h a i r - l i k e protruberance and a p e g - l i k e knob, both a r i s i n g from p i t s ( P l a t e 1 6 a ) . The m a x i l l a r y palp i s also rounded, but covered with numerous p a p i l l a e and some peg- l i k e s t r u c t u r e s , again i n p i t s ( P l a t e 16 - 38 -b). Each p a p i l l a has i t s own t o r o i d or "doughnut-shaped" c o l l a r ( P l a t e 16 c ) . One other s t r u c t u r e of probable sensory f u n c t i o n was l o c a t e d near the mouth, at the base of the pseudocephalic p r o t r u s i o n ( P l a t e 16 d ) . Th i s i s probably the sensory p a p i l l a described by Bhatia (1939). This p a p i l l a was u n l i k e the m a x i l l a r y p a p i l l a e , being l a r g e r (approximately 10 mu i n diameter) w i t h a conspicuous pore at i t s apex. Because i t s v i s i b i l i t y depended on good extension of the t h o r a c i c segments, t h i s p a p i l l a was seen only once. - 39 -P l a t e V\_. Syrphid Larva J A) S.E.M. of a second i n s t a r M. venablesi l a r v a , a n t e r i o r dorsal sur-face. Prominent cupped h a i r s (ch) are arranged i n rows circum-f e r e n t i a l l y around the l a r v a . C u t i c u l a r spines (cs) are numerous between rows of cupped h a i r s . B) S.E.M. of a second i n s t a r £5. opinator l a r v a w i t h a n t e r i o r extended. A n t e r i o r c u t i c l e i s stretched smooth showing prominent long cupped h a i r s (ch). Pseudocephalic lobes ( p l ) are seen extended from the buccal c a v i t y . C) S.E.M. of a second i n s t a r M. venablesi l a r v a , d o r s a l c u t i c l e . Note the t u b e r c u l a r warts (tw) on the c u t i c l e . D) S.E.M. of a t h i r d i n s t a r M. venablesi l a r v a w i t h long cupped h a i r s (ch) standing amid numerous sm a l l e r c u t i c u l a r spines ( c s ) . Legend ch P l cupped h a i r s pseudocephalic lobes c u t i c u l a r spines t u b e r c u l a r warts cs tw - 3«A -- 40 -Plate 15. Syrphid Larva 2 A) S.E.M. of the ventral surface of a second instar M. venablesi la r v a with pairs of pseudopods (pp) and probable l a t e r a l mucous glands (mg) . B) S.E.M. of a probable ventral mucous gland (mg) opening on the ventral surface of a second instar M. venablesi l a r v a . C) Anterior S.E.M. view of S. opinator larva head showing oral opening (o) and two pseudocephalic lobes ( p l ) . D) S.E.M. of pseudocephalic lobe of S. opinator larva showing d e t a i l of the maxillary lobe (ml) and the antennal lobe ( a l ) . Legend pp pseudopods mg mucous glands o o r a l opening pl pseudocephalic lobe ml maxillary lobe a l antennal lobe - 41 -Plate 16. Syrphid Larva 3 A) S.E.M. of the antennal lobe on the pseudocephalic lobe of a larv a of S. opinator. Note two sensory structures; the peg (pg) and hair (h) both a r i s i n g from p i t s . B) S.E.M. of the maxillary lobe on the pseudocephalic protrusion of the larva of S. opinator. Note two types of sensory structure; a peg in a p i t (pg), and p a p i l l a e (p). C) S.E.M. of the maxillary lobe p a p i l l a e on the pseudocephalic lobe of a larva of S. opinator. Note the "doughnut " c o l l a r structure around each p a p i l l a (p). D) S.E.M. of a p a p i l l a of probable sensory function (sp) near the base of the pseudocephalic protrusions of the larv a of S. opinator. Note the prominent c e n t r a l pore. Legend al antennal lobe peg i n a p i t hair in a p i t maxillary lobe p a p i l l a e Pg h ml P sp sensory p a p i l l a -41 tr - 42 -Discuss i o n Antennal S e n s i l l a Antennal s e n s i l l a o f the syrphids s t u d i e d here contained no apparent mechanoreceptors or contact chemoreceptors. A l l receptors s t u -died appeared t o be o l f a c t o r y i n nature. T h i s s p e c i a l i z a t i o n has been reported f o r other higher Di p t e r a ( S l i f e r & Sekhon 1964; White & Bay 1980). The antennae of the Brachyceran Di p t e r a d i f f e r s t r i k i n g l y from those of Nematoceran forms (e.g. mosquitoes). Species of higher D i p t e r a studied f o r antennal o l f a c t o r y s t r u c t u r e s i n c l u d e the c a l l i p h o r i d s Phor- mia regina (Dethier et a l . 1963) and C a l l i p h o r a v i c i n a (Kaib 1974), and the sarcophagid, Sarcophaga argyrostoma ( S l i f e r & Sekhon 1964). The absence of chemosensory s e n s i l l a on the dipteran a r i s t a has been noted by others (Bay & P i t t s 1976; Dethier et a l . 1963; Lewis 1971; White & Bay 1980). No s t r u c t u r e of a chemosensitive nature has been reported on the scape or p e d i c e l . S e t i f e r o u s plaques present on at l e a s t 29 species of muscoid Di p t e r a have been e l i m i n a t e d as o l f a c t o r y r e ceptors (Greenberg & Ash 1972). Other studies have assumed chemorecep-t o r s are l a c k i n g from these two basal subsegments (Bay & P i t t s 1976; White & Bay 1980). In the present S.E.M. study of syrphids, no s u p e r f i -c i a l s t r u c t u r e w i t h apparent chemosensory c a p a b i l i t i e s was found on the a r i s t a , p e d i c e l or scape. J_. P i t t e d - S u r f a c e d , Multiporous S e n s i l l a - 43 -Thin-walled, p i t t e d multiporous s e n s i l l a of types I and II (MPP or s e n s i l l a basiconica) found i n t h i s study, bear a resemblance to those commonly found i n other Diptera (Bay & P i t t s 1 9 7 6 ; Dethier et al. 1 9 6 3 ; Lewis 1 9 7 1 ; S l i f e r & Sekhon 1 9 6 4 ; White & Bay 1 9 8 0 ) in s i z e , wall t h i c k -ness, number of neurons and d e n d r i t i c branching. Their pore structure, however, i s unusual. Whereas other f l i e s studied a l l had pore k e t t l e s and pore tubules associated with each c u t i c u l a r perforation, pores of the present s e n s i l l a widen into a bell-shaped c a v i t y continuous with the lumen of the sensillum. Consequently there i s no pore tubule to com-plete the connection through c u t i c l e . This arrangement of pores how-ever, was reported f o r s e n s i l l a trichodea of the sand f l y , Culicoides  furans (Chu-Wang et al_. 1 9 7 5 ) . The absence of a d e n d r i t i c sheath has been observed in the previously mentioned s e n s i l l a of C. furans (op.cit) and also in the face f l y , Musca autumnalis (Bay & P i t t s 1 9 7 6 ) . The v a r i a t i o n in pattern of d e n d r i t i c branching within the s e n s i l -lum was also observed by Lewis ( 1 9 7 1 ) in Stomoxys c a l c i t r a n s . Accord-ing to Lewis, a f i n e l y branching dendrite in an o l f a c t o r y sensillum i s l i k e l y to be more s e n s i t i v e than a l a r g e r unbranched one. This d i f f e r -ence therefore may indicate v a r i a b l e s e n s i t i v i t y i n the receptor c e l l s . The functional s i g n i f i c a n c e of c o n c e n t r i c a l l y layered dendrites has not been elucidated, but i t has been suggested that the increased surface area s i m i l a r l y increases the receptive surface (Mclver 1 9 7 2 ) . Various lamellated and " r o l l e d " arrangements of dendrites have been reported in thin-walled pegs ( s e n s i l l a basiconica) on the palps of female c u l i c i n e mosquitoes (Mclver 1 9 7 2 b ) , i n pegs in p i t s ( s e n s i l l a coeloconica) on the antennae of c u l i c i n e mosquitoes (Mclver 1 9 7 3 ) , i n the bulb-shaped sen-s i l l a of the palps of C_. furans (Chu-Wang et al. 1 9 7 5 ) as well as in the - 44 -apparently o l f a c t o r y c l a v a t e s e n s i l l a ( w i t h p i t t e d surface) on the antenna of S. c a l c i t r a n s (Lewis 1971). Only i n the sandfly and c u l i c i n e mosquitoes are s e n s i l l a with l a m e l l a t e dendrites known to f u n c t i o n as carbon d i o x i d e r e c e p t o r s (Chu-Wang et a l . 1975; Mclver 1973). 2. Grooved-Surface, Multiporous S e n s i l l u m The grooved pegs i n t h i s study were surface s t r u c t u r e s , never found i n p i t s , as they have been f o r C. furans (Chu-Wang et a l . 1975) and Anopheles stephensi (Boo & Mclver 1976). T h i s surface arrangement i s a l s o common among other f l i e s studied (Bay & P i t t s 1976; Dethier ejt a l . 1963; Lewis 1971; S l i f e r & Sekhon 1964; White & Bay 1980). The number of grooves found here, 10-12, i s small compared w i t h the ranges reported f o r other D i p t e r a , 10-16. Pores are a common f e a t u r e i n these s e n s i l l a , and only i n one s p e c i e s , Aedes aeqypti (Mclver 1974) do te r m i -n a l pores replace pores along the grooves. Groove-pores have no pore tubules (Zacharuk 1980) but are considered o l f a c t o r y receptors because of n e u r o p h y s i o l o g i c a l evidence ( K e l l o g 1970 f o r those w i t h t e r m i n a l pores, and A l t n e r et a l . 1973, i n Zacharuk 1980, f o r multiporous t y p e s ) . The number of unbranched d e n d r i t e s , 3, found i n the present study i s wit h i n the range o f 1 t o 5 found i n other dipterans s t u d i e d . In only one dipteran have dendrites been found t o branch i n these s e n s i l l a ( i n A. stephensi , Boo & Mclver 1976). MPG s e n s i l l a found here were also shorter than most by about h a l f , g i v i n g them a short stout appearance. As i n comparable s e n s i l l a p r e v i o u s l y s t u d i e d , the d e n d r i t i c sheath i n these MPG s e n s i l l u m extended a l l or part of the way up i n t o the peg 1ume n. - 45 -Grooved-surfaced MP s e n s i l l a are a common fe a t u r e on dipteran antennae, whether these are bulbous, as i n the syrphid antenna, or f l a g e l l a r , as in antennae o f mosquitoes and b l a c k f l i e s (Boo & Mclver 1976; Mclver 1974; Mercer & Mclver 1973). The funct i o n s of grooved pegs have been studied n e u r o p h y s i o l o g i c a l l y i n mosquitoes. K e l l o g g (1970) found grooved pegs of A. aegypti responded t o anmonia, a n i s o l e , to water vapour. T h i s l a t t e r o b s e r v a t i o n , however, could not be confirmed f o r e i t h e r males o r females by Davis (1977) or Davis & Sokolove (1976). In females, the MPG s e n s i l l u m has responded to l a c t i c acid (Davis & Soko-love 1976), f a t t y acids and e s s e n t i a l o i l s (Lacher 1967) and commercial r e p e l l e n t s (Davis & Rebert 1972). In males, t h i s same s e n s i l l u m did not respond to the r e p e l l e n t s (Davis 1977). Mophologically s i m i l a r types of s e n s i l l a , t h e r e f o r e , do not n e c e s s a r i l y f u n c t i o n s i m i l a r l y , a phenomenon that has a l s o been noted i n the tobacco horn worm, Manduca sexta ( S t a d l e r & Hanson 1975). In t h i s c a t e r p i l l a r , a group of apparent con-t a c t chemoreceptors on the m a x i l l a e responded to o l f a c t o r y odours of t h e i r food p l a n t , whereas a morp h o l o g i c a l l y i d e n t i c a l second group gave no response u n t i l they came i n t o contact w i t h the food source. S t a d l e r & Hanson (1975) concluded that receptor s e n s i t i v i t y v a r i e d among sen-s i l l a , which may e x p l a i n the sex d i f f e r e n c e s in responses t o r e p e l l e n t s by male and female A. aegypti . 3_. Thick-Walled, Multiporous S e n s i l l a The thickness of the c u t i c u l a r w a l l s of these s e n s i l l a i s a c t u a l l y not much greater than those of the other multiporous s e n s i l l a on the antennae (0.18 um as opposed to 0.15 um). The d i f f e r e n c e a r i s e s from the form of the pores. Because o f the area taken up by the bell-shaped - 46 -inner pore (see P l a t e 4 d ) i n the MPP types I and I I s e n s i l l a , many p a r t s of the c u t i c l e are much thinner than the reported mean t h i c k n e s s . In c o n t r a s t , the pores were fewer and much narrower throughout t h e i r l e n g t h i n the " t h i c k - w a l l e d " s e n s i l l a , so that the th i c k n e s s of most parts of the i n t e r v e n i n g c u t i c l e i s c l o s e to the mean value (see P l a t e 7 d, e ) . In f a c t , our " t h i c k - w a l l e d " MPP s e n s i l l a f a l l j u s t short of the range given by Zacharuk ( 1980) f o r w a l l t h i c k n e s s of t h i s type o f s e n s i l l u m (0.2-1.0 um). Although t h i s MPP s e n s i l l u m was not tapered t o a p o i n t , i t appears from the l i t e r a t u r e t h a t , i n other ways, i t i s most s i m i l a r to the sen-s i l l a t r i c h o d e a o f other authors. These s e n s i l l a t r i c h o d e a tend to be " h a i r s " , sometimes pointed , sometimes b l u n t , and often curved. U n i -porous s e n s i l l a are sometimes included i n t h i s category but only those MPP s e n s i l l a reported f o r dipterans are considered i n t h i s d i s c u s s i o n . Lengths of these s e n s i l l a vary, from 11 um (A. ae g y p t i , Mclver 1978) to 83 um (Phormia r e g i n a , Dethier et al_. 1963) and often one species has two or more len g t h s (Lewis 1971; Mclver 1978; Mclver & S i e m i c k i 1979). Dendrites in t h i s s e n s i l l u m were unbranched. They have been reported branched (Bay & P i t t s 1976; Mclver 1978; Mclver & S i e m i c k i 1979) and unbranched (Dethier et a l . 1963; Lewis 1971; White & Bay 1980) i n comparable s e n s i l l a . In the present study the pore channel was the same diameter throughout (approx. 222 A) but t h i s i s u s u a l l y not the case i n other t h i c k - w a l l e d MPP s e n s i l l a . White & Bay (1980) reported a "V" shaped channel widening t o the i n t e r i o r i n Haematobium i r r i t a n s  i r r i t a n s , as d i d Bay & P i t t s (1976) f o r M. autumnalis. The absence of pore tubules noted i n syrphids was also recorded i n A. aegypti (Mclver - 47 -1978; Mclver & S i e m i c k i 1979) but pores are present i n H. i. i r r i t a n s (White & Bay 1980) and M. autumnalis (Bay & P i t t s 1976). Even density of pores v a r i e s considerably i n the species i n which i t has been reported (2-20 per square micron, Zacharuk 1980). C l e a r l y , t h i s type o f s e n s i l l u m , f o r which there i s l e a s t i n f o r m a t i o n , i s also the most v a r i -a ble, according t o the l i t e r a t u r e . Because of the porous nature of t h e i r c u t i c l e , these s e n s i l l a are classed as o l f a c t o r y (Zacharuk 1980) but I cannot speculate f u r t h e r on f u n c t i o n , because o f the v a r i a b i l i t y recorded i n the l i t e r a t u r e . Uniporous O v i p o s i t o r S e n s i l l u m The uniporous s e n s i l l u m described from the syrphid o v i p o s i t o r i s a t y p i c a l l y gustatory o r contact chemosensillum of the t h i c k - w a l l e d type of S l i f e r (1970) and the UPP (uniporous w i t h simple p i t pore) chemosen-s i l l a type o f Zacharuk (1980). In some UPP s e n s i l l a , the d e n d r i t i c sheath encloses the dendrites in the d e n d r i t i c channel (e.g. F e l t & Vande Berg 1976). In others (e.g. Cook 1972) as w e l l as these syrphids, i t i s fused w i t h the inner surface o f the d e n d r i t i c c a n a l (see P l a t e 11) and there i s no separate s e n s i l l a r channel. The o v i p o s i t o r of the f a c e f l y , M. autumnalis , has been studied f o r contact chemo receptors (Hooper et al_. 1972). One receptor very s i m i l a r to the one described herein was present. I t was s h o r t e r , 35 um, than that on M. v e n a b l e s i , 58.3 pm, but contained 5 neurons, one being mechanoreceptive. The other four continued unbranched to the t e r m i n a l pore. There was a second s e n s i l l u m w i t h 3 d e n d r i t e s , a l l enclosed i n t h e i r own d e n d r i t i c sheaths t o the terminal pore. Other Dip t e r a that - 48 -have shown evidence of contact chemoreceptors on t h e i r o v i p o s i t o r s are: the blow f l y , L u c i l i a cuprina (Rice 1976); the c a r r o t f l y , P s i l a rosae; and the cabbage r o o t f l y , D e l i a b r a s s i c a e (Behan & Ryan 1977). L a r v a l S e n s i l l a Syrphid l a r v a e appear to have gustatory receptors on t h e i r m a x i l -l a r y lobe as w e l l as in the buccal c a v i t y when viewed w i t h the scanning e l e c t r o n microscope. This arrangement confirms Bhatia's ( 1939) report based on l i g h t microscopy. I t also supports the behavioural observation that S. c o r o l l a e l a r v a e have d e f i n i t e feeding preferences (Ruzicka 1976). When given a choice of d i f f e r e n t aphids, these l a r v a e con-s i s t e n t l y chose the same prey. They avoided C a v a r i e l l a t h e o b o l d i , which i s t o x i c to them ( o p . c i t ) . Ruzicka (1976) concluded that l a r v a e chose t h e i r food on the basis of aphid "morphological p r o t e c t i o n " and " n u t r i -t i o n a l s u i t a b i l i t y " , or t a s t e . The l a r v a l antennal lobes seen i n the present study a l s o possessed s t r u c t u r e s of p o s s i b l e sensory c a p a c i t y , although behavioural evidence of o l f a c t o r y a b i l i t i e s in l a r v a e i s l a c k -i n g . I t i s p o s s i b l e that these antennal s e n s i l l a also are used i n assessing the s u i t a b i l i t y o f prey at short ranges (e.g. a few mm). - 49 -CHAPTER 2. OVIPOSITIONAL BEHAVIOUR OF A PH IDOPHAGOUS SYRPHIDS - 50 -In t r o d u c t i o n Much study has been devoted t o the o v i p o s i t i o n behaviour o f a d u l t syrphids because o f t h e i r p o t e n t i a l f o r he l p i n g t o c o n t r o l aphid popula-t i o n s (Schneider 1969). F i e l d s tudies have shown that o v i p o s i t i n g females have height preferences (Chandler 1968d), and th a t there are p r e d i c t a b l e numerical r e l a t i o n s h i p s between the s i z e o f an aphid colony and the number of eggs l a i d i n or near i t (Chandler 1968b; Dixon 1959; Yakhontov 1966). Syrphid females are not deterred from o v i p o s i t i n g i n a colony by the presence of other eggs or l a r v a l d e f e c a t i o n (Chandler 1968c). Although the presence of flowers i n an area encourages females t o search there f o r o v i p o s i t i o n s i t e s (Schneider 1969), Chandler (1968c) found that flowers i n the immediate v i c i n i t y o f a plant did not i n f l u -ence the o v i p o s i t i o n preference of syrphids i n l a r g e outdoor cages. Some syrphids o v i p o s i t p r e f e r e n t i a l l y on c e r t a i n aphid s p e c i e s , but t h i s has more to do w i t h h a b i t a t requirements than w i t h prey s p e c i f i c i t y (Dusek & Laska 1966). In laboratory s t u d i e s , the s i t e preferences of o v i p o s i t i n g syr-phids have been studied by a number of i n v e s t i g a t o r s . The s p a t i a l l o c a -t i o n and exposure of an aphid colony, e.g. on v e r t i c a l s u r f a c e s , are important (Dusek & Laska 1966; Sanders 1980). Syrphid females are a t t r a c t e d to an o p t i c a l p a t t e r n resembling aphids painted on host p l a n t s (Chandler 1968b). Syrphus spp. were very p a r t i c u l a r about the s i z e o f aphid patch i n which they would o v i p o s i t , a v o i d i n g d e n s i t i e s both too low and too high (op. c i t ) . - 51 -Host plant e f f e c t s have, at times, seemed unimportant (Bombosch & Volk 1966; Peschken 1965) but the controversy seems t o have been resolved by Chandler (1966, 1968a). When he worked w i t h s e v e r a l s y r p h i d s p e c i e s , he found that one group r e q u i r e d only the host plant f o r ovipo-s i t i o n , whereas aphids were the most important stimulus f o r another group. Female age and previous "aphid abstinence" are also f a c t o r s (Schneider 1969). I t i s p o s s i b l e that c l o s e r examination of the sp e c i e s , t h e i r ages, and the l e n g t h of time they have been deprived of o v i p o s i t i o n s t i m u l i may expla i n these apparent d i f f e r e n c e s in r e s u l t s . Colour, as i t r e l a t e s t o the o v i p o s i t i o n s i t e , has sometimes been shown to be unimportant (Bombosch & Volk 1966; Peshken 1965) but has also d i r e c t l y a f f e c t e d the number of eggs l a i d on aphid-smeared g l a s s rods (Dixon 1959). Chandler (1968a) considered i t important. Peshken (1965) concluded that the most important g u i d i n g p r i n c i p l e i n the o r i e n -t a t i o n o f gravid female S. c o r o l l a e was negative p h o t o t a x i s . More r e c e n t l y , Sanders (1980), confirmed that S . c o r o l l a e p r e f e r r e d t o ovipo-s i t in shaded c o l o n i e s when given a choice between c o l o n i e s in l i g h t and shade. Aphids and t h e i r "exudates" (honeydew and alarm pheromone) have perhaps the great e s t importance in o v i p o s i t i o n a l a t t r a c t i o n . Volk (1964 i n Schneider 1969) i s o l a t e d , but did not c h a r a c t e r i z e , a s i n g l e chemical from "aphids and t h e i r exudates" that could induce syrphids t o o v i p o s i t . Honeydew alone was also shown to be a s u f f i c i e n t stimulus f o r S. c o r o l -lae to o v i p o s i t on an a r t i f i c i a l substrate (Bombosch & Volk 1966). When honeydew was the o v i p o s i t i o n stimulus, Bombosch and Volk (1966) con-cluded t h a t the p o s i t i o n of the egg was determined by the female's body - 52 -p o s i t i o n and the t e x t u r e of the substrate as detected by the o v i p o s i t o r . When aphids themselves were the stimulus, the o v i p o s i t o r was used t o s e l e c t the s i t e , r e g a r d l e s s of substrate t e x t u r e . The o v i p o s i t o r was a l s o i m p l i c a t e d as a s t e r e o t a c t i c organ in Chandler's (1968a) study. Dixon (1959) was able to induce S. l u n i g e r to o v i p o s i t on green, white, and black g l a s s rods smeared w i t h aphids. These syrphids a l s o l a i d eggs on opaque green v i a l s c o n t a i n i n g aphids, apparently i n response to odour alone. The aphids in the v i a l s were e q u a l l y e f f e c t i v e dead or a l i v e , thus e l i m i n a t i n g p o s s i b l e a u d i t o r y cues from l i v e aphids. Adult green lacewings (Chrysopidae) searching f o r aphids are faced w i t h the same problems c o n f r o n t i n g female syrphids. Honeydew has been shown to be an important f a c t o r f o r chrysopids a l s o , perhaps even more so than f o r syrphids, since honeydew and p o l l e n c o n s t i t u t e adult food (Hagen et a l . 1976). Hagen et a l . (op. c i t ) f i e l d - t e s t e d ten c o n s t i t u e n t amino acid s of honeydew and found tryptophan to be the a t t r a c t i v e source (or kairomone) f o r Chrysopa carnea. Because of the low v o l a t i l i t y of tryptophan, van Emden & Hagen (1976) also tested a number of o x i d a t i o n products of tryptophan i n an olfactometer. Female C_. carnea responded maximally to indoleacetaldehyde whereas they were not a t t r a c t e d t o the precursor of indoleacetaldehyde, indolealdehyde. Since i n d o l e a c e t a l -dehyde i s more v o l a t i l e than tryptophan, van Emden & Hagen (op. c i t ) postulated that the slow o x i d a t i o n of tryptophan under f i e l d c o n d i t i o n s would supply a constant source of the a t t r a c t i v e component, indoleacetaldehyde, f o r searching lacewings. The common f a c t o r , honey-dew, i n the a r r a y of substances a t t r a c t i n g both syrphids and chrysopids suggests that tryptophan o x i d a t i o n products should be tested as p o s s i b l e chemical a t t r a c t a n t s f o r syrphids. - 53 -There are some apparent discrepancies in the l i t e r a t u r e concerning important visual and chemosensory responses i n syrphids. In th i s chapter, I intend to investigate t h i s problem, f i r s t with olfactometer experiments designed to eliminate v i s u a l and gustatory cues. Secondly, following Dixon's (1959) example, I s h a l l observe syrphid responses to glass rods pretreated with various components of the t o t a l aphid-infested-plant o v i p o s i t i o n complex. The experiments with rods w i l l com-plement the olfactometer studies by o f f e r i n g various combinations of o l f a c t o r y , gustatory and visual cues, which are s t i l l not so complex a stimulus as an aphid-infested plant. The responses of mated and unmated males and females w i l l be observed to determine whether the responses are innate, or re l a t e d to the ins e c t s ' p h y s i o l o g i c a l ( i . e . gravid) state. Materials and Methods Olfactometer Experiments A modified version of Osgood & Kempster's (1971) olfactometer was used i n these experiments (Plate 17). In the present model, a i r was f i l t e r e d through charcoal before entering the test chambers (or a f r e s h supply was used while a i r from within the system was evacuated from the room). A l l j o i n t s in the clear p l a s t i c were sealed with masking tape to maintain the desired air flow. White paper was placed in front of the lower part of the test chambers to eliminate any v i s u a l cues. Coloured, translucent (or transparent) paper was f i x e d over the a i r mixing zone between the test chambers and f l i g h t area. The a i r speed was maintained at a constant flow rate for a l l tests with a s q u i r r e l fan and rheostat set at 30 u n i t s . - 55 -Plate 17 Olfactometer Apparatus This f i g u r e shows the p l e x i g l a s s olfactometer w i t h i t s large f l y -i n g chamber ( f c ) f o r i n t r o d u c t i o n of i n s e c t s , the a i r mixing zone (am) over which was placed coloured paper, mesh screens (cm), and two sti m u l u s chambers ( s c ) , with i t s a i r c i r c u l a t i o n system; charcoal f i l t e r ( c f ) and f a n ( s f ) . Legend f c large f l y i n g chamber am a i r mixing zone sc stimulus chamber cm copper mesh screen cf charcoal a i r f i l t e r s f s q u i r r e l f a n and rheostat - 56 -The olfactometer was placed under two warm and two cool f l u o r e s -cent l i g h t s with th e i r long axes oriented to the long axis of the o l f a c -tometer. From one to ten insect s were introduced f o r one t r i a l . P r e l -iminary experiments showed that two hours was s u f f i c i e n t for the insects to respond to a stimulus. This period was adopted as the minimum f o r each experiment. Only f i r s t - and second-generation laboratory-reared inse c t s were used. Individuals were never used more than twice and none was ever tested twice in a 24-hour period. Only a few in d i v i d u a l s were tested twice with the same stimulus. Mated females, no matter what t h e i r response, were also tested f o r their "egg-laying response" after removal from the olfactometer. They were placed i n d i v i d u a l l y i n v i a l s contain-ing aphids on a l e a f and were l e f t for 4 hours (or overnight i f the t r i a l concluded l a t e in the day) after which any eggs l a i d were counted. Unmated females and males were returned d i r e c t l y to the i r maintenance cages after an experiment. Age, generation, mating status, time o f day, duration of experiment, type of stimulus, and type of paper used over the mixing zone were recorded. Most experiments were conducted in a rearing room that had the ranges of temperature and humidity already noted (Chap.1 Materials and Methods). A few experiments were conducted i n a smaller, environmen-t a l l y c o n t r o l l e d room with comparable conditions of l i g h t , temperature and humidity. The Z te s t was used here as a common s t a t i s t i c f o r binomial exper-iments (eg. response vs no response) and i s based on the normal approxi-mation. The s t a t i s t i c was calculated; Proportion observed (Po) - Pro-portion expected (Pe) divided by standard error of the parameter. I f Z - 57 -i s l a r g e , the difference Po - Pe must be s i g n i f i c a n t l y greater than zero and i f Z i s small, the difference i s l e s s than zero. The hypothesis tested i s then, HO : Po = Pe and HI : Po = Pe. Tables of Z values are used to determine i f HO i s rejected, and P< 0.01 i s the p r o b a b i l i t y that the observed r e s u l t occurred by chance. In these experiments, Pe was calculated from the o v e r a l l response to c o n t r o l chambers in a l l experiments. Coloured Glass Rod Experiments Nine mm diameter glass tubing was cut into 40 cm lengths and coloured paper (also used f o r coloured flowers-see Appendix 1) was cut, r o l l e d and inserted into the tube. The tubing was f i t t e d into a 2.5 cm high black rubber cork or a block of p l a s t i c i n e covered with parafilm. The r e s u l t i n g rod was 41.5 cm high. The open top was sealed with a small piece of masking tape, to protect the paper from the treatments applied to the tubing. In four preliminary experiments, six coloured rods (blue, green, yellow, red, black, white) were presented i n a c y l i n d r i c a l acetate sleeve atop a p l a s t i c standard plant pot of 15 cm diameter. Each rod had five large pea aphids (A. pisum) smeared onto the top 1/3 with f o r -ceps (carcasses removed). O v i p o s i t i n g female E. v o l u c r i s were i n t r o -duced i n d i v i d u a l l y so that t h e i r behaviour could be continuously observed f o r two or four hours. On four days, the six coloured rods were painted on the top 1/3 with f l u i d from 3-400 pea aphids crushed in 2 ml d i s t i l l e d water (carcasses removed) and presented to mixed cages of o v i p o s i t i n g M. venablesi and E_. v o l u c r i s with no other o v i p o s i t i o n - 58 -stimulus. Rods were removed a f t e r 8 h and examined f o r eggs l a i d . F o l -lowing these experiments, p a i r s of untreated green rods were presented to syrphids of both species in maintenance cages and any a c t i v i t i e s were recorded f o r two hours. The types of a c t i v i t i e s noted were taken to be r e l a t e d to the presence o f the v e r t i c a l rods, not to any chemical stimulus on the g l a s s . A c t i v i t i e s r e l a t e d to o v i p o s i t i o n a l behaviour of female syrphids were observed over periods of s e v e r a l hours a f t e r the f l i e s were presented w i t h l i v i n g o v i p o s i t i o n p l a n t s . In a l l remaining experiments, p a i r s of rods were introduced to 70 X 60 X 50 cm cages of adult syrphids (M. venablesi and E_. v o l u c r i s ) i n which the numbers of each sex were approximately equal. Actual numbers, age, o v i p o s i t i o n a l s t a t u s , s p e c i e s , and time o f day were noted. Obser-vat i o n was continuous f o r one-half to two hours, but r e s u l t s were usu-a l l y recorded f o r a one-hour p e r i o d . I n d i v i d u a l v i s i t s were monitored f o r s p e c i f i c a c t i v i t i e s and t h e i r d u ration was recorded. O c c a s i o n a l l y , when a c t i v i t y was too intense to monitor i n d i v i d u a l s , counts of numbers, sex, and main a c t i v i t i e s were made at one-minute i n t e r v a l s . Six hours of observation were devoted t o each of the rod c o n d i t i o n s unless otherwise noted. Results were expressed i n frequency of v i s i t s to experimental and c o n t r o l rods. Frequency of v i s i t s f o r i n d i v i d u a l observation hours w i t h i n a group were compared by the Chi-squared t e s t ( to assess d i f f e r -ences i n time of day, s p e c i e s , and any i n d i v i d u a l d i f f e r e n c e s that occurred) before being combined. The Chi-squared t e s t was used w i t h i n s p e c i f i e d confidence l i m i t s to determine whether the recorded number of v i s i t s t o the paired c o n t r o l and tre a t e d rods d i f f e r e d s i g n i f i c a n t l y from e q u a l i t y . M a t e r i a l s and methods of i n d i v i d u a l experiments are described below. Crushed-bean Rods A young broad bean plant, 15 cm high, with two pa i r s of leaves, was cut j u s t above s o i l l e v e l and chopped into f i n e pieces in a small beaker containing 10 ml d i s t i l l e d water. The plant material was then crushed and mixed with the water and painted onto a green rod with a camel-hair brush. The c o n t r o l rod was painted with d i s t i l l e d water applied with another brush. Both rods were removed after 30 minutes of observation and re-painted to keep them wet during the t r i a l . Tryptophan and Indoleacetaldehyde Rods Saturated tryptophan (1.18% i n water) and 0.5% indoleacetaldehyde solutions were made up with d i s t i l l e d water and stored at 4 degrees C. A green rod was painted with both of these solutions and allowed to dry. The control rod was untreated. After a one-hour observation period, each rod was removed and cleaned. Honeydew Rods Rods to be treated were placed f o r at l e a s t 12 hours in a cage of broad beans heavily infested with pea aphids . If a rod was used f o r two experiments in one day i t was returned to the aphid cage between experiments. At the end of each day the treated rod was cleaned. Con-t r o l rods were untreated. Dead Aphids Glued to Rods - bO -Pea aphids were k i l l e d by f r e e z i n g . Immediately before the obser-vation period, a green rod was painted i n three places with clear n a i l p o l i s h and a t h i c k 3.6 x 1.0 cm mat of aphids was placed on each patch before the n a i l p o l i s h dried. Control rods were treated only with n a i l p o l i s h . Observations were l i m i t e d to a maximum of one hour to insure that the aphids remained fresh. Black Spots Black spots were applied with a washable f e l t marker to three places on a rod to simulate three "colonies". The s i z e of each colony was 3.6 X 1.0 cm, s i m i l a r to the patches of dead aphids described above. Control rods were not marked. Aphids i n Glass V i a l s A 5 cm glass v i a l containing 2 0 pea aphids caged with cotton wool and gauze was attached by an e l a s t i c band 1 0 cm from the top of each rod. The v i a l s were placed perpendicular to the rods. Control v i a l s were empty. This t e s t was based on the assumption that the f l i e s could see the aphids moving about inside the experimental v i a l , while o l f a c -tory and gustatory cues were reduced by the cotton plug. - 61 -Results and Discussion Olfactometer Experiments A i r Flow Controls Air-flow c o n t r o l experiments were conducted on 73 i n s e c t s , with and without green paper over the mixing zone (55 with, 18 without) and with the stimulus chambers empty. There was no response to either stimulus chamber. In subsequent t r i a l s , when one chamber contained a stimulus and the other acted as a blank co n t r o l , only 2 of 305 (0.66%) insects responded to the control chamber. Only one control t e s t s i t u a -t i o n gave r i s e to any response in the absence of o l f a c t o r y s t i m u l i . Twenty-seven insects were tested with pairs of colours over the mixing zone (green, with yellow or with red). Two females and one male entered the empty stimulus chambers, one per colour, a response frequency of 11.11%. These two response frequencies were used as expected values i n the Z tests of the various responses to o l f a c t o r y s t i m u l i . 2. Colour Over Mixing Zone Fourteen mated, f i r s t generation females previously exposed to an o v i p o s i t i o n plant (5 M. venablesi, 9 E_. v o l u c r i s ) were exposed to a heavily i n f e s t e d broad bean plant in one of the olfactometer stimulus chambers. Insects were exposed to the stimulus from 2 to 5 1/2 hours with no coloured paper over the mixing zone. None of the insects responded by searching for the o v i p o s i t i o n a l stimulus. Subsequent to these experiments, green paper was placed over the mixing zone. When the - 62 -insects were retested under these conditions they responded to the infested bean plant by searching f o r and finding t h e i r way into the stimulus chamber. This change i n the i r behaviour suggests that, without some visual stimulation, such as the colour of a po t e n t i a l host plant, these females could not or would not respond s o l e l y to odours with which they were already f a m i l i a r . Visual and olfactory cues thus seem to be linked to one another. 3_. Real Flower Olfactory Stimuli A l l insects used i n these t r i a l s were from f i r s t - or second-laboratory-reared generations. Their ages varied from one day to four weeks, with or without feeding experience on flowers. Two types of flower were employed, mock orange (Philadelphus sp.) with a very strong fragrance, and a mix of garden annuals, including those used i n the real-flower preference studies (Appendix 1 ). The fragrance of t h i s flower mixture was not so strong as the mock orange. Responses were recorded when each flower type was tested against an empty control chamber in Table I. - 63 -Table I_. Olfactometer Response of Syrphids Exposed to Real Flowers 1 _ . Response to Mock Orange Insects used Insects responding 2_. Response Species Sex Number .. venablesi males 3 2 females 15 5 . v o l u c r i s males 0 0 females 7 0 to Garden Mix Insects used Insects responding Species Sex Number !. venablesi males 5 1 f emales 7 1 • vo l u c r i s males 6 0 f emales 11 1 To t a l s : males females 14 40": 3 7 - 64 -The t o t a l response was 18.5%. The Z t e s t showed that there was a s i g n i f i c a n t response to flowers (P< 0.01 that the response occurred by chance), but that the observed difference in response to d i f f e r e n t flowers was due to chance. Also, response differences between males and females were not s i g n i f i c a n t ; both responded to flower odours to the same degree. Previous experience with flowers did not a f f e c t the response. These t r i a l s showed that syrphids respond to the odour of t h e i r food source - flowers. Although this response has not been previously reported in the l i t e r a t u r e , i t i s an established fact f o r bees (von F r i s c h 1969) and these two groups display convergent evolution in a number of t r a i t s , including behaviour, appearance and habitat choice (Bishop & Chung 1972). Kaib (1974) showed that there are separate flower odour receptors on the antenna of the c a l l i p h o r i d , C. v i c i n a , used for adult food recognition, whereas the meat odour receptors are used f o r o v i p o s i t i o n s i t e recognition. Kaib's r e s u l t s suggest that syr-phids might also use d i f f e r e n t s e n s i l l a to recognize food and o v i p o s i -t i o n s i t e s . 4. Uninfested Bean Plants; Whole and Crushed To test for o l f a c t o r y attractiveness of plants without aphids, one chamber of the olfactometer was charged with a whole broad-bean plant , or one crushed in d i s t i l l e d water was presented on saturated paper t i s -sues ("Kimwipes") in a p e t r i d ish. Forty-two mated i n d i v i d u a l s (11 ti\ and 7 0 of E. v o l u c r i s , and 19 0* and 5 Q M. venablesi) were tested with the whole plant and 10 (M. venablesi, 5 0" and 5 Q) with the crushed - 65 -bean. Control chambers remained empty when the whole plant was used. Water-soaked tissues i n a p e t r i dish were the control for the crushed pi ant. There was only one response from a female ( E_. voluc r i s ) to the control chamber in the crushed bean t r i a l s . There was no evidence that either sex of these species would search further in response to the odour of whole or crushed bean plants. 5^. Aphids as a Stimulus Since aphids constitute a major aspect of an o v i p o s i t i o n s i t e , groups of aphids were presented i n the olfactometer to 10 mated and o v i -p o s i t i n g E_. v o l u c r i s females. Three to four hundred potato aphids (Macrosiphum euphorbia) were placed i n a 10 X 12 cm nylon bag and suspended from the c e i l i n g of the test chamber. A control nylon bag was s i m i l a r l y suspended i n the control chamber. Since there was no response after two hours, the females were l e f t i n the chamber f or an ad d i t i o n a l 1.25 hours. They s t i l l did not respond, but h a l f of them l a i d eggs when tested with aphids i n a v i a l after t h i s experiment was concluded. Either the stimulus i n the olfactometer was not s u f f i c i e n t l y strong, or aphid odour alone was not enough to e l i c i t search f l i g h t s for an o v i p o s i t i o n s i t e . In S. luniger , at l e a s t , aphid odours alone were reported to have stimulated o v i p o s i t i o n (Dixon 1959). 6. Tryptophan and Indoleacetaldehyde as Olfactory Stimuli - 66 -Twenty-two unmated female E. v o l u c r i s , 18 mated females (10 M. venablesi, 8 _E. vo l u c r i s ) and 1 0 unmated males ( E_. vo l u c r i s ) were tested i n the olfactometer with a mixture of 1.18% tryptophan and 0.5% indoleacetaldehyde solutions i n d i s t i l l e d water on saturated tissues ("Kimwipes") . The control was water-saturated tissues ("Kimwi pes") . The mated females had been exposed to an ov i p o s i t i o n plant either 24 or 48 hours before the experiment. Two mated females (one of each species) responded, but no male or unmated female reacted. Five of the mated females which did not respond were then tested with one chamber contain-ing an infested bean plant and one empty chamber. Three (1 E_. v o l u c r i s , 2 M. venablesi) responded to the infested bean. Two responses in 18 i s s i g n i f i c a n t when tested by the Z t e s t . But the fact that three of five female nonresponders l a t e r responded to a r e a l o v i p o s i t i o n stimulus suggests that the stimulus from the chemicals in the chamber was r e l a t i v e l y weak. Responders were from the 2-week and 3-4 week age groups, while mated nonresponders were either newly emerged or from the 2-week age group. Age might have affected t h e i r response. Although there was a demonstrated response, these r e s u l t s are not so s t r i k i n g as those of van Emden and Hagen (1976) with green lacewings. The lacewings were much more responsive during olfactometer t r i a l s . 7_. Infested Host Plants as Olfactory Stimuli F l i e s i n several physiological states were tested with the "nor-mal" o v i p o s i t i o n stimulus, a broad-bean plant i n f e s t e d with either pea-or black-bean aphids. "Unmated" r e f e r s to insects never exposed to the opposite sex. "Mated" refers to insects kept with both sexes in cages - 67 -i n which matings were observed. "Naive" r e f e r s t o i n s e c t s not p r e v i -ously exposed to any part of the o v i p o s i t i o n a l stimulus. "Experienced" i n s e c t s are those p r e v i o u s l y exposed t o the o v i p o s i t i o n stimulus or some part of i t . "Deprived" r e f e r s t o "experienced" i n s e c t s not exposed to an o v i p o s i t i o n plant i n t h e i r cages f o r 24 or 48 hours p r i o r to t e s t i n g . 7. Unmated Naive I n s e c t s : Never Exposed to Aphids or P l a n t s F i f t y - f i v e unmated females (43 M. v e n a b l e s i , 12 E. v o l u c r i s ) and 4 unmated males (3 M. v e n a b l e s i , 1 E. v o l u c r i s ) , a l l l e s s than one week o l d , were tested w i t h a h e a v i l y i n f e s t e d bean plant i n the olfactometer. Two female M. venablesi responded. This response i s s i g n i f i c a n t (P< 0.01, Z t e s t ) suggesting that there i s some innate response t o o v i p o s i -t i o n s t i m u l i even before there i s a p h y s i o l o g i c a l need t o o v i p o s i t . The few males t e s t e d did not respond. 7.2. Unmated Experienced I n s e c t s : P r e v i o u s l y Exposed to P l a n t s and  Honeydew, but not Aphids Ten female and 19 male, unmated E. v o l u c r i s were exposed i n the olfactometer to a i r passed over a broad-bean pla n t i n f e s t e d w i t h pea aphids. These i n s e c t s had never been exposed to aphids, but had been exposed to a bean plant w i t h honeydew i n t h e i r cage p r i o r to the e x p e r i -ment. Two females and 4 males responded to the stimulus. Although sam-p l e s i z e s were too small t o compare the sexes, the Z t e s t of the com-bined response was s i g n i f i c a n t (P< 0.01). T h i s r e s u l t shows that p r e v i -ous exposure to at l e a s t part of the stimulus improves r e c o g n i t i o n of the stimulus, even before there i s a p h y s i o l o g i c a l need to o v i p o s i t . - 68 -For males, the r e c o g n i t i o n would not have been o v i p o s i t i o n a l , but was probably i n response to odours associated w i t h a food source (honeydew). Females might have been responding t o the food source, the o v i p o s i t i o n a l cues, o r both. M a t e d Naive Females: Never Exposed to Aphids or P l a n t s Nine mated E. v o l u c r i s females were exposed to an i n f e s t e d broad-bean pla n t i n the olfactometer. There was no green paper over the mix-ing zone. F i v e mated M. venablesi females were s i m i l a r l y exposed to an o v i p o s i t i o n p l a n t , but the mixing zone was covered w i t h green paper. No response occurred i n the absence of green paper as one would expect from e a r l i e r p r e l i m i n a r y experiments. One insec t responded when the green paper was present. Though the sample s i z e i s s m a l l , one response out of f i v e i s s i g n i f i c a n t (P< 0.01, Z t e s t ) . Because even unmated f l i e s responded d i f f e r e n t l y a f t e r p r i o r expo-sure to host p l a n t s and honeydew, mated and o v i p o s i t i n g females were te s t e d along w i t h t h e i r r e s p e c t i v e males in the next set of experiments, a f t e r they had had varying previous experience w i t h host f a c t o r s . M a t e d Experienced I n s e c t s : O v i p o s i t i o n Plant i n Cage Immediately  P r i o r to Experiment F o r t y - f i v e mated females (18 M. v e n a b l e s i , 27 E. v o l u c r i s ) were removed from a cage c o n t a i n i n g an o v i p o s i t i o n plant and t e s t e d i n the olfactometer w i t h a s i m i l a r p l a n t . Thirteen i n s e c t s (8 M. Venablesi, 5 E_. v o l u c r i s ) responded t o t h i s s t i m u l u s , a s i g n i f i c a n t response (P< 0.01, Z t e s t ) . - 69 -Twenty-nine mated E. v o l u c r i s males were al s o t e s t e d i n a s i m i l a r sequence. Their l a c k of response was unexpected, s i n c e unmated males had responded to the same stimulus a f t e r previous exposure to i t . 7_.J?_. Experienced I n s e c t s : O v i p o s i t i o n Plant Deprived- 2A_ or _4_8 Hours T h i r t y - f i v e mated and o v i p o s i t i o n p l a n t - d e p r i v e d females (25 M. ven a b l e s i , 10 E. v o l u c r i s ) and t e n s i m i l a r l y c o n ditioned M. venablesi males were exposed to an o v i p o s i t i o n plant i n the olfactometer. Three male and f i v e female M. venablesi responded, a s i g n i f i c a n t r e s u l t (P< 0.001, Z t e s t ) . O v i p o s i t i o n p l a n t d e p r i v a t i o n thus may increase the sen-s i t i v i t y of these i n s e c t s or lower t h e i r t h r e s h o l d f o r searching behaviour. The f o l l o w i n g contingency t a b l e i l l u s t r a t e s the d i f f e r e n t responses of o v i p o s i t i n g females and mated but no n - o v i p o s i t i n g females not p r e v i o u s l y exposed to an o v i p o s i t i o n p l a n t . NO RESPONSE not deprived deprived o v i p o s i t i n g females 12 3 no n o v i p o s i t i n g females 22 21 RESPONSE not deprived deprived o v i p o s i t i n g females 8 5 n o n o v i p o s i t i n g females 6 2 - 70 -Chi-squared t e s t s of pairwise i n t e r a c t i o n s with the computer pro-gramme NED. 3WAY (UBC Computing Center Document 1 3.2), showed a s i g n i f i -cant i n t e r a c t i o n between o v i p o s i t i n g and non-ovipositing females and the i r respective responses. There was a s i g n i f i c a n t (P< 0.01, Chi-squared t e s t ) r e l a t i o n s h i p between the o v i p o s i t i o n a l status of the female and the degree of response. On the other hand, there was no s i g -n i f i c a n t response related to plant deprivation. Since the 1-2 day period of o v i p o s i t i o n - p l a n t deprivation was rather short in r e l a t i o n to the l i f e s p a n of the insects, a longer deprivation period possibly might have had more e f f e c t . Table II l i s t s the indices of attractiveness calculated for each of the o l f a c t o r y s t i m u l i (species combined) by the following equation: number responding to stimulus INDEX= ' #in outer chamber + #in stimulus chamber + #in control chamber Apart from the o v i p o s i t i o n plant, mock orange was the most a t t r a c -t i v e stimulus. The insects most sens i t i v e to the o v i p o s i t i o n plant were mated experienced females and mated deprived males. Although the mated experienced females were responding to o v i p o s i t i o n a l s t i m u l i , the males were probably only responding to an odour they had learned to associate with food ( i n the form of honeydew). In any event, the r e s u l t s show that males, besides possessing the same antennal s e n s i l l a types as females, are capable of responding to at l e a s t some o f the o l f a c t o r y cues to which females respond. - 71 -Table I I . A t t r a c t i v e n e s s Indices f o r A l l S t i m u l i Presented to Syrphids i n the Olfactometer STIMULUS INDEX A. Flowers mock orange 0.280 garden mix 0.103 B. Others bean plant 0.000 crushed bean 0.000 aphids 0.000 tryptophan/indoleacetaldehyde unmated females 0.000 mated females 0.111 C. O v i p o s i t i o n P l a n t Offered t o ; unmated naive females 0.036 unmated experienced males 0.211 unmated experienced females 0.200 mated naive females 0.200 mated experienced females 0.289 mated deprived females 0.139 mated deprived males 0.300 - 72 -O v i p o s i t i o n Rod Experiments P r e l i m i n a r y Experiments In four t r i a l s of i n d i v i d u a l f i r s t - g e n e r a t i o n _£_. v o l u c r i s exposed to s i x coloured g l a s s rods smeared w i t h pea aphids, none o f the females l a i d eggs or showed any r e c o g n i t i o n of the rods as an o v i p o s i t i o n a l stimulus. Because of the absence of response i n these females, s i m i -l a r l y t r e a t e d s e t s of rods i n s i x co l o u r s were presented to f i r s t -generation o v i p o s i t i n g females of both species in t h e i r maintenance cages. In four days of exposure and observation, no eggs were l a i d on the g l a s s rods, nor did syrphids spend any time searching the rods. These r e s u l t s do not match Dixon's (1959) f i n d i n g s that such tre a t e d rods induced o v i p o s i t i o n in S_. c o r o l l a e . In f a c t , i n Dixon's experiments, green rods promoted sign i f i c a n t l y more l a i d eggs than did white or black. This could be a species d i f f e r e n c e or perhaps might be an e f f e c t of l a b o r a t o r y r e a r i n g on S. c o r o l l a e . The two species studied i n my experiments were never more than one generation removed from the w i l d s t a t e . S. c o r o l l a e , on the other hand, i s e a s i l y reared i n the la b o r a t o r y and thus i s commonly kept i n l a b o r a t o r i e s f o r many genera-t i o n s (see r e f s . i n Schneider 1969). There i s no informat i o n in Dixon's (1959) paper regarding the number of generations her stock of S. c o r o l - lae had been i n the l a b o r a t o r y . I n d i v i d u a l f l i e s in a cage o f o v i p o s i t i n g M. venablesi and E. v o l u c r i s made 47 v i s i t s t o two green g l a s s rods during two hours. A c t i v i t i e s observed included l a n d i n g without hovering (18), hovering then l a n d i n g ( 5 ) , landing on top (27), f a s t walking (11), preening ( 9 ) , - 73 -and r e s t i n g (3o). Two v i s i t s also included t a s t i n g . These a c t i v i t i e s were assumed to be related to the presence of v e r t i c a l glass rods, not to sources of food or ov i p o s i t i o n s i t e s . In addition to these a c t i v i t i e s , the following responses also occurred on r e a l o v i p o s i t i o n plants: (a) the substrate was tasted with the labellum and t h i s t a s t i n g was accompanied by slow searching and f r e -quent turning; (b) p r i o r to o v i p o s i t i o n , the abdomen was frequently wagged up and down while the ovipositor was extended and dragged on the substrate. These a d d i t i o n a l , and more intense a c t i v i t i e s thus could be related to s p e c i f i c o v i p o s i t i o n a l s t i m u l i in the ovi p o s i t i o n sequence. A l l these a c t i v i t i e s could be divided into three categories: a c t i v i t y p r i o r to and including landing; a c t i v i t y related to the stimulus; and unrelated a c t i v i t y . In the f i r s t category, recorded a c t i v i t i e s included: hovering with landing hovering without landing landing without hovering In the second category, a c t i v i t i e s recorded included: repeated take-off with hovering and landing again t a s t i n g walking slowly turning abdominal waggling ov i p o s i t o r extension egg laying - 74 -Recorded a c t i v i t i e s unrelated to the stimulus i n c l u d e d : l a n d i n g on top walking f a s t preening r e s t i n g On rods i n p r e l i m i n a r y experiments, and on t r e a t e d rods i n subse-quent experiments, the a c t i v i t i e s noted were i n t e r p r e t e d as f o l l o w s . Hovering without l a n d i n g i n d i c a t e d some r e c o g n i t i o n , e i t h e r v i s u a l or o l f a c t o r y , of the object being i n v e s t i g a t e d . Landing a f t e r hovering was the next step in the a t t r a c t i o n of i n s e c t s to a rod. Repeated t a k e - o f f s w i t h hovering and landing occurred when the stimulus on the rod a t t r a c t e d a large number of i n s e c t s , and thus i t co u l d be r e l a t e d to the degree of e x c i t a t i o n generated by the stimulus. Landing without hover-ing was the most common means of s e t t l i n g on a rod, but where the l a n d -ing took place on the rod was not always r e l a t e d t o the treatment. For example, l a n d i n g on top of the rod was u s u a l l y associated w i t h r e s t i n g and preening behaviour and was r a r e l y followed by i n v e s t i g a t i o n of the r e s t of the rod. Thus, l a n d i n g on the top was p r i m a r i l y a response to a choice perching p o s i t i o n . Males landed on top most f r e q u e n t l y , and would o f t e n challenge each other f o r a r e s t i n g spot on the top of the rod. " T a s t i n g " r e f e r r e d t o p e r s i s t e n t l a b e l l a r contact w i t h the rod l a s t i n g longer than two seconds. Females were more l i k e l y than males t o i n c l u d e t a s t i n g i n t h e i r v i s i t s , and the most frequent or p e r s i s t e n t t a s t i n g was associated w i t h the rods t h a t r e c e i v e d the most v i s i t s . Tasting seemed to be as s o c i a t e d w i t h the previous treatment of the rod. - 75 -Walking slowly, with turns, usually accompanied t a s t i n g and seemed to be part of the searching sequence i n i t i a t e d by t a s t i n g . Walking fast usu-a l l y occurred during short v i s i t s , and was not always associated with the stimulus on the rod. The speed of searching might be modified on a treated rod after some apparent recognition (by t a r s a l gustation, o l f a c -t i o n , v i s i o n , or t a s t i n g ) . Without this apparent recognition, some unrelated behaviour, such as preening or r e s t i n g , followed. Abdominal movements (waggling) only accompanied t a s t i n g with slow turning and thus probably indicated a further increase in response to the stimulus. Ovipositor extension and egg laying were the culmination of the sequence. Preening (unless i t involved the ovipositor after egg laying) and r e s t i n g were non-specific a c t i v i t i e s unrelated to the stimulus, as were landings on top of the rod. They tended to occur on control rods as frequently as on experimental rods. The experiments in which pairs of green glass rods were presented to syrphids were grouped f or comparison into four categories, depending on the sensory cues a v a i l a b l e : A. Visual Colour Preference 1. green vs yellow rods (both untreated) 2. green vs yellow rods (both honeydew treated) B. Gustatory and Olfactory Stimuli without Visual Cues 1. crushed broad-bean vs water control 2. tryptophan and indoleacetaldehyde vs water control 3. honeydew vs untreated control 4. tryptophan and indoleacetaldehyde vs honeydew - 76 -C. Visual Stimuli with Limited or without Gustatory or Olfactory Cues 1. black marker spot "colonies" vs untreated control 2. l i v e aphids i n a glass v i a l vs empty v i a l c o n t r o l D. Visu a l , Gustatory and Olfactory Cues 1. dead aphids glued to rod vs glue control 2. dead aphids glued to rod vs honeydew In experiments with i d e n t i c a l conditions, Chi-squared analysis revealed that there were no differences between the frequencies of morn-ing and afternoon v i s i t s among males, females, or the sexes combined. Nor were there any s i g n i f i c a n t differences in duration of v i s i t s (P<0.05, a n a l y s i s of variance) f o r either sex i n morning or afternoon. For these reasons, experiments with i d e n t i c a l rod conditions were grouped together and the combined r e s u l t s are reported here. K A c t i v i t i e s Unrelated to the Stimulus As expected, a c t i v i t i e s not related to the stimulus on a rod did not vary from experimental to control rods, except in two cases. These both involved insects doing more fast walking (Chi-squared, P< 0.001), once on a tryptophan- and indoleacetaldehyde-treated rod (vs honeydew) and once on a control rod (3 colonies of fresh dead aphids vs c o n t r o l ) . In both cases, females included f a s t walking in their v i s i t s more f r e -quently than did males. This difference did not occur in any other experiment. Unrelated a c t i v i t i e s w i l l not be discussed further. 2. V i s u a l Colour Preference - 77 -2.J_. Green vs Yellow Rods (both untreated) Observations on each species included one hour i n the morning and another i n the afternoon. Chi-squared a n a l y s i s showed that there was no d i f f e r e n c e in the frequency of v i s i t s to yellow ( 2 1 ) and green ( 1 9 ) rods. F i g u r e 1 shows the frequency of s t i m u l u s - r e l a t e d a c t i v i t i e s on each c o l o u r . In these experiments, most a c t i v i t y was of the n o n s p e c i f i c s o r t , and there was no preference f o r e i t h e r rod i n any a c t i v i t y . _•-2-1 Preen vs Yellow Rods (both honeydew trea t e d ) Observations included three and one-half hours i n the morning and two and one-half hours i n the afternoon. Two of the morning hours were w i t h E. v o l u c r i s , and the remainder with M . v e n a b l e s i . A l l afternoon observations used M. ve n a b l e s i . V i s i t d u r a t i o n f o r males on the yellow rods was sho r t e r but not s i g n i f i c a n t l y so, than that on the green rods. F i g u r e 2 shows the frequency of a l l s t i m u l u s - r e l a t e d a c t i v i t i e s on each c o l o u r . Most a c t i v i t y involved l a n d i n g , t a s t i n g , slow searching w i t h t u r n s , and repeated t a k e - o f f s w i t h hovering and f u r t h e r landings. Each colour had the same l e v e l o f a c t i v i t y . There was no hovering without l a n d i n g on e i t h e r c o l o u r , i n d i c a t i n g l i t t l e i f any v i s u a l i n t e r e s t . - 78 -Figure j_. Syrphid A c t i v i t y on Untreated Green and Yellow Glass Rods Frequency of landing (a) and stimulus-related a c t i v i t i e s (b) observed in 2 h of study with both sexes of M. venablesi and E. v o l u c r i s presented with two untreated glass rods, one green and one yellow. Legend a. 1 hovering without landing 2 hovering with landing 3 landing without hovering b. 1 repeated take-offs with hovering and landing 2 t a s t i n g 3 slow walking 4 with turning 5 abdominal waggling 6 o v i p o s i t o r extension - 1 ^ • CH- *b > r> LT >• O >-O CO I T ) H r o A o u e n b a - i j o o r o CM - 79 -F i g u r e Syrphid A c t i v i t y on Honeydew-Treated Green and Yellow Glass Rods Frequency of landing (a) and stimulus-related a c t i v i t y (b) observed in 5 h of study with both sexes of M. venablesi and E. v o l u c r i s presented with two honeydew-treated glass rods, one green and one y e l -low . Legend 1 hovering without landing 2 hovering with landing 3 landing without hovering 1 repeated take-offs with hovering and landing 2 t a s t i n g 3 slow walking 4 with turning 5 abdominal waggling 6 o v i p o s i t o r extension - 80 -In these experiments, colour by i t s e l f was not important f o r e i t h e r s p e c i e s , whether or not i t was associated w i t h food (honeydew). When both rods were tre a t e d w i t h a substance c o n t a i n i n g feeding as w e l l as o v i p o s i t i o n a l s t i m u l i , there was some i n d i c a t i o n that v i s i t duration might have decreased s l i g h t l y f o r males on the yellow rod, but there was s t i l l no o v e r a l l s i g n i f i c a n t d i f f e r e n c e . Since honeydew may be present on both green and yellow p l a n t s , one would expect the f l i e s t o have no marked colour preference while f o r a g i n g . In these experiments, the i n s e c t s accepted food wherever they found i t . T h i s r e s u l t d i f f e r s from the p r e v i o u s l y noted a t t r a c t i o n of some syrphid species to yellow (Dixon 1959; U s e 1949; Peschken 1965). Dixon ( 1959) concluded that yellow f l o w e r s , by v i r t u e o f t h e i r c o l o u r , c o u l d be more a t t r a c t i v e t o syrphids than other c o l o u r s . In the present experiments, yellow rods, by v i r t u e o f t h e i r colour alone, were not more a t t r a c t i v e than green, w i t h or without a s s o c i a t i o n w i t h feeding or o v i p o s i t i o n a l cues. My r e s u l t s are also contrary to those obtained f o r S. c o r o l l a e ( Bombosch & Volk 1966) i n t hat M. venablesi and E_. v o l u c r i s would not o v i p o s i t on honeydew without other inducement. 3. Gustatory and O l f a c t o r y S t i m u l i without V i s u a l Cues 3.K Crushed Broad Bean vs D i s t i l l e d Water C o n t r o l A l l i n s e c t s were t h i r d - and f i f t h - g e n e r a t i o n l a b - r e a r e d M. vena- b l e s i . Observations included three morning and four afternoon hours. There was a s i g n i f i c a n t (P< 0.001, Chi square) preference f o r crushed bean, as can be seen i n the f o l l o w i n g t a b l e : control crushed bean male v i s i t s 8 0 128 female v i s i t s 64 142 A s i g n i f i c a n t l y greater number of insect s (Figure 3) landed on the treated rods, repeatedly took-off, then hovered and landed, and waggled thei r abdomens up and down. Females v i s i t e d the experimental rods more often than males, and tasted and walked slowly and turned more often. Even on the control rods, females tasted s i g n i f i c a n t l y more often than males. - 82 -Figure 3. Syrphid A c t i v i t y on Green Glass Rods; Crushed Broad-bean Treated vs Untreated Co n t r o l Frequency of l a n d i n g (a) and s t i m u l u s - r e l a t e d a c t i v i t i e s (b) observed i n the 7 h when males and females of M. venablesi and E. v o l u c r i s were presented w i t h two green g l a s s rods, one untreated c o n t r o l and one t r e a t e d w i t h crushed broad-bean l i q u i d . S i g n i f i c a n t d i f f e r e n c e s (P<0 001, Chi square) between c o n t r o l and t r e a t e d rods are i n d i c a t e d by an a s t e r i s k above the " t r e a t e d " column. Legend a. 1 hovering without l a n d i n g 2 hovering w i t h l a n d i n g 3 landing without hovering b. 1 repeated t a k e - o f f s w i t h hovering and l a n d i n g 2 t a s t i n g 3 slow walking 4 w i t h t u r n i n g 5 abdominal waggling 6 o v i p o s i t o r extension 7 o v i p o s i t i o n - 83 -Both males and females showed much more in t e r e s t i n the crushed-bean than in the control rod, but females tended to taste and search longer on t h i s treated rod. Although a crushed plant does not the same stimulus as an intact plant, the sex differences in response during these t r i a l s i n d i c a t e that plant tastes or odours are involved i n the o v i p o s i t i o n sequence. Considering the negative r e s u l t s obtained in olfactometer t r i a l s with both whole and crushed bean, I suggest that the gustation of the host plant i s important i n s e l e c t i n g an o v i p o s i t i o n s i t e . Host-plant odour alone w i l l not i n i t i a t e search f l i g h t s for o v i -p o s i t i o n s i t e s . On the other hand, the taste of a crushed host plant on an a r t i f i c i a l substrate w i l l i n i t i a t e slow methodical searching involv-ing a number of the elements of an o v i p o s i t i o n a l s i t e search ( i . e . stimulus re l a t e d a c t i v i t i e s ) . 3.2. Tryptophan and Indoleacetaldehyde vs Untreated Control In these t r i a l s , females were observed f o r f i v e and one quarter hours (2.5 morning and 2.75 afternoon) and males were watched f o r one hour. Since females of M. venablesi and E_. v o l u c r i s shared the same cages, the r e s u l t s f o r females were combined. The r e s u l t s also showed that the frequency and duration of v i s i t s by older i n s e c t s (2-3 weeks), did not d i f f e r from those by i n s e c t s 1-2 weeks o l d . There were 39 v i s i t s to the control rods and 64 to the treated rods, but Chi-squared analysis showed no s i g n i f i c a n t difference (P>0.1D) between them. Figure 4 shows the frequency of each stimulus related a c t i v i t y during female v i s i t s . Treated rods showed c o n s i s t e n t l y more of each type of s p e c i f i c a c t i v i t y . Only i n slow walking, however, were the - 81 -d i f f e r e n c e s s u f f i c i e n t l y great to be s t a t i s t i c a l l y s i g n i f i c a n t (P< 0 . 0 0 1 ) . - 85 -Figure 4. Female Syrphid A c t i v i t y on Green Glass Rods; Tryprophan and Indoleacetaldehyde Treated vs Water Control Frequency of landing (a) and stimulus-related a c t i v i t i e s (b) observed i n 5.5 h of study with females of M. venablesi and E_. volucris presented with two green glass rods, one untreated and one treated with tryptophan and indoleacetaldehyde. Legend hovering without landing hovering with landing landing without hovering repeated take-off with hovering and landing tasting slow walking with turning abdominal waggling ovipositor extension 2 3 b. 1 2 3 4 5 6 — ro w _ & a* o o o Frequency o o - 86 -During the one-hour observation on males there were only four v i s i t s t o each rod. There were two n o n s p e c i f i c top landings on the experimental rod. Only one o f the other two v i s i t s t o t h i s rod involved t a s t i n g , which was not followed by searching. The remaining v i s i t s t o c o n t r o l and experimental rods a l l involved n o n s p e c i f i c a c t i v i t i e s , such as preening, walking f a s t or r e s t i n g . D i f f e r e n c e s were not so s t r i k i n g between c o n t r o l and t r e a t e d rods i n t h i s experiment, but females d i d v i s i t the t r e a t e d rod more than the males (averaging 19 v i s i t s per hour vs the males' 4 per hour) and spent t h e i r time there i n s t i m u l u s - r e l a t e d a c t i v i t y . Since the chemicals used i n t h i s treatment are important components of the aphid product, honey-dew, the preference females d i s p l a y e d f o r the t r e a t e d poles i s e x p l i c a -b l e i n terms of h o s t - f i n d i n g behaviour. The r e s u l t s of these e x p e r i -ments confirm the a t t r a c t i o n , however s l i g h t , f o r these chemicals observed i n olfactometer t r i a l s . The work of Hagen e_t aJL. (1976) and van Emden and Hagen (1976), on the a t t r a c t i o n of these chemicals f o r green lacewings suggested t h i s experiment. Syrphids seem s e n s i t i v e to at l e a s t some o f the same components of honeydew that a t t r a c t lacewings. This should not be s u r p r i s i n g i n view of the e c o l o g i c a l s i m i l a r i t i e s between the two kinds of i n s e c t s . 3.3 Honeydew vs Untreated C o n t r o l Cages c o n t a i n i n g equal numbers of both sexes of M. venablesi and E. v o l u c r i s were observed f o r three morning and th r e e afternoon hours. During h a l f of these experiments, an o v i p o s i t i o n plant i n f e s t e d w i t h pea aphids remained i n the cage, while the other h a l f did not i n c l u d e a - 87 -pl a n t . The presence of the pla n t made no s t a t i s t i c a l d i f f e r e n c e to the responses of males or females t o the g l a s s rods. A l l observations t h e r e -f o r e c o u l d be combined. Both sexes v i s i t e d the honeydew-treated rod more often than the c o n t r o l (P< 0.001). Actual frequencies of v i s i t s t o each rod are as f o l l o w s : honeydew c o n t r o l male v i s i t s 73 40 female v i s i t s 74 27 Fi g u r e 5 shows the frequency of stimulus r e l a t e d a c t i v i t i e s on each rod. The f o l l o w i n g a c t i v i t i e s occurred w i t h s i g n i f i c a n t l y greater frequency on the honeydew-treated rods: l a n d i n g without hovering; repeated t a k e - o f f s w i t h hovering and l a n d i n g ; t a s t i n g ; walking s l o w l y . Mean duration of v i s i t s to the treated rod was not longer than the aver-age v i s i t to the c o n t r o l rod, because repeated t a k e - o f f s w i t h hovering before subsequent lan d i n g s i n e v i t a b l y produced s h o r t e r , though more f r e -quent, v i s i t s . - 88 -Figure 5. Syrphid A c t i v i t y on Green Glass Rods; Honeydew-Treated vs Untreated Control Frequency of l a n d i n g (a) and s t i m u l u s - r e l a t e d a c t i v i t i e s (b) observed during the 6 h of study i n which males and females of M. vena-b l e s i and E. v o l u c r i s were presented w i t h two green g l a s s rods, one untreated and one t r e a t e d w i t h aphid honeydew. S i g n i f i c a n t d i f f e r e n c e s between c o n t r o l and t r e a t e d rods are i n d i c a t e d by an a s t e r i s k above the t r e a t e d column. Legend 1 hovering without l a n d i n g 2 hovering w i t h l a n d i n g 3 l a n d i n g without hovering 1 repeated t a k e - o f f s w i t h hovering and l a n d i n g 2 t a s t i n g 3 slow walking 4 w i t h t u r n i n g 5 abdominal waggling 6 o v i p o s i t o r extension • • - 89 -Honeydew was very a t t r a c t i v e to both males and females. As i n experiments w i t h green and yellow honeydew-treated rods, no o v i p o s i t i o n occurred, confirming that honeydew alone i s not s u f f i c i e n t stimulus t o induce o v i p o s i t i o n i n M. venablesi or E. v o l u c r i s . There i s a high pro-p o r t i o n of sugar in honeydew (Schneider 1969) which would provide an a t t r a c t i v e food source f o r both sexes, and i t i s a known food source f o r syrphids (Schneider 1969). In e f f e c t , male v i s i t s provided a d d i t i o n a l data f o r assessing the o v i p o s i t i o n a l a t t r a c t i o n o f honeydew f o r females. For example, i f more females than males were a t t r a c t e d t o a rod that had o v i p o s i t i o n a l as w e l l as feeding a t t r a c t a n t s , then the surplus could be a t t r i b u t e d to the o v i p o s i t i o n a l a t t r a c t a n t . In these experiments, females v i s i t e d the t r e a t e d rod as o f t e n as males but v i s i t e d the con-t r o l rod only h a l f as o f t e n as males. In other words, they favoured the honeydew rod more than the males d i d . The next step therefore would be to t e s t each of these s t i m u l i on separate rods presented simultaneously to both sexes. The next set of experiments provided such a t e s t . 3_.k_ Honeydew ys_ Tryptophan and Indoleacetaldehyde Treated Observations included two morning hours w i t h both sexes of E. v o l u c r i s , and one afternoon hour w i t h both sexes of M. v e n a b l e s i . There was no sex d i f f e r e n c e in t o t a l v i s i t s among the treatments (HD 81, T&I 79). Furthermore, ne i t h e r males alone nor females alone showed any preference f o r e i t h e r treatment. On tryptophan- and indoleacetaldehyde- t r e a t e d rods, however, there were s i g n i f i c a n t l y more female than male v i s i t s (P< 0.001 Chi-square). No such d i f f e r e n c e occurred on honeydew-treated rods. - 90 -Figure 6. Syrphid A c t i v i t y on Green Glass Rods; Honeydew-Treated vs Tryptophan and Indoleacetaldehyde Treated Frequency of l a n d i n g (a) and s t i m u l u s - r e l a t e d a c t i v i t i e s (b) observed i n the 3 h during which male and female M. venablesi and E_. v o l u c r i s were presented w i t h two green g l a s s rods, one honeydew-treated and the other t r e a t e d w i t h tryptophan and indoleacetaldehyde. Legend a. 1 hovering without l a n d i n g 2 hovering w i t h l a n d i n g 3 landing without hovering b. 1 repeated t a k e - o f f s w i t h hovering and l a n d i n 2 t a s t i n g 3 slow walking 4 w i t h t u r n i n g 5 abdominal waggling 6 o v i p o s i t o r extension -IV Ob -- 91 -Fi g u r e 6 shows the frequency of s t i m u l u s - r e l a t e d a c t i v i t y on both rods. S i g n i f i c a n t l y more slow walking and t u r n i n g occurred on honeydew-treated rods (P< 0.001). Females on tryptophan and indoleacetaldehyde rods engaged i n s i g n i f i c a n t l y more landings without hovering, and also tasted s i g n i f i c a n t l y more often than males (P< 0.001). The sex d i f f e r e n c e i n the response to the tryptophan- and indoleacetaldehyde-treated rods i s good evidence that tryptophan and/or indoleacetaldehyde are gustatory steps i n the sequence of s t i m u l i t h a t readies a female f o r o v i p o s i t i o n . The e a r l i e r evidence from olfactome-t e r t r i a l s i n d i c a t e d that there was also an o l f a c t o r y response which, however, was far from maximal. The gustatory response to these treated g l a s s rods was f a r stronger. L a b e l l a r g u s t a t i o n of honeydew and i t s com-ponents was p r e v i o u s l y suggested by Chandler (1966) but to date there was no experimental evidence. Dixon ( 1959) reported proboscis extension a f t e r a l i g h t i n g , but without the proboscis o r o v i p o s i t o r touching the s u b s t r a t e . My r e s u l t s show that l a b e l l a r g u s t a t i o n may of t e n be a major event i n the sequence during which the o v i p o s i t i o n a l a t t r a c t a n t s , t r y p -tophan and indoleacetaldehyde, i n honeydew are recognized by a searching f emale. V i s u a l With L i m i t e d or No Gustatory or O l f a c t o r y Cues _4.J_. Black Marker Spot "Colonies" ys Untreated C o n t r o l There were three hours of morning and three hours of afternoon observation of c o n t r o l and black-spotted rods. Two hours in each period were devoted to the females of both s p e c i e s , while the sexes were mixed - 92 -for the remainder of the observations. One morning experiment was done in the presence of an o v i p o s i t i o n plant, but there was no difference i n the frequency of v i s i t s to the rods, whether or not plants were present. Since the species were combined i n cages, no species d i f f e r e n c e was con-sidered. When o v e r a l l frequency of v i s i t s to co n t r o l and black spotted rods were compared, there was no s i g n i f i c a n t difference between the sexes. Figure 7 shows the frequency of stimulus related a c t i v i t i e s on each rod. The most frequent a c t i v i t i e s on both rods were the non-s p e c i f i c ones. Since there was nothing but aphid-like spots to e l i c i t slow searching, t a s t i n g , or abdominal waggling, such behaviour was rare. Note, however, that most of these a c t i v i t i e s occurred on the "spotted" rod. Very d i f f e r e n t behaviour occurred at the spots. Fourteen of seven-teen cases of hovering without landing occurred i n front of the spots. Of the 25 landings without preliminary hovering on the spotted rod, 15 were d i r e c t l y onto spots. Landings without hovering were s i g n i f i c a n t l y more frequent on the spotted rod than the control (P< 0.001, Chi-square) . - 93 -Figure 7_. Syrphid A c t i v i t y on Green Glass Rods; Black Marker Spot-Treated vs Untreated Control Frequency of landing (a) and stimulus-related (b) a c t i v i t i e s i n the 6 h during which male and female M. venablesi and E. v o l u c r i s were presented with two green glass rods, one untreated and one marked with 3 "colonies" of black marker spots that simulated aphids. Legend 1 hovering without landing 2 hovering with landing 3 landing without hovering 4 hovering at spots without landing 5 landing on spots 1 repeated take-offs with hovering and landin 2 t a s t i n g 3 slow walking 4 wi th turning 5 abdominal waggling 6 o v i p o s i t o r extension - 94 -T h i s treatment d i d not e l i c i t the same type o f response as the t a s t e or odour experiments. The v i s u a l responses t o the spots were unique to t h i s experiment. The c o n s i d e r a b l e amount of hovering i n f r o n t of the spots i n d i c a t e d v i s u a l a t t r a c t i o n . Many more i n s e c t s landed on the spotted than on the c o n t r o l rod, but a l a c k of gustatory and/or o l f a c t o r y cues a f t e r l a n d i n g cut short the searching sequence. Even so, i t i s noteworthy that females v i s i t e d longer than males (e.g. females 6.8 min. +12.2, males 1.9 min. + 4.2). Females were more p e r s i s t e n t i n f o l l o w i n g cues not d i r e c t l y associated w i t h food; i . e . p a t t e r n s of spots on a green stem-like background. These r e s u l t s confirm the observation of Chandler (1968b) that female syrphids were a t t r a c t e d to an o p t i c a l p a t t e r n resembling aphids, e s p e c i a l l y i f the colour c o n t r a s t e d w i t h the p l a n t . In close-range searching, t h e r e f o r e , v i s u a l cues of patterns resembling aphids on stems are important t o the o v i p o s i t i o n a l sequence. These r e s u l t s are c o n t r a r y to Dixon's (1959) conclusion that v i s u a l per-ception of aphids i s of l i t t l e , i f any importance, to o v i p o s i t i n g a p h i -dophagous syrp h i d s . 4^ 2_. L i v e Aphids i n a Glass V i a l vs Empty V i a l C o n t r o l This experiment was designed t o t e s t the v i s u a l response to l i v e , moving aphids without t h e i r normal odours. Both sexes of E. v o l u c r i s were observed f o r one hour i n the morning and one hour i n the afternoon. There was no d i f f e r e n c e in the numbers of male or female v i s i t s to the c o n t r o l or experimental rods. - 95 -Figure 8_. Syrphid A c t i v i t y on Green Glass Rods; Live Aphids in an Attached V i a l vs Empty V i a l Frequency of landing (a) and stimulus-related (b) a c t i v i t i e s observed during 2 h while male and female M. venablesi and E. v o l u c r i s were presented with two green glass rods with attached clear glass v i a l s , one empty and the other containing l i v e pea aphids. Legend a. 1 hovering without landing 2 hovering with landing 3 landing without hovering 4 land on v i a l b. 1 repeated take-offs with hovering and landing 2 t a s t i n g 3 slow walking 4 with turning 5 abdominal waggling 6 ov i p o s i t o r extension - 96 -Figure 8 shows the frequency of stimulus-related a c t i v i t i e s on each type o f rod. Most a c t i v i t y was nonspecific. There was no s i g n i f i -cant difference f or any a c t i v i t y between experimental and control rods. Landing on v i a l s was noted separately from landing on rods, but aphids in the v i a l had no s i g n i f i c a n t e f f e c t e i t h e r . In f a c t , more in s e c t s landed on the control v i a l than on the v i a l with aphids. V i a l s f u l l of l i v e aphids did not a t t r a c t males or females. The f l i e s gave no sign that they could recognize aphids i n these containers. In f a c t , the marker spots drew more attention than did the r e a l aphids "out of context". This r e s u l t , i n contrast to the previous experiments, seemed to indicate that Dixon (1959) was correct i n assuming that visual perception of aphids was unimportant to the o v i p o s i t i o n a l sequence of aphidophagous syrphids. In combination with my foraging r e s u l t s , how-ever, t h i s present r e s u l t suggests that, even i f visual perception of aphids alone may not be a s u f f i c i e n t stimulus, the addition of a green stem behind the pattern of aphids makes the pattern recognizable to females searching f o r o v i p o s i t i o n s i t e s . 5. V i s u a l With Gustatory and Olfactory Cues 5_.J_. Dead Aphids Glued to a Rod vs Control (glue) Although "colonies" of fresh dead aphids were the closest labora-tory approximation to a r e a l o v i p o s i t i o n stimulus, dead aphids could not produce honeydew. Consequently most of the t a s t e / o l f a c t o r y a t t r a c t a n t s in honeydew were absent from these t e s t s . The following tests of both sexes of the two species occupied f i v e hours in the afternoon and one-- 97 -h a l f hour i n the morning. A t o t a l o f 154 males and 152 females were observed. As befo r e , h a l f of the experiments were conducted w i t h an o v i p o s i t i o n p l a n t i n the cage and the other h a l f without one. There was no s i g n i f i c a n t d i f f e r e n c e i n the responses t o the g l a s s rods w i t h or without the o v i p o s i t i o n p l a n t . Because there was no d i f f e r e n c e , a l l hours of observation could be combined. Control rods r e c e i v e d fewer (65) v i s i t s than aphid rods (89), but t h i s d i f f e r e n c e was not s i g n i f i c a n t . Although females v i s i t e d both rods more oft e n than males, t h i s d i f f e r e n c e was s i g n i f i c a n t o n l y on the aphid rods (P< 0.001 Chi-square). F i g u r e 9 shows the frequency o f s t i m u l u s - r e l a t e d a c t i v i t i e s on each type o f rod. S i g n i f i c a n t l y more syrphids hovered, then landed, on the aphid rods than on the c o n t r o l rods. Other a c t i v i t i e s that occurred s i g n i f i c a n t l y more often on t r e a t e d rods were, slow walking and t u r n i n g , and abdominal waggling. O v i p o s i t o r extension and egg l a y i n g only took place on the "aphid" rods. - 98 -Figure 9_. Syrphid A c t i v i t y on Green Glass Rods; Dead Aphids Glued to a_ Rod vs Glue Control Frequency of landing (a) and stimulus-related (b) a c t i v i t i e s observed i n the 5.5 h when male and female M. venablesi and E_. v o l u c r i s were presented with two green glass rods, one glue-control and one with three "colonies" of fresh l y k i l l e d aphids glued to i t . Legend hovering without landing hovering with landing on aphids landing without hovering landing without hovering on aphids repeated take-offs with hovering and landing t a s t i n g slow walking with turning abdominal waggling o v i p o s i t o r extension o v i p o s i t i o n 2 3 4 b. 1 2 3 4 5 6 7 - 99 -On rods with f r e s h l y k i l l e d aphids, females d i s p l a y e d most of the behavioural sequence during which they l o c a t e an o v i p o s i t i o n s i t e and begin to l a y eggs. Even though most i n d i v i d u a l s seemed disturbed by the s u b s t r a t e , o c c a s i o n a l l y one would l a y an egg, but most probing with the o v i p o s i t o r was not followed by egg l a y i n g . Since the surface t e x t u r e of smooth g l a s s or n a i l p o l i s h d i f f e r s g r e a t l y from the surface of a l e a f , i t must d i f f e r even more from a l e a f surface coated w i t h honeydew. A f t e r a l l o w i n g f o r such d i f f e r e n c e s in t e x t u r e , however, the gustatory s e n s i l l a on the o v i p o s i t o r might s t i l l have been the major l i m i t i n g f a c -t o r , because there was l i t t l e , i f any, honeydew on the g l a s s that held the dead aphids. The f a c t that some eggs were l a i d d i r e c t l y on the dead aphids suggests that gustatory s t i m u l i were indeed the most important l i m i t i n g f a c t o r f o r most of these caged females. Honeydew-contaminated aphids seem to have provided a s u f f i c i e n t stimulus to s a t i s f y the. egg-l a y i n g requirements of a few of these females. One important f i n d i n g i n these experiments i s that aphids alone, without t h e i r host plant could release o v i p o s i t i o n . The species i n v e s -t i g a t e d here belong t o the group f o r which the aphid i s the paramount stimulus (Chandler 1 9 6 8 a ) . I must disagree w i t h Chandler's (1966) suggestion, however, that the v i s u a l stimulus of aphid appendage move-ment i s a p r e r e q u i s i t e f o r o v i p o s i t i o n . A l s o , I would add to the gusta-tory c a p a c i t i e s of the o v i p o s i t o r s e n s i l l a of these s p e c i e s , the a b i l i t y to recognize the release chemicals i n honeydew as w e l l as aphids. 5.2. Dead Aphids Glued to a Rod vs Honeydew Treatment - 1 0 0 -There were three morning and three afternoon hours of observation of second- and f i f t h - g e n e r a t i o n lab reared M. venablesi a d u l t s exposed to t h i s combination of rods. Cages contained between 13 and 3 1 males and 14 and 30 females but there were equal numbers (+ or - 1) of each sex i n each cage. A l l i n s e c t s were two weeks o l d or l e s s . Honeydew-treated rods received s i g n i f i c a n t l y (P< 0.001 Chi-square) more v i s i t s (283) than did aphid rods (195). Males, however, showed a s i g n i f i c a n t preference f o r honeydew rods, whereas females v i s i t e d each type w i t h the same frequency. F i g u r e 10 shows the frequency of s t i m u l u s - r e l a t e d a c t i v i t i e s on each type o f rod. There were no s i g n i f i -cant sex d i f f e r e n c e s i n any s t i m u l u s - r e l a t e d a c t i v i t y on the honeydew rods. On aphid rods, however, males hovered without l a n d i n g (both at the rod and above aphids) s i g n i f i c a n t l y more o f t e n than females ( C h i -squared, P< 0.001). In a l l other a c t i v i t i e s r e l a t e d to the stimulus, females showed s i g n i f i c a n t l y (P< 0.001, Chi-squared) greater a c t i v i t y than males. As expected, there were also great d i f f e r e n c e s between rods. In a l l l a n d i n g and s t i m u l u s - r e l a t e d a c t i v i t i e s , honeydew and aphid rods a t t r a c t e d s i g n i f i c a n t l y d i f f e r e n t amounts of a c t i v i t y (Chi-squared, P< 0.001 f o r a l l ) . Aphid rods had more hovering, w i t h and without l a n d i n g , and t h i s a c t i v i t y was focused on places where there were aphids. Ovipo-s i t o r extension and attempted o v i p o s i t i o n occurred more f r e q u e n t l y on aphid rods. — 101 — Figure 10. Syrphid A c t i v i t y on Green Glass Rods; Dead Aphids Glued to a^  Rod vs a Honeydew-Treated Rod Frequency of landing (a) and stimulus-related (b) a c t i v i t i e s observed during the 6 h of observation in which male and female M. vena- b l e s i and E_. v o l u c r i s were presented with two green glass rods, one treated with honeydew and one with three "colonies" of freshly k i l l e d aphids glued to i t . Legend a. 1 hovering without landing 2 hovering with landing 3 landing without hovering b. 1 repeated take-offs with hovering and landing 2 t a s t i n g 3 slow walking 4 with turning 5 abdominal waggling 6 o v i p o s i t o r extension 7 o v i p o s i t i o n -1 Ol ft - 1 02 -S u c c e s s f u l o v i p o s i t i o n occurred only on aphid rods. In c o n t r a s t , honeydew-treated rods r e c e i v e d more v i s i t s i n v o l v i n g l a n d i n g without hovering, repeated t a k e - o f f s w i t h hovering and l a n d i n g , t a s t i n g , slow walking, t u r n i n g , and abdominal waggling. In part these increases r e f l e c t the greater number of v i s i t s t o the honeydew. Females in t h i s t e s t d i s p l a y e d the same behavioural sequence i n response to the f r e s h l y k i l l e d aphids as in the l a s t set of observations w i t h dead aphids on rods. Some females succeeded i n l a y i n g eggs, although not without some confusion when the o v i p o s i t o r probed the sub-s t r a t e . The f a c t that males p r e f e r r e d honeydew rods, whereas females did not, once again provided a " c o n t r o l " f o r assessing the feeding stimulus that honeydew provided. Males were not so a t t r a c t e d to aphid rods, and t h e i r a c t i v i t i e s on each type o f rod d i f f e r e d g r e a t l y . S t i m u l u s - r e l a t e d a c t i v i t y that occurred f r e q u e n t l y on one rod occurred only i n f r e q u e n t l y , i f at a l l , on the other. For example, the v i s u a l importance of aphids was apparent i n the hovering responses t o the aphid rod, whereas there was v i r t u a l l y no hovering before l a n d i n g on the honeydew rod. In the f o l l o w i n g three t a b l e s , male and females responses t o a l l p a i r s of rods are summarized. The percentage o f v i s i t s that included each a c t i v i t y i s given f o r each experiment. In l a n d i n g a c t i v i t i e s (Table I I I ) , most lan d i n g s occurred without hovering. Unrelated a c t i v i -t i e s (Table IV) showed no p a r t i c u l a r trend i n sex d i f f e r e n c e s i n r e l a -t i o n to rod treatment. S t i m u l u s - r e l a t e d a c t i v i t i e s (Table V) on honey-dew rods were high f o r both sexes. Male and female d i f f e r e n c e s on the other rods are apparent i n the tabulated percentages . compared. T a b l e H I . P e r c e n t a g e o f v i s i t s t o g l a s s o v i p o s i t i o n r o d s t h a t  i n c l u d e d s p e c i f i c l a n d i n g a c t i v i t i e s E x p e r i m e n t L a n d i n g A c t i v i t y No. hover w. hover l a n d w/o H. on L. on s p o t s / t o t a l h ours l a n d w/o l a n d hover s p o t s v i a l v i s i t s I V i s u a l c o l o u r p r e f . m f , m f m f m f m f m f c A. g r e e n 2 0 0 8 0 33 42 12 7 vs y e l l o w 2 0 0 0 8 63 61 8 13 I ( u n t r e a t e d ) B. green 6 4 3 0 0 95 94 75 63 vs y e l l o w 6 3 1 0 0 97 98 75 86 (honeydew) G u s t a t o r y & O l f a c t o r y A. c r u s h e d bean 7 9 17 6 2 35 6 128 142 vs c o n t r o l 7 4 17 9 13 75 17 80 64 B. T r y p t o p h a n / I n d o l e . * 5 23 20 13 6 47 53 47 15 vs c o n t r o l 5 1 1 29 11 0 57 43 28 7 C. honeydew 6 0 5 0 0 80 84 74 73 vs c o n t r o l 6 5 7 5 4 68 85 40 27 D. Tryp./Indole. 3 0 4 0 0 80 78 20 54 vs honeydew 3 0 2 0 0 97 90 29 52 V i s u a l A. black spots 6f 0 0 12 0 15 19 61 27 0 47 26 32 vs c o n t r o l 2m 8 1 4 15 18 38 23 13 22 B. aphid v i a l s 2 0 1 1 11 0 56 67 22 22 9 9 vs c o n t r o l 2 0 0 14 0 43 29 29 64 7 14 V i s u a l , Gust. O l f . A. dead aphids 6 9 25 5 3 24 16 21 68 vs c o n t r o l 6 0 5 0 5 46 59 24 41 B. dead aphids 6 1 1 66 45 1 1 23 14 88 107 vs honeydew 6 3 0 0 1 82 95 153 130 * females o n l y used, "male" column is M. venablesi and "female" column i s E. v o l u c r i s . ~ T a b l e IV. P e r c e n t a g e o f v i s i t s t o g l a s s o v i p o s i t i o n r o d s t h a t i n c l u d e d s p e c i f i c s t i m u l u s - r e l a t e d a c t i v i t i e s E x p e r i m e n t S t i m u l u s - R e l a t e d A c t i v i t y r e p e a t e d t a s t s i ow t u r n - abdo. o v i . eg Igs & TH&L i n g walk i n g wag g l e e x t . a t t e m p t s C o l o u r p r e f . " m f m f m f m f m f m f a e A. g r e e n 0 0 33 29 0 0 0 0 0 0 0 0 0 vs y e l l o w 0 8 13 38 0 15 13 0 0 0 0 0 0 ( u n t r e a t e d ) B. green 45 63 91 87 89 86 88 86 0 6 0 0 0 vs y e l l o w 64 55 72 84 87 94 85 88 0 3 0 0 0 (honeydew) Gust.& O l f . A. bean 23 47 54 94 41 82 35 75 18 27 6 1 1 vs water 4 6 5 60 0 3 1 20 0 2 0 0 0 B. T r y p / I n d o l e . * 11 13 53 53 43 20 17 33 11 27 0 0 0 vs water 11 0 32 43 0 14 4 43 0 14 0 0 0 C. honeydew 24 23 62 75 66 75 49 53 1 14 1 0 0 vs c o n t r o l 8 7 20 26 3 1 5 5 7 0 7 0 0 0 D. Tryp/Indole. 0 2 50 63 20 28 5 15 0 0 0 0 0 vs honeydew 3 6 79 79 69 73 38 46 0 4 0 0 0 V i s u a l A. black spots 0 0 4 9 0 9 0 9 0 6 0 0 0 vs c o n t r o l 0 0 8 9 0 0 0 0 0 0 0 0 0 B. a p h i d / v i a l 0 1 1 0 67 11 0 0 0 0 0 0 0 0 vs v i a l 14 0 14 21 0 11 0 0 0 0 0 0 0 Vis/Gu s t / O l f A. aphids 0 18 33 47 14 38 9 37 0 19 16 8 1 vs glue 0 1 46 44 0 2 4 0 0 0 0 0 0 B. aphids 5 29 17 29 8 68 9 64 8 64 33 9 3 vs honeydew 58 62 84 98 82 95 79 97 57 85 5 0 0 * females only used, "male" column i s M venablesi and "female" column i s E. v o l u c r i s . - 105 -Table V_. Percentage of v i s i t s to glass o v i p o s i t i o n rods that  included a c t i v i t i e s unrelated to the stimulus Experiment A c t i v i t y land walk on top f a s t preen r e s t Colour pref. m f m f m f m f A. green 58 57 33 29 25 42 33 42 vs yellow 37 30 25 30 37 46 37 8 (untreated) B. green 1 5 4 5 5 6 8 13 vs yellow 0 1 1 2 6 7 16 6 (honeydew) Gust. & O l f a c t . A. crushed bean 36 6 1 1 18 24 6 25 3 vs water 75 17 6 45 45 41 58 25 B. Trypto./Indole.* 17 6 32 40 17 26 6 13 vs water 21 29 32 86 29 43 21 57 C. honeydew 5 8 8 1 18 19 1 1 11 vs c o n t r o l 25 7 28 33 18 15 38 52 D. Trypto./Indole. 20 19 30 63 25 15 25 9 vs honeydew 3 8 21 2 14 6 14 13 V i s u a l A. black spots 31 19 4 31 35 47 35 41 vs c o n t r o l 38 58 2 3 18 62 41 23 45 B. aphids v i a l 1 1 11 56 44 44 56 11 78 vs v i a l 0 0 29 36 43 36 43 57 V i s . Gust. & O l f . A. dead aphids 9 15 5 6 9 29 14 28 vs glue 50 24 50 46 25 34 33 27 B. dead aphids 45 9 6 9 18 13 41 6 vs honeydew 15 4 0 1 14 8 17 5 * females o n l y , "male " column i s M. venablesi and "female" column i s E. v o l u c r i s . - 106 -Dis c u s s i o n and Conclusions In olfactometer experiments, food odours (e.g. mock orange) had the next highest a t t r a c t i v e index a f t e r the complex that c o n s t i t u t e s the " s u i t a b l e " o v i p o s i t i o n s i t e . Syrphids then, can respond t o the odour o f a flower before (or even without) seeing the as s o c i a t e d object or i t s c o l o u r . O l f a c t o r y o v i p o s i t i o n a l s t i m u l i were not so r e a d i l y recognized at such long ranges however. Only tryptophan and indoleacetaldehyde e l i c i t e d searching behaviour, and then only i n mated females. But even these females d i s p l a y e d only a weak response. O v i p o s i t i n g i n s e c t s could not recognize c e r t a i n i n d i v i d u a l components of the o v i p o s i t i o n stimulus (whole or crushed bean plant and whole aphids) even when green paper covered the mixing zone. And without the v i s u a l input by green paper, not even the combined s t i m u l i c o u l d be recognized. These l i m i t e d c a p a c i -t i e s suggest that a complex i n t e r a c t i o n of the v a r i o u s components of an o v i p o s i t i o n s i t e i s involved i n the r e c o g n i t i o n process. In most in s t a n c e s , a s i n g l e source o f information i s inadequate, and doubled s t i m u l i are s c a r c e l y more h e l p f u l . The timing of the p r e r e q u i s i t e com-b i n a t i o n of s t i m u l i , may however, be l e s s c r i t i c a l than t h e i r composi-t i o n - sometimes they may act almost simultaneously; o r they may act i n sequence as some l a b o r a t o r y t e s t s showed. Not only do the components of the o v i p o s i t i o n s i t e i n t e r a c t i n complex ways, but p h y s i o l o g i c a l state also a f f e c t s the behavioural responses. Schneider (1969) noted t h a t , as syrphids age or are deprived of aphids, t h e i r o v i p o s i t i o n a l responses become l e s s s e l e c t i v e . And Chandler (196oa) suggested t h a t , as syrphids age, t h e i r o l f a c t o r y a b i l -i t y to s e l e c t aphids may d i m i n i s h , thereby unmasking more p r i m i t i v e - 1 07 -hos t - p l a n t r e c o g n i t i o n sequences. But i n the experiments w i t h M. vena- b l e s i and E. v o l u c r i s , p h y s i o l o g i c a l state of the insect a l s o a f f e c t e d responses t o the o v i p o s i t i o n p l a n t . Male responses d i f f e r e d i n r e l a t i o n to t h e i r previous experience w i t h the stimulus. Mated and experienced, but deprived, males were more responsive to the o v i p o s i t i o n plant than unmated, experienced males. Among females, those which were mated and also experienced proved to be the most responsive. Next came unmated, experienced i n d i v i d u a l s , and then mated but naive females. Unmated, naive females were the l e a s t responsive. While v a r i o u s elements of the o v i p o s i t i o n a l s i t e on g l a s s rods were s u f f i c i e n t l y a t t r a c t i v e to syrphids t o e l i c i t some steps i n the o v i p o s i t i o n sequence, only aphids could carry that sequence to com-p l e t i o n . M. venablesi and E_. v o l u c r i s thus belong t o Chandler's (1968a) t r u l y aphidophagous group, that depends on aphids as the paramount stimulus f o r o v i p o s i t i o n . But even when the end r e s u l t , o v i p o s i t i o n , occurred i n these l a b o r a t o r y t e s t s , the i n s e c t s that l a i d eggs s t i l l probed f ar longer w i t h t h e i r o v i p o s i t o r s f o r a s u i t a b l e spot than f l i e s exposed to a l l a v a i l a b l e cues in a n a t u r a l s i t u a t i o n . In the l a b o r a t o r y t e s t s , the missing chemical and t a c t i l e cues were r e s p o n s i b l e f o r the confused responses of the caged f l i e s . A few of the c o n s t i t u e n t s t i m u l i t e s t e d (e.g. the host p l a n t , tryptophan & indoleacetaldehyde) produced somewhat d i f f e r e n t responses in the olfactometer and on the green g l a s s rods, thereby f u r t h e r strengthening the suggestion that some components of the t o t a l stimulus may have d i f f e r e n t t a r g e t s at d i f f e r e n t stages i n the behavioural sequence, and th e r e f o r e can r a r e l y t r i g g e r an appropriate behavioural response when they are presented alone. - 1 08 -A heirarchy of behavioural responsiveness can be derived for the s t i m u l i tested by using the frequencies with which stimulus-related a c t i v i t i e s occurred on the treated rods. The sequence, i n decreasing order of a t t r a c t i v e n e s s , i s as follows: (1 ) real aphids (2) honeydew (3) crushed bean (4) typtophan and indoleacetaldehyde (5) aphid mimicking spots The p o s s i b i l i t y that prolonged rearing in the laboratory may influence the responses of gravid syrphids has not been considered in previous work. After t r i a l rearings for several months, and in the l i g h t of the report by Chambers (1977, tha t c i t e d evidence of behavioural changes as well as genetic changes in mass reared i n s e c t s ) , I preferred to use only f i r s t - and second-generation laboratory-reared insec t s for behavioural experiments. In p a r t i c u l a r , my concern was that cage-reared syrphids might, over several generations, be selected f o r t h e i r d o c i l i t y , as they would never need to search f o r o v i p o s i t i o n s i t e s or food, and would usually be kept in r e l a t i v e l y crowded conditions. Consequently t h e i r response to normal o v i p o s i t i o n a l cues might d i f f e r from wild or nearly wild stock. The experiment i n which crushed bean painted on glass rods was exposed to t h i r d - and f i f t h - g e n e r a t i o n l a b -reared M. venablesi was the only one in which ov i p o s i t i o n took place in the absence of r e a l aphids. Although only one female responded thus, she was a product of prolonged l a b - r e a r i n g . The f a c t that the number of lab-reared generations has not been considered i n other studies may account f o r some of the reported discrepancies in behaviour (e.g. Dixon 1959, Peshken 1965, Chandler 1968a). - 1 1 0 -CHAPTER 3. NEUROPHYSIOLOGY OF SYRPHID ANTENNAL AND OVIPOSITOR SENSILLA - 1 1 1 -Int r o d u e t i o n A l l i n s e c t chemoreceptors are primary neurons; t h a t i s , they l a c k synapses between t h e i r e x t e r n a l i n t e r f a c e w i t h the environment and the c e n t r a l nervous system. Consequently, s e l e c t e d input t o the b r a i n can be d i r e c t l y analysed by r e c o r d i n g e l e c t r o p h y s i o l o g i c a l ^ from the chernoreceptor. For example, i t i s p o s s i b l e to make d i r e c t recordings of the DC p o t e n t i a l i n the antennal nerve by means of electroantennogrammes (EAG). Rather than r e f l e c t i n g the response of i n d i v i d u a l r e c e p t o r s , how-ever, the EAG r e f l e c t s the summation of receptor p o t e n t i a l s of neurons i n the antenna and thus measures the s e n s i t i v i t y of the e n t i r e o l f a c t o r y system. The f i r s t such study to use EAGs i n t h i s way d e a l t w i t h the responses of the s i l k moth, Bombyx mori, t o o l f a c t o r y s t i m u l i (Schneider 1957). Since then, EAGs have been e x t e n s i v e l y used f o r sex pheromone a n a l y s i s (e.g. Schneider 1963) and more r e c e n t l y f o r i d e n t i f y i n g host plant odours recognized by pest species; e.g. p i n e o i l components used by the wood wasp (Simpson 1976), green plant v o l a t i l e s used by the Colorado potato b e e t l e ( V i s s e r 1979), and grass odours t o which l o c u s t s respond (Boeckh et a l . 1965). R e s u l t s are most u s e f u l when corroborated by behavioural responses t o the substances t o which receptors are shown to be s e n s i t i v e ; as i n the responses of l o c u s t s (Kennedy & Moorhouse 1969), and the Colorado potato beetle ( V i s s e r & N i e l s e n 1977) t o plant substances. Recordings from the s i n g l e receptor have al s o been used to study d i f f e r e n c e s i n s p e c i f i c i t y between various r e c e p t o r s . Lacher (1967, 1971) recorded responses of t h i n walled setae on the antennae of A. aegypti to s e v e r a l substances found i n human body odour, as w e l l as - 1 1 2 -recording spontaneous b a s e - l i n e a c t i v i t y . Davis (1976) recorded responses to o v i p o s i t i o n a l a t t r a c t a n t s i n a t r i c h o i d s e n s i l l u m on the antennae o f female A. a e g y p t i , while grooved s e n s i l l a i n females (Davis & Rebert 1972) but not males (Davis 1977), responded to r e p e l l e n t s . Other r e c e p t o r s were found to respond t o l a c t i c acid (Davis & Sokolove 1976), and even thermal changes (Davis & Sokolove 1975). The honeydew component of o v i p o s i t i o n s i t e s has been shown to be a t t r a c t i v e not only t o syrphids but als o to other aphid predators (Ben-saad & Bishop 1976). The green lacewing, C. carnea, a f t e r f i r s t being a t t r a c t e d to tryptophan i n a r t i f i c i a l honeydew sprayed on f i e l d crops (Hagen et a l . 1976), was a t t r a c t e d to t h i s chemical and some of i t s decomposition products i n an olfactometer (van Emden & Hagen 1976). Because green lacewings responded to these o l f a c t o r y a t t r a c t a n t s , i t i s not unreasonable to expect that other aphid predators, i n c l u d i n g s y r -phids, may a l s o respond to these components. These chemicals, p r e v i o u s l y t e s t e d i n the behavioural experiments j u s t d e s c r i b e d , were assessed w i t h n e u r o p h y s i o l o g i c a l t e s t s o u t l i n e d i n t h i s chapter. V i s s e r ' s (1979) s t u d i e s on EAG responses of Colorado potato bee-t l e s showed that the 6-carbon chain a l c o h o l s and aldehydes to which the beetles responded also e l i c i t e d EAG responses from many other phyto-phagous i n s e c t s , i n c l u d i n g the tobacco horn worm Manduca sexta, a shoot borer Hypsipyle g r a n d e l l a , the ermine moth Yponomeuta sp., t h e l o c u s t S c h i s t o c e r c a g r e g a r i a , and the beetle Adoxophyes orana. V i s s e r ( o p . c i t ) therefore concluded that t h i s group of compounds was of importance to the host s e l e c t i o n process among phytophagous i n s e c t s . Six-carbon chains have a l s o been shown to be important t o the b l o w f l y , C. v i c i n a , - 1 1 3 -(Kaib 1974) i n which one type o f antennal receptor f o r meat odours i s e s p e c i a l l y s e n s i t i v e t o 6-carbon aldehydes, a l c o h o l s and ketones. Thus the s e n s i t i v i t y to 6-carbon chain compounds may be even more widespread than V i s s e r (1979) suggested. I f i n s e c t s from diverse f a m i l i e s u t i l i z e the same types of v o l a t i l e compounds during host f i n d i n g , then they may also be used by other nonphytophagous i n s e c t s besides the b l o w f l y . For example, there are phytophagous as w e l l as aphidophagous s y r p h i d s , both of which could use v o l a t i l e components of green p l a n t s t o l o c a t e t h e i r host p l a n t s . Among the i n s e c t s , contact or gustatory chemoreceptors are found i n p a r t i c u l a r l y high numbers on the t a r s i , l a b e l l a and o v i p o s i t o r . These r e c e p t o r s can be stimulated i n d i v i d u a l l y w i t h s o l u t i o n s of chemi-c a l s . L a b e l l a r h a i r s have been shown to be s e n s i t i v e t o s a l t s , sugars, and water i n Phormia (Dethier 1963; Rees 1968) and the l a b e l l a r h a i r s of i n s e c t s that feed at flowers ( b l o w f l i e s , b u t t e r f l i e s , and the honeybee) are a l s o s e n s i t i v e t o s a l t s and sugars (Schoonhoven 1968). M a x i l l a r y gustatory s e n s i l l a o f lepidopterous l a r v a e are s e n s i t i v e t o sugars, amino a c i d s , and g l y c o s i d e s . S e n s i t i v i t y v a r i e s w i t h the species and involves both n u t r i t i v e and n o n - n u t r i t i v e plant products (Dethier & Kuch 1971). Such studies again suggest groups of compounds th a t syrphids may also u t i l i z e . O v i p o s i t o r gustatory receptors have o n l y r e c e n t l y been recognised (Behan & Ryan 1977; Hooper et a l . 1972) and only i n f r e q u e n t l y studied by s i n g l e s e n s i l l u m recordings (Rice 1976). Since some com-ponents of the o v i p o s i t i o n s i t e are among the compounds p r e v i o u s l y men-t i o n e d , o v i p o s i t o r - h a i r recordings should improve our understanding of the process of s e l e c t i o n of o v i p o s i t i o n s i t e s . In t h i s chapter, I use the EAG technique on female s y r p h i d antennae to t e s t the v o l a t i l e - 1 1 4 -substances used i n the behavioral studies w i t h the olfactometer and green g l a s s rods (Chap. 2). Contact chemoreceptors on other body p a r t s were test e d f o r t h e i r responses t o the n o n v o l a t i l e components of these substances as w e l l . M a t e r i a l s and Methods Electroantennogrammes were obtained i n Dr. B.K. M i t c h e l l ' s l a b o r a -tory i n the Department of Entomology, U n i v e r s i t y of A l b e r t a . Antennae from f r e s h l y decapitated l a b o r a t o r y - r e a r e d M. venablesi females were excised at the f i r s t antennal segment. The t i p of the t h i r d segment was then removed before the base was i n s e r t e d i n the end of a g l a s s m i c r o p i -p e t t e f i l l e d w i t h 0.01M NaCl. The p i p e t t e was placed over a s i l v e r / s i l v e r c h l o r i d e electrode mounted on a L e i t z d i s s e c t i n g micro-scope. The second (recording) e l e c t r o d e , i n another m i c r o p i p e t t e con-t a i n i n g 0.01M NaCl, was then placed w i t h i n the hole i n the t i p of the t h i r d antennal segment. Si g n a l s from the r e c o r d i n g electrode passed through a micro-a m p l i f i e r and could be viewed on a Tektronix 549 storage o s c i l l o s c o p e or recorded on a Harvard #4 86, 12-speed chart recorder. Responses were recorded on 35mm Ektachrome s l i d e f i l m , u s i n g the o s c i l l o s c o p e storage f u n c t i o n , and on photographic paper i n a Cine-scope Recorder connected to a second Tektronix o s c i l l o s c o p e . F i g u r e 11 shows the electroantenno-gramme apparatus. S o l u t i o n s t o be t e s t e d were a p p l i e d by m i c r o p i p e t t e u n t i l they saturated one h a l f c i r c l e of a 1 cm disc of f i l t e r paper. The wet f i l t e r paper was then placed i n a 3 ml d i s p o s a b l e Stylex syringe equipped w i t h i t s needle and 12 cm of p l a s t i c tubing having the same bore as the needle. A f t e r the d e s i r e d q u a n t i t y o f a i r was drawn i n t o the s y r i n g e , the tube was sealed to allow the t e s t compound t o saturate the a i r . - 1 1 6 -Figure 1_1 . Electroantennogramme Apparatus The electroantennogramme apparatus w i t h d i s s e c t e d antenna (AN), e l e c t r i c a l c i r c u i t r y ( electrodes RE and RF), a m p l i f i e r (MA), a i r d e l i v e r y system, d e l i v e r y tube (DT), s y r i n g e (SY), t e s t sample (SF) and stimulus removal funnel ( F ) , a l l enclosed i n a copper mesh cage (CS), w i t h the recording equipment, an o s c i l l o s c o p e (OS), o u t s i d e . Legend AN antenna RE re c o r d i n g electrode RF reference e l e c t r o d e MA m i c r o a m p l i f i e r OS o sc i l lo sc ope DT d e l i v e r y tube SY syr inge SF sample on f i l t e r paper F funnel CS copper screen PA p u r i f i e d a i r -|( (oft- -- 117 -An "L" shaped glass tube, tapered at both ends, and wi t h a 5 mm diameter hole at the elbow, was used to d e l i v e r the stimulus. A sm a l l aquarium a i r pump was connected by s o f t rubber t u b i n g to an a c t i v a t e d charcoal f i l t e r which i n turn was connected t o the short arm of the L-shaped tube. The tube was p o s i t i o n e d i n a clamp stand so that the t i p of the long s i d e o f the "L" was only a few mm from the excised antenna and pointed d i r e c t l y a t i t . The a i r forced over the preparation by the pump was c a r r i e d away by a vacuum l i n e attached to a p l a s t i c funnel on the other side o f the microscope. The pump provided a constant rate of a i r flow throughout a l l experiments. Test substances were introduced i n t o the a i r stream by i n s e r t i n g the tubing of the syringe i n t o the hole at the elbow of the d e l i v e r y tube, and i n j e c t i n g the syringe contents q u i c k l y and smoothly. A l l equipment, e x c l u s i v e o f the re c o r d e r s , was enclosed i n a copper mesh cage to e l i m i n a t e extraneous e l e c t r i c a l i n t e r f e r e n c e . Room temperature was 22°C. Preparations of o v i p o s i t o r and l a b e l l a r h a i r s were made by e x c i s -ing whole abdomens and extended mouthparts from f r e s h l y k i l l e d young M. venablesi or E. v o l u c r i s females, and mounting them on a micr o p i p e t t e f i l l e d w i t h 0.01M NaCl or phosphate b u f f e r . T h i s p i p e t t e was then mounted on a s i l v e r e l e ctrode on a L e i t z micromanipulator arm under a L e i t z compound microscope ( m a g n i f i c a t i o n 384X). I n d i v i d u a l h a i r s were lo c a t e d by f o c u s i n g . Test s o l u t i o n s were placed i n another m i c r o p i p e t t e (bore 4 microns at t i p ) mounted on the second e l e c t r o d e . The i n d i v i d u a l h a i r s were then stimulated by moving the micromanipulator. Responses were viewed on a Tektronix 536 O s c i l l o s c o p e , sound recorded on a P h i l i p s M i n i l o g 4 (0-2 500 HZ) tape recorder, and v i s u a l l y recorded on a Honeywell 1858 CRT V i s i c o r d e r on Kodak d i r e c t p r i n t Linagraph 2165 - 1 1 8 -paper. As b e f o r e , a l l equipment, e x c l u s i v e of the re c o r d e r s , was enclosed i n a copper-mesh cage to e l i m i n a t e extraneous e l e c t r i c a l i n t e r f e r e n c e . Figure 12 shows the design o f the apparatus f o r i n d i v i -dual h a i r r e c o r d i n g s . Room temperature and humidity were c o n t r o l l e d at 16*C and 60-70 % r e l a t i v e humidity to prolong the u s e f u l l i f e o f s p e c i -mens. Figure 12. Single Hair Recording Apparatus This apparatus was used f o r recording responses of s i n g l e o v i p o s i -t o r hairs (OP) with the test solutions (TS) in a micropipette. The e l e c t r i c a l c i r c u i t r y and microamplifier (MA) were a l l enclosed i n a copper mesh cage (CS) while the recording equipment, a tape recorder (TR), o s c i l l o s c o p e (OS), and u l t r a v i o l e t s e n s i t i v e chart recorder (UV) were outside. Legend OP o v i p o s i t o r pad with chemosensitive h a i r s TS t e s t solution RE recording electrode RF reference electrode MA microamplifier CS copper screen TR tape recorder with sound production OS oscilloscope UV u l t r a v i o l e t s e n s i t i v e chart recorder - 1 2 0 -Results Electroantennogramme Recordings Two female M. venablesi were tested with the f o l l o w i n g chemicals: green plant v o l a t i l e substances, t r a n s - 2 - h e x e n - 1 - o l , c i s - 2 - h e x e n - 1 - o l , t r a n s - 3 - h e x e n - 1 - o l , c i s - 3 - h e x e n - 1 - o l , c i s - 3 - h e x e n y l - a c e t a t e (Roth chemi-c a l s ) , and hexanol-1 (Sigma Chemicals); crushed c a r n a t i o n p e t a l s ; crushed pea aphids; two other v o l a t i l e substances of plant o r i g i n , m e t h y l s a l i c y l a t e and amylacetate ; honeydew and some o f i t s components, tryptophan, indolealdehyde, and indoleacetaldehyde (American S c i e n t i f i c Chemicals). A p o s i t i v e response or antennal nerve d e p o l a r i z a t i o n showed on the o s c i l l o s c o p e as a d i p in the baseli n e followed by a recovery to the o r i -g i n a l l e v e l . Syrphid antennae proved t o be e x c e l l e n t preparations f o r EAGs. Baseline noise was low and the s i g n a l was s t a b l e , d r i f t i n g only r a r e l y . Preparations l a s t e d up to 8 h, a l l o w i n g d u p l i c a t e t e s t i n g of many substances. C o n t r o l s of dry (dry f i l t e r paper d i s c in the syringe) and moist a i r (wet d i s c i n the syringe) e l i c i t e d no response, or only a s l i g h t response when an i n j e c t i o n was too quick or uneven ( F i g . 13 A&B). Dry and moist c o n t r o l s were tested a f t e r every 8 or 9 s t i m u l a t i o n s . A l l experimental f i l t e r paper d i s c s were saturated w i t h the t e s t s o l u t i o n . V a r i a b l e q u a n t i t i e s of t e s t molecules could be administered by va r y i n g the volume of saturat e d a i r i n j e c t e d . The nerves were allowed one minute to recover between consecutive s t i m u l a t i o n s s i n c e p r e l i m i n a r y t r i a l s showed that no greater recovery time was required to obtain — 121 — maximal response during subsequent s t i m u l a t i o n s . Preparations v a r i e d s l i g h t l y i n t h e i r magnitude o f response, but there was r a r e l y any question whether there had been a response. Ten r e p l i c a t e s of each of the green plant v o l a t i l e s e l i c i t e d p o s i t i v e responses i n both antennal p r e p a r a t i o n s . Responses to trans-2-hexen-1-o l and trans-3-hexen-1-ol are shown i n Figure 13 C&D. Dose response curves f o r these pl a n t v o l a t i l e s were constructed f o r one antennal preparation ( F i g . 15 A & B). The cis-isomers o f these two compounds showed no d i f f e r e n c e i n response ( F i g . 13D & 14A). The dose response curves f o r these isomers are shown i n F i g . 17 B & C. P o s i t i v e responses were al s o obtained f o r hexanol-1 ( F i g . 14B) and cis-3-hexenylacetate ( F i g . 14c). Twenty to t h i r t y pea aphids ground w i t h 2 ami of d i s t i l l e d water produced enough l i q u i d to soak a f i l t e r paper d i s c . Antennae were sub-jected to 2 ml of saturated a i r from t h i s s y r i n g e . T h i s treatment was repeated twice. Each time a s l i g h t r e a c t i o n was recorded ( F i g . 14E). The mean response i s p l o t t e d i n F i g . 17D. The l i q u i d from crushed white car n a t i o n p e t a l s was used to soak a f i l t e r paper d i s c . One antenna was given graded t e s t s w i t h t h i s s timulus i n amounts of 0.4, 2.0 ( F i g . 14D), and 3 ml. I t responded with r e s p e c t i v e d e f l e c t i o n s of 0.23, 0.46, and 1.24 mv. The response to 2 ml i s shown in F i g u r e 17D. Two other plant v o l a t i l e s , m e t h y l s a l i c y l a t e and amylacetate, had strong odours e a s i l y d e t e c t a b l e by humans. These substances were each test e d once. There were responses to 2 ml q u a n t i t i e s of both chemicals - 1 22 -( F i g . 15 A&B). The magnitudes of the responses are shown i n Figure 17D f o r comparison with other substances t e s t e d . Honeydew was c o l l e c t e d by p l a c i n g a p l a s t i c p e t r i d i s h l i d under a h e a v i l y i n f e s t e d broad bean l e a f o v e r n i g h t . The r e s u l t i n g s o l i d s were d i s s o l v e d i n 0.1 ml d i s t i l l e d water and used t o saturate a f i l t e r paper d i s c . When 2 ml of saturated a i r from t h i s syringe were passed over one antenna, there was only a very s l i g h t d e f l e c t i o n of the b a s e l i n e ( F i g . 15C). Honeydew components, tryptophan, indolealdehyde (both saturated i n d i s t i l l e d water) and indoleacetaldehyde (0.01% i n d i s t i l l e d water, as used f o r o v i p o s i t o r h a i r s ) were applied t o antennae 3, 5 and 2 times r e s p e c t i v e l y , w i t h no response ( F i g . 15 D&E). Thus, while plant sub-stances and crushed aphids e l i c i t e d EAG responses from female M. vena- b l e s i antennae, honeydew, some o f i t s components, and i t s o x i d a t i o n pro-ducts did not. - 1 2 3 -Figure 1 3 - Electroantennogramme Responses 1 Responses to 2 ml q u a n t i t i e s of a i r saturated w i t h each of the f o l l o w i n g substances: A) moist a i r c o n t r o l B) dry a i r c o n t r o l C) trans-2-hexen-1-ol D) c i s - 2 - h e x e n - 1 - o l E) trans-3-hexen-1-ol The d e f l e c t i o n distance down i n d i c a t e s the q u a n t i t a t i v e response of the antennal nerve. The form of the d i p i n d i c a t e s the smoothness of the a p p l i c a t i o n . One second i s i n d i c a t e d by two dashes of the l i n e under each response re c o r d i n g . The arrow i n d i c a t e s the point i n time of a p p l i c a t i o n . E - 1 24 -F i g u r e 14. Electroantennogramme Responses 2 Responses t o 2 ml q u a n t i t i e s of a i r saturated w i t h each of the f o l l o w i n g substances: A) cis-3-hexen-1-ol B) hexanol-1 C) cis-3-hexenyl acetate D) crushed c a r n a t i o n p e t a l s E) crushed aphids See Figure 13 c a p t i o n f o r i n t e r p r e t i v e d e t a i l s . • - 1 25 -Fi g u r e 15. Electroantennogramme Responses _3 Responses to 2 ml q u a n t i t i e s of a i r saturated w i t h each of the f o l l o w i n g substances: A) m e t h y l s a l i c y l a t e B) amylacetate C) honeydew ( i n water) D) indolealdehyde E) indoleacetaldehyde See Figure 13 c a p t i o n f o r i n t e r p r e t i v e d e t a i l s . B - 126 -Figure 16. Green Pl a n t V o l a t i l e Substances Dose Response Curves 1 Dose response curves f o r three green plant v o l a t i l e s in mv d e f l e c -t i o n u n i t s against dose of saturated a i r administered by syringe to a constant a i r -flow over female M. venablesi or IE. v o l u c r i s antennae prepared f o r electroantennogramme responses. A) trans-2-hexen-1-ol B) trans-3-hexen-1-ol C) hexanol-1 mv mv A. 1.00 0.75 0.50 0.25 B. 1.00 0.75 0.50 0.25 A' A ' I I I I I I 5 I 1.5 2 2.5 3 dose in ml. • i« i .5 I 1.5 2 Z5 3 dose in mi. mv C 1.00 0.75 0.50 0.251 i i 5 I 1.5 2 25 3 dose in ml. - 127 -Figure 17. Green Plant V o l a t i l e Substances Responses and Dose Response Curves A-C Dose response curves f o r three cis-isomers of green plant v o l a -t i l e s i n mv d e f l e c t i o n units against the dose of saturated a i r administered by syringe into a constant a i r flow over female M. venablesi or E. v o l u c r i s antennae prepared f o r electroantenno-gramme responses. D EAG responses in mv d e f l e c t i o n s for 2 ml quantities of a i r saturated with the following substances; aa amylacetate cp crushed carnation petals a crushed aphids ms methylsalicylate h honeydew mv A. 1.00 0.75 0.50 0.25 I 15 2 2.5 3 B. j 1.00 0.75-mv 0.50 0.25 1.00 0.75 mv 0.50 0.25 .5 I 1.5 2 2.5 3 A •5 I 1.5 2 2b mv D. 0.30 020 0.10 aa cp ms - 1 28 -S i n g l e O v i p o s i t o r Hair Recordings O v i p o s i t o r h a i r s of both M. venablesi and E_. v o l u c r i s females were stimulated w i t h the f o l l o w i n g substances: honeydew, tryptophan, indoleacetaldehyde, ala n i n e (as one o f the a d d i t i o n a l amino a c i d s found i n honeydew) and sucrose (one sugar found i n honeydew). Each substance was used to s t i m u l a t e numerous contact chemoreceptive h a i r s on each p r e p a r a t i o n . I n i t i a l l y 0.08 M phosphate b u f f e r or 0.05 M NaCl ( F i g . 18A) were used as c o n t r o l s , but there was a strong chemoreceptor response to both s o l u t i o n s . Since t h i s r e a c t i o n appeared to be a response to s a l t , suc-c e s s i v e l y l e s s concentrated NaCl s o l u t i o n s (down to 0.01 m i l l i m o l a r mM) were t e s t e d . The response did not weaken p r o p o r t i o n a l l y , and remained unchanged even when p o s s i b l e osmolarity e f f e c t s were e l i m i n a t e d by usin g 1 mM NaCl i n 0.1 M glucose. T h i s response apparently was not to the s a l t but to the water, since i t could be s u c c e s s f u l l y depressed w i t h 10% di m e t h y l s u l f o x i d e (DMSO), which has a molecular s t r u c t u r e s i m i l a r to water and thus could compete f o r receptor s i t e s . This new procedure e f f e c t i v e l y blocked the response to water without k i l l i n g c e l l s , and thus provided the necessary c o n t r o l s . The co n c e n t r a t i o n of 10% DMSO was the lowest t h a t was e f f e c t i v e i n maintaining c o n t r o l b a s e l i n e readings between receptor s t i m u l a t i o n s . For subsequent t e s t s , t h e r e f o r e , a l l substances were made up i n 1.0 mM NaCl, with and without 10% DMSO. Hairs t o be tested were f i r s t s t i m ulated w i t h 1.0 mM NaCl, then w i t h 1.0 mM NaCl i n 10% DMSO, then w i t h the t e s t s o l u t i o n i n NaCl, with and without 10% DMSO. A f t e r that sequence was completed they were test e d again wit h 1.0 mM NaCl t o ensure - 1 29 -t h a t the DMSO had not adversely a f f e c t e d any c e l l s or t h e i r r e c e p t o r s i t e s . Four responsive o v i p o s i t o r preparations from each species were used. As noted, s e v e r a l h a i r s from each preparation were s t i m u l a t e d by the t e s t substances. Test concentrations used were: 0.01 M i n d o l e a c e t a l -dehyde, 1.18% tryptophan ( s a t u r a t e d ) , 0.1 M a l a n i n e , and 0.1 M sucrose. Indolealdehyde was not t e s t e d . Honeydew was c o l l e c t e d w i t h a drop of 1.0 mM NaCl from a g l a s s microscope s l i d e l e f t overnight under a bean l e a f h e a v i l y i n f e s t e d w i t h pea aphids. The o v i p o s i t o r h a i r responses to honeydew, tryptophan, indoleacetaldehyde, a l a n i n e , and sucrose are shown i n Figures 18 B,C,D, and 19 A,B r e s p e c t i v e l y . A t y p i c a l i n s e c t chemoreceptor response con-s i s t s of two stages, phasic and t o n i c . The i n i t i a l response (phasic) has c l o s e l y spaced s p i k e s . This changes to a l e s s frequent f i r i n g p a t t e r n , the t o n i c response. P e r i o d i c i t y decreases w i t h continued s t i m u l a t i o n , but nerve recovery time i s s h o r t . Mechanoreceptor responses, which some-times occurred when the h a i r was moved during attempts to s t i m u l a t e i t , were f a r l e s s r e g u l a r ( F i g . 19C). Only q u a l i t a t i v e responses were recorded, s i n c e only one concentration of each stimulus was used. L a b e l l a r and T a r s a l Hair Recordings There were two l a b e l l a r preparations f o r each species. These preparations were test e d w i t h 1.0 mM and 10.0 mM NaCl, and 0.01 M indoleacetaldehyde, 0.1 M a l a n i n e , 1.18% tryptophan, and 0.1 M sucrose ( a l l i n 1.0 mM NaCl). As before, honeydew was c o l l e c t e d i n 1.0 mM NaCl from a microscope s l i d e placed under a broad bean plant i n f e s t e d w i t h - 1 30 -pea aphids. In one female E. v o l u c r i s that was <1 week o l d , a few short but otherwise u n i d e n t i f i e d h a i r s ( F i g . 19D) responded t o 0.1 M sucrose i n 1.0 mM NaCl. There were no responses or response a r t i f a c t s to the other substances. Another young E_. v o l u c r i s female was tested f o r t a r s a l h a i r s e n s i -t i v i t y t o 0.1 M sucrose i n 1 mM NaCl. Stimulus a r t i f a c t s occurred, i n d i c a t i n g good contact between h a i r t i p and t e s t s o l u t i o n . No response was recorded on any of s e v e r a l h a i r s t e s t e d . Those p a r t i c u l a r h a i r s apparently were not s e n s i t i v e to sucrose. - 131 -Figure 18. O v i p o s i t o r Hair E l e c t r i c a l Responses A) E l e c t r i c a l response of the o v i p o s i t o r UP contact chemoreceptor to 0.05M NaCl. T h i s i s a t y p i c a l chemoreceptor c e l l response and i n t h i s r e c e p t o r , shows the response to water by the water s e n s i t i v e c e l l . B) E l e c t r i c a l response of the o v i p o s i t o r UP chemoreceptor to honey-dew . C) E l e c t r i c a l response of the o v i p o s i t o r UP chemoreceptor to t r y p t o -phan, an amino ac i d component of aphid honeydew. The response i s not so c l e a r as that t o water, but was c o n s i s t e n t l y present i n t h i s form each time a s e n s i l l u m was t e s t e d . D) E l e c t r i c a l response of the o v i p o s i t o r UP chemoreceptor to indoleacetaldehyde, an o x i d a t i v e product of tryptophan. The arrow i n each t r a c i n g i n d i c a t e s the stimulus a r t i f a c t , o r the point at which the t e s t substance makes contact w i t h s e n s i t i v e c e l l s . - Cbi ft -A - 1 32 -Figure 1 9 . Ovipositor Hair E l e c t r i c a l Responses 2_ E l e c t r i c a l response of the o v i p o s i t o r UP chemoreceptor to the amino acid alanine. E l e c t r i c a l response of the ovipositor UP chemoreceptor to sucrose. Again, the response i s not so clear as that to alanine, but was consistent i n s e n s i l l a repeatedly tested. E l e c t r i c a l response of the ovipositor UP chemoreceptors to mechan-i c a l stimulation (with chemical stimulation a l s o ) . The t y p i c a l mechanoreceptor response i s a series of short bursts of a c t i v i t y . E l e c t r i c a l response of a l a b e l l a r contact chemoreceptor to sucrose. As in the sucrose response in ov i p o s i t o r h a i r s , the form was not t y p i c a l but remained consistent with repeated stimula-t i o n s . - 1 3 3 -D i s c u s s i o n The disc o v e r y of a water receptor on the o v i p o s i t o r was unex-pected, since there i s no suggestion i n the l i t e r a t u r e that these s y r -phids might r e q u i r e water or high humidity f o r o v i p o s i t i o n . F r e s h l y excreted honeydew i s wet, however, and a c e r t a i n amount of water vapour i s a s s o c i a t e d w i t h stomata on the undersides of lea v e s . In a d d i t i o n , many members of the M i l e s i i n a e o v i p o s i t i n t r e e r o t holes and sap oozes (Maier & Waldbauer 1979a), and E r i s t a l i s tenax o v i p o s i t s i n p o l l u t e d water (Borrer et al_. 1976). I t seems p o s s i b l e that the syrphid water receptor was evolved f o r the obvious needs of the M i l e s i i n a e but was re t a i n e d by the aphidophagous forms to confir m the presence of f r e s h honeydew (and thus l i v e aphids) and/or a favourable aphid h a b i t a t on a healthy l e a f . I t s importance in aphidophagous syrphids i s evidenced by the demonstration that at l e a s t one of t h e i r o v i p o s i t o r ' s four chemosen-s i t i v e neurons remains s e n s i t i v e to water. Evans & M e l l o r (1962) reported a water re c e p t o r i n long t r i c h o i d s e n s i l l a on the la b e l l u m of Phormia terranova, and Rees (1970) stu d i e d i t s mode o f a c t i o n . I f water i s an important element i n o v i p o s i t i o n s i t e s e l e c t i o n , then one might expect to f i n d an o l f a c t o r y s e n s i t i v i t y , i n a d d i t i o n to the s e n s i t i v e contact s e n s i l l a on the o v i p o s i t o r . O l f a c t o r y water-vapour s e n s i l l a have i n f a c t been demonstrated e l e c t r o -p h y s i o l o g i c a l l y on the t a r s i of the brown dog t i c k , Rhipicephalus  sanguineus (Haggart & Davis 1980). Behavioural evidence has suggested the presence of hygrothermic receptors on many other i n s e c t s (see Dethier 1963) but my r e s u l t s suggest t h a t they are l a c k i n g on sy r p h i d antennae. Indeed, water vapour was used as a "moist a i r c o n t r o l " i n EAG - 1 34 -recordings ( F i g . 1 3 ) . Host-plant s e l e c t i o n by phytophagous i n s e c t s i s to a l a r g e extent chemosensory. Some plant-feeders are very s p e c i f i c , showing a f i n e degree of d i s c r i m i n a t i o n , whereas others show l i t t l e s p e c i f i c i t y (Schoonhoven 1 9 6 8 ) . The sensory impact of h o s t - p l a n t odours i s one of the f i r s t and most important t r i g g e r s s e t t i n g o f f the feeding process of a phytophagous i n s e c t ( o p . c i t ) . A l l of the common green pl a n t v o l a t i l e s t e s t e d here induced a response by the antenna o f the aphidophagous M. ven a b l e s i , j u s t as they have f o r many kinds of phytophagous i n s e c t s ( V i s s e r 1979) . Consequently, f o r the f i r s t time, the present study has demonstrated that the antennae o f an aphidophagous i n s e c t can also respond to v o l a t i l e plant substances. The behavioural response, however, was not so c l e a r - c u t . Apparently, the odour of a host p l a n t , e i t h e r whole or crushed, i n an olfactometer, by i t s e l f w i l l not normally e l i c i t o v i p o s i t i o n - s i t e searching behaviour by an aphidophagous s y r p h i d . The tas t e of crushed bean, on the other hand, was of more i n t e r e s t t o ovipo-s i t i n g females than males, inducing them to d i s p l a y at l e a s t some a c t i v i t i e s u s u a l l y r e l a t e d to searching f o r o v i p o s i t i o n s i t e s . The absence of an EAG response to tryptophan and i n d o l e a c e t a l -dehyde was somewhat s u r p r i s i n g , since there was a minor behavioural response during the olfactometer experiments (Chap.2). Although no EAG has been performed on green lacewings, the work of van Emden & Hagen (1976) s t r o n g l y suggests the presence of antennal o l f a c t o r y s e n s i l l a s e n s i t i v e to honeydew components. In c o n t r a s t , the r e s u l t s obtained here suggest t h a t , i n syrphids, the responding s e n s i l l a are not loc a t e d on the antennae. The primary f u n c t i o n of the o v i p o s i t o r h a i r s that did - 135 -respond i s not o l f a c t o r y , but gustatory. Nevertheless, some of these h a i r s might be s e n s i t i v e enough to respond t o air-borne molecules p r i o r to c o n t a c t . T h i s a d d i t i o n a l c a p a c i t y i s known to occur i n some contact chemoreceptors (Zacharuk 1980) but u n f o r t u n a t e l y the recording system used here c o u l d not demonstrate i t . In the present system, the r e c o r d -ing electrode was i n s i d e the stimulus p i p e t t e and thus could record nerve a c t i v i t y only a f t e r contact w i t h the h a i r t i p . To detect increased nervous a c t i v i t y p r i o r to contact, the e l e c t r o d e must be i n s e r t e d through the side w a l l of the h a i r . My r e s u l t s therefore cannot r e s o l v e the question of o l f a c t o r y reception by the o v i p o s i t o r . Although the r e s p e c t i v e EAG responses to crushed c a r n a t i o n p e t a l s and crushed aphids may have been p a r t l y due to flower and aphid odours, they were more probably due to the small concentrations of the same green plant substances which e l i c i t e d the e a r l i e r antennal responses. Both s t i m u l i would be expected to contain these compounds. V o l a t i l e flower odours induced searching behaviour i n both males and females i n the olfactometer t r i a l s (Chap.2). Moreover, Kaib (1974) i d e n t i f i e d three d i f f e r e n t types of flower odour receptors on the antenna of the b l o w f l y , C_. v i c i n a . T h i s evidence suggests that syrphids a l s o have spe-c i a l i z e d r e c e p t o r s f o r flower odours, but the EAG response could not be d i s t i n g u i s h e d i n my experiments from a p o s s i b l e response to any crushed plant m a t e r i a l . The EAG response to m e t h y l s a l i c y l a t e i s probably important because t h i s compound i s found i n p l a n t s , though i t i s not so common as the green-plant v o l a t i l e s . U nlike these v o l a t i l e s , i t s chemical core s t r u c -t u r e i s c y c l i c , rather than s t r a i g h t - c h a i n e d . The o n l y s t r u c t u r a l e l e -- 136 -ment i t has i n common with the v o l a t i l e s i s an exposed hydroxyl group, which may be an important requirement at receptor s i t e s . For example, V i s s e r (1979) changed the p o s i t i o n of the terminal hydroxyl group during t e s t s of s e v e r a l short-chain a l c o h o l s and aldehydes on the Colorado potato b e e t l e , and found t h a t the EAG response, while s t i l l present, was much reduced a f t e r the change. I f the exposed hydroxyl group i s not i n v o l v e d , then the strong odour of m e t h y l s a l i c y l a t e might be r e a d i l y detected by a more general chemical sense. The form of the response, which was d i f f e r e n t from that of other chemicals t e s t e d ( F i g . 15A), pro-v i d e s some evidence f o r t h i s a l t e r n a t i v e p o s s i b i l i t y . When exposed to m e t h y l s a l i c y l a t e , the stimulated nerve took much longer to recover, i n d i c a t i n g that t h i s p a r t i c u l a r molecule continued to s t i m u l a t e the receptor s i t e longer than the other types of molecules. Weak responses to m e t h y l s a l i c y l a t e have been demonstrated i n antennae of the Colorado potato b e e t l e ( V i s s e r 1979) and i n the b l u n t - t i p p e d s e n s i l l a t r i c h o d e a , type I I , of female A. aegypti (Davis 1976). The contact chemoreceptors on the o v i p o s i t o r were s e n s i t i v e to the components of aphid honeydew tested i n these t r i a l s . Amino acid s are known to s t i m u l a t e some in s e c t gustatory r e c e p t o r s (Dethier & Kuch 1971). Sugars are a l s o commonly detected by contact chemoreceptors (Dethier 1963; Schoonhoven 1968) but these sugar receptors have been i d e n t i f i e d most of t e n on l a b e l l a . The l a b e l l u m of these syrphids a l s o demonstrated a s e n s i t i v i t y to sugar, although the response was not so c l e a r and t y p i c a l o f i n s e c t chemosensory responses as were some o f the o v i p o s i t o r r e c o r d i n g s . In aphidophagous, as opposed to phytophagous, sy r p h i d s , however, s e n s i t i v i t y of the o v i p o s i t o r to sugars could be most u s e f u l during the f i n a l stages of l o c a t i n g a s u i t a b l e place to o v i p o s i t . - 1 37 -F i n a l s e l e c t i o n of an o v i p o s i t i o n s i t e appears to involve the amino acids as well as sugars in honeydew, and perhaps moisture associated with f r e s h honeydew and/or the l e a f surface, i n addition to the t a c t i l e cues provided by the mechanoreceptors. - 1 38 -DISCUSSION AND SUMMARY - 139 -Female aphidophagous syrphids respond to a number of stimuli in the course of their complex oviposition sequence. Some of the f i r s t cues are probably visual. Syrphids approach oviposition sites from some distance (Dixon 1959), perhaps responding to the predominant colour of such sites - green - and the form of the vegetation (Chandler 1967). Although the importance of colour has been debated (Bombosch & Volk 1966; Peshken 1965; Dixon 1959; Chandler 1968a), light intensity at least, has been shown to be important, since ovipositing syrphids prefer shaded colonies (Peshken 1965; Sanders 1979). Negative phototaxis, how-ever, comes into play mainly after location of a suitable host plant, whereas vegetation colour, i f important, would be one of the f i r s t cues ut i l i z e d . Behavioural evidence from E. tenax (Use 1949) and M. venablesi (R. Smith pers. comm. 1981) shows that at least these two species have some colour vision. Electrophysiologically, Bishop (1974) demonstrated that E_. tenax has receptors in its compound eye that are sensitive not only to ultraviolet but also to wavelengths in the visible range. Behavioural evidence from my olfactometer studies suggests that without some appropriate visual stimulus, female syrphids cannot recognize the odours of their oviposition site. Thus, visual colour stimuli are important and must be closely linked with olfactory stimuli in the early stages of oviposition-site recognition. - 1 40 -O v i p o s i t i o n s i t e s also comprise a range of chemical s t i m u l i of which honeydew has been accorded most c r e d i t as the source of o l f a c t o r y s t i m u l a t i o n (Schneider 1 9 6 9 ; Bombosch & Volk 1 9 6 6 ; Dixon 1 9 5 9 ) . The more recent work of Hagen et a l . ( 1 9 7 6 ) and van Emden & Hagen ( 1 9 7 6 ) on green lacewings, however, suggests that the a c t u a l a t t r a c t a n t s are two components of honeydew, tryptophan and indoleacetaldehyde. The absence of an EAG response to honeydew i n my experiments w i t h M. venablesi and E_. v o l u c r i s t h e r e f o r e was somewhat s u r p r i s i n g , though i t was confirmed by the i n s e n s i t i v i t y of the antenna to both of the components of honey-dew t e s t e d , tryptophan and indoleacetaldehyde. The EAG f i n d i n g s were con t r a r y to the observed responses, however s l i g h t , o f mated females i n the olfactometer to tryptophan and indoleacetaldehyde. When syrphids were exposed to these chemicals painted on green g l a s s rods, there was a d e f i n i t e behavioural response by females, but not males ( F i g . 4 ) . Since the EAG records do not f i t the patter n emerging from other data, other o l f a c t o r y r e c e p t o r s , not on the antenna, must have been r e s p o n s i b l e f o r the observed behaviour. As noted e a r l i e r , some contact chemoreceptors can a l s o respond to a very strong o l f a c t o r y stimulus. The contact chemoreceptors on the o v i p o s i t o r are a p o s s i b l e a l t e r n a t i v e s i t e , as they d i s p l a y e d s e n s i t i v i t y to both chemicals. Concentrations of t r y p t o -phan and indoleacetaldehyde i n the olfactometer were higher than those expected to occur n a t u r a l l y , so they may have t r i g g e r e d an o l f a c t o r y response i n these contact chemoreceptors. Such a dual f u n c t i o n of con-t a c t chemoreceptors has already been shown f o r the b l o w f l y (Dethier 1 9 7 2 ) . - 141 -Honeydew i s not the only component of the o v i p o s i t i o n complex, however. In olfactometer t r i a l s , the stimulus was not complete without the p l a n t . Most of the green-plant v o l a t i l e s t e s t e d w i t h s y r p h i d anten-nae were s t r a i g h t c h a i n , 6-carbon saturated and unsaturated a l c o h o l s formed by o x i d a t i v e degradation of plant l i p i d s . They have been reported as v o l a t i l e components of numerous plant species in v a r i o u s f a m i l i e s ( V i s s e r and Ave 1978). The Colorado potato b e e t l e , the migra-tory l o c u s t , and the c a r r o t r u s t f l y are among the i n s e c t s known to be s e n s i t i v e to 6-carbon a l c o h o l s and aldehydes (Guerin and V i s s e r 1980). With evidence that common green-plant v o l a t i l e s are e x p l o i t e d by other i n s e c t s , and w i t h behavioural evidence that some syrphids use plant cues almost e x c l u s i v e l y f o r l o c a t i n g o v i p o s i t i o n a l s i t e s (Chandler 1968a), i t i s reasonable to expect aphidophagous syrphids t o have the c a p a c i t y to detect green-plant v o l a t i l e s (see Chap. 3, EAG experiments). That they did not respond to crushed broad bean i n the olfactometer (Chap. 2) sug-gests that some plan t odours by themselves may not be a s u f f i c i e n t s timulus to e l i c i t a complete behavioural response i n these s p e c i e s . Chandler (1968a) suggested t h a t , i n the s y r p h i d s ' e v o l u t i o n , the phytophagous h a b i t was the o l d e s t , whereas the entomophagous h a b i t was more recent. The EAG r e s u l t s in t h i s study support h i s suggestion that the c a p a c i t y to recognize host p l a n t s without aphids i s s t i l l present i n entomophagous s p e c i e s . Chandler (1968a) suggested t h a t t h i s e x i s t i n g response to host p l a n t s i s normally masked i n t r u l y aphidophagous s p e c i e s , but that t h i s masking d e t e r i o r a t e s under adverse c o n d i t i o n s . The f a c t that s p e c i f i c responses do diminish w i t h aging, or a f t e r long periods when the f l i e s are deprived of contact w i t h hosts (Schneider 1969), i s f u r t h e r support f o r h i s hypothesis. - 1 42 -In olfactometer observations during t h i s study, neither p l a n t s alone nor aphids alone e l i c i t e d the behavioural response of searching i n the two species t e s t e d (Chap. 2). I t i s s i g n i f i c a n t that there was no response to aphids without the plant (and perhaps honeydew). Even though green-plant v o l a t i l e s are s t i l l important components of the stimulus that t r i g g e r s searching f o r hosts by entomophagous s y r p h i d s , the complete o v i p o s i t i o n a l response seems to depend on a combination of s t i m u l i . The evidence presented here favours m u l t i p l e stimulus r e c o g n i -t i o n r a t h e r than a masking of the response to a host p l a n t . The EAG response to crushed aphids may very w e l l have been due t o the plant com-ponents ingested by the aphids. That response c e r t a i n l y was a p p r o p r i -a t e l y s mall when compared w i t h those of the other s t i m u l i (Fig.14E). Some f u r t h e r support f o r the suggestion that green-plant v o l a t i l e s are an important component of the o v i p o s i t i o n stimulus comes from the ovipo-s i t i o n experiments w i t h the green g l a s s rods. Crushed bean painted on a green g l a s s rod a t t r a c t e d many more syrphids than the c o n t r o l rod, and the a c t i v i t i e s on t h i s t r e a t e d rod were r e l a t e d to the stimulus ( F i g . 3 ) . Females v i s i t e d the t r e a t e d rod s i g n i f i c a n t l y more often than males ( F i g . 3 ) . Much t a s t i n g was i n v o l v e d i n these v i s i t s , which s t r o n g l y sug-gests that the host plant was also c o n t r i b u t i n g gustatory i n f o r m a t i o n . T h i s increased a t t e n t i v e n e s s to the host plant may be another r e l i c of the a n c e s t r a l phytophagous h a b i t . Since t h i s component does not seem, to be completely masked i n entomophagous s p e c i e s , i t i s a v a i l a b l e to act w i t h i n the stimulus combination suggested above. - 1 4 3 -The odour and/or ta s t e of crushed aphids on a g l a s s rod was expected to enhance o v i p o s i t i o n , as reported by Dixon ( 1959 ) . My r e s u l t s w i t h g l a s s rods, however, agreed w i t h r e s u l t s obtained i n the olfa c t o m e t e r , t h a t the odour of aphids could not i t s e l f induce o v i p o s i -t i o n i n these species. The d i f f e r e n c e in response to these g l a s s rods and those on which dead aphids were glued was probably i n l a r g e part due to the v i s u a l stimulus of r e a l aphids. Again then, two types of st i m u l u s were l i n k e d , and combined s t i m u l i had to be presented before a s p e c i f i c behaviour was re l e a s e d . Whole aphids on a green g l a s s rod induced o v i p o s i t i o n without support from p l a n t s t i m u l i . In these experiments, the plant element of the complex was missing and presumably so were most of the honeydew-r e l a t e d s t i m u l i . Since "normal" o v i p o s i t i o n behaviour continued u n t i l the o v i p o s i t o r probed f o r an appropriate s i t e , o l f a c t o r y , v i s u a l , and perhaps some gustatory cues were s u f f i c i e n t l y s t r o n g t o elim i n a t e the dependence on c e r t a i n of the plant s t i m u l i . At the stage o f o v i p o s i t o r probing, however, there were behavioural i n d i c a t i o n s that some stimulus was missing. In the syrphids studied, the o v i p o s i t o r was s e n s i t i v e to a l a n i n e , sucrose, water, honeydew, tryptophan and indoleacetaldehyde ( F i g . 18). So the o v i p o s i t o r i s not only a s t e r e o t a c t i c organ, as e v i -denced by the presence of a mechanoreceptor i n each of the gustatory h a i r s ( P l a t e 10 ) ; i t i s also capable o f r e c o g n i z i n g chemical s t i m u l i which i d e n t i f y the o v i p o s i t i o n s i t e . These chemical s t i m u l i are r e l a t e d to the presence of honeydew. The g l a s s rod lacked honeydew, though i t was probably present on aphids, and they were where some females chose to o v i p o s i t . There was also an absence o f l e a f moisture and f r e s h honeydew which the water receptor might otherwise have detected. - 1 44 -Perhaps these missing p a r t s of the stimulus are important f o r o v i p o s i t -ing females that have a wider choice of s i t e s , but when d e p r i v a t i o n or l i m i t e d choice occurs as i n the forgo i n g l a b o r a t o r y t e s t s , the impor-tance o f the water stimulus may be diminished along w i t h some o f the plant s t i m u l i . L a b e l l a r contact chemoreceptors, had they been as a c c e s s i b l e as o v i p o s i t o r chemoreceptors, would probably also have displayed some o f the same s e n s i t i v i t i e s as the o v i p o s i t o r h a i r s . Behavioural evidence f o r l a b e l l a r gustatory perception of these chemicals was abundant i n the o v i p o s i t i o n - r o d s t u d i e s . Both males and females t a s t e d more of t e n on honeydew- and crushed bean-treated g l a s s rods than on c o n t r o l s . Females spent s i g n i f i c a n t l y more time t a s t i n g and searching on the tryptophan-and indoleacetaldehyde-treated rods than on c o n t r o l s ( F i g . 4 ) . In other experiments, when both tryptophan- and indoleacetaldehyde- t r e a t e d rods were simultaneously presented w i t h honeydew-treated rods, s i g n i f i c a n t l y more females than males were a t t r a c t e d t o and searched and tasted the tryptophan- and indoleacetaldehyde-treated rods ( F i g . 6 ) . L a b e l l a r gusta-t i o n thus seems to be a major step i n the behavioural sequence f o r d e t e c t i n g the o v i p o s i t i o n a l a t t r a c t a n t s , tryptophan and i n d o l e a c e t a l -dehyde. In summary, a flow diagram showing p o s s i b l e combinations of s t i m u l i and responses of an o v i p o s i t i n g aphidophagous female i s presented to e x p l a i n the behavioural sequence in o v i p o s i t i o n . - 1 45 -Are you hungry? yes Feed no • Do you see the ri g h t colour? ~ 1 A no 4. yes Keep searching yes Do you smell a p o t e n t i a l s i t e ? plants? honeydew? aphid? no- Keep searching Hover search Is there an o l f a c t o r y gradient ? T no-Is i t highest on the dark side of a leaf? yes ir Do you see an aphid pattern' yes no Land and taste with labellum Do you taste a plant? honeydew? yes Taste any aphids Are they fresh? Is honeydew f r e s h ? L -yes Test taste substrate with o v i p o s i t o r •V ~ Are l a b e l l a r taste cues confirmed ? | Is the plant healthy' Does the substrate f e e l r i g h t ? no • yes yes yes Oviposit } \ Take o f f / preen} - 146 -LITERATURE CITED A d l e r , V.E. and M. Jacobson. 1972. E l e c t r o p h y s i o l o g i c a l responses of four species of pest Lepidoptera to s y n t h e t i c acetates and a l c o h o l s . J . Econ. Entomol. 65: 1582-1585. Bay, D.E. 1974. O l f a c t o r y responses and f i n e s t r u c t u r e of the f l a g e l l a r sense organs of the face f l y , Musca autumnalis Degeer. D i s s . Abst. I n t . B. 35: 2238. Bay, D.E. and C.W. P i t t s . 1976. Antennal o l f a c t o r y s e n s i l l a of the face f l y , Musca autumnalis Degeer (D i p t e r a : Muscidae). I n t . J. Insect Mor-phol. & Embryol. 5: 1-16. Behan, M. and M.F. Ryan. 1977. Sensory receptors on the o v i p o s i t o r of the c a r r o t f l y P s i l a rosae ( D i p t e r a : P s i l i d a e ) and the cabbage root f l y D e l i a b r a s s i c a e ( D i p t e r a : Anthomyiidae). B u l l . Entomol. Res. 67: 383-389. Bellamy, R.W. and R.Y. Zacharuk. 1976. St r u c t u r e of the l a b i a l palp of a l a r v a l e l a t e r i d and of si n u s c e l l s a s s o c i a t e d w i t h i t s s e n s i l l a . Can. J. Zool. 54: 2118-2128. Bensaad, A.A. and G.W. Bishop. 1976. E f f e c t of a r t i f i c i a l honeydews on in s e c t communities in potato f i e l d s . E n v i r . Entomol. 5: 453-457. B h a t i a , M.L. 1939. Bio l o g y , morphology and anatomy of aphidophagous s y r -phid l a r v a e . P a r a s i t o l o g y 31: 78-129. Bishop, L.G. 1974. An u l t r a v i o l e t photoreceptor i n a dipteran compound eye. J . Comp. P h y s i o l . 91: 267-275. Bishop, L.G. and D.W. Chung. 1972. Convergence of v i s u a l sensory capa-b i l i t i e s i n a p a i r o f Batesian mimics. J . Insect P h y s i o l . 18: 1501-1508. Boeckh, J . , K.E. K a i s s l i n g , and D. Schneider. 1965. Insect o l f a c t o r y r e c e p t o r s . Cold Spring Harb. Symp. Quant. B i o l . 30: 263-280. Bombosch, S. and St. Volk. 1966. S e l e c t i o n of the o v i p o s i t i o n s i t e by Syrphus c o r o l l a e Fabr. Pages 117-119. i_n I. Hodek. ed. The ecology o f aphidophagous i n s e c t s . Academia Pub. House, Prague.. Boo, S.K. 1980. Fine s t r u c t u r e of the antennal sensory h a i r s i n female Anopheles stephensi. Z. Parasitenkd 61: 161-172. Boo, K.S. and S.B. Mclver. 1975. Fine s t r u c t u r e of sunken t h i c k - w a l l e d pegs ( s e n s i l l a ampullacea and coeloconica) on the antennae o f mosqui-toes. Can. J . Zool. 53: 262-266. - 1 47 -Boo, K.S. and S.B. Mclver. 1976. Fine s t r u c t u r e of surface and sunken grooved pegs on the antenna o f female Anopheles stephensi ( D i p t e r a : C u l i c i d a e ) . Can. J. Zool. 54: 235-244. B o r r e r , D.J., D.M. Delong and CA. T r i p l e h o r n . 1976. An i n t r o d u c t i o n to the study of i n s e c t s . 4th ed. Holt Rinehart Winston, N.Y. Braverman, Y. and P.E. H u l l e y . 1979. The r e l a t i o n s h i p between the numbers and d i s t r i b u t i o n of some antennal and p a l p a l sense organs and host preference i n some C u l i c o i d e s ( D i p t e r a : Certatopogonidae) from Southern A f r i c a . J. Med. Entomol. 15: 419-424. Chambers, D.L. 1977. Q u a l i t y c o n t r o l i n mass r e a r i n g . Ann. Rev. Ento-mol. 22: 289-308. Chandler, A.E.F. 1966. Some aspects of host pla n t s e l e c t i o n i n aphido-phagous Syrphidae. Pages 113-115. i n I. Hodek, ed. Ecology of aphido-phagous i n s e c t s . Academia Pub. House, Prague. Chandler, A.E.F. 1967. O v i p o s i t i o n responses by aphidophagous Syrphidae ( D i p t e r a ) . Nature 213: 736. Chandler, A.E.F. 1968a. Some host-plant f a c t o r s a f f e c t i n g o v i p o s i t i o n by aphidophagous Syrphidae ( D i p t e r a ) . Ann. Appl. B i o l . 61: 415-423. Chandler, A.E.F. 1968b. The r e l a t i o n s h i p between aphid i n f e s t a t i o n s and o v i p o s i t i o n by aphidophagous Syrphidae ( D i p t e r a ) . Ann. Appl. B i o l . 61: 425-434. Chandler, A.E.F. 1968c. Some f a c t o r s i n f l u e n c i n g the occurrence and s i t e of o v i p o s i t i o n by aphidophagous Syrphidae ( D i p t e r a ) . Ann. Appl. B i o l . 61: 435-446. Chandler,A.E.F. 1968d. Height preferences f o r o v i p o s i t i o n of aphido-phagous Syrphidae ( D i p t e r a ) . Entomophaga 13: 187-195. Chandler, A.E.F. 1969. Locomotory behaviour o f f i r s t i n s t a r l a r v a e o f aphidophagous Syrphidae (Diptera) a f t e r contact w i t h aphids. Anim. Behav. 17: 673-678. Cook A.G. 1972. The u l t r a s t r u c t u r e of the A1 s e n s i l l a on the p o s t e r i o r surface o f the clypeo-labrum o f Locusta m i g r a t o r i a m i g r a t o r i o i d e s (R and F ) . Z. Z e l l f o r s c h . Mikrosk. Anat. 134: 539-554. Chu-Wang, I . , R.C. A x t e l l , and D.L. K l i n e . 1975. Antennal and pa l p a l s e n s i l l a o f the sandfly C u l i c o i d e s furens (Poey) (Diptera: Ceratopo-gonidae). I n t . J. Insect Morphol. & Embryol. 4: 131-149. Davis, E.E. 1976. A receptor s e n s i t i v e t o o v i p o s i t i o n s i t e a t t r a c t a n t s on the antennae of the mosquito Aedes aeg y p t i . J. Insect P h y s i o l . 22: 1371-1 376. i - 148 -Davis, E.E. 1977. Response of antennal receptors of male Aedes aegypti mosquitoes. J. Insect P h y s i o l . 23: 613-617. Davis, E.E. and C S . Rebert. 1972. Elements of o l f a c t o r y receptor cod-ing i n the yellow f e v e r mosquito, Aedes ae g y p t i . J. Econ. Entomol. 65: 1058- 1061. Davis, E.E. and P.G. Sokolove. 1975. Temperature responses of the antennal thermoreceptors of the mosquito, Aedes ae g y p t i . J. Comp. Phy-s i o l . 96: 223-233. Davis, E.E. and P.G. Sokolove. 1976. L a c t i c a c i d - s e n s i t i v e r e c e p t o r s on the antennae of the mosquito, Aedes ae g y p t i . J. Comp. P h y s i o l . 105: 43-54. Deth i e r , V.G. 1963. The physiology of i n s e c t senses. John Wiley & Sons Inc., N.Y. D e t h i e r , V.G. 1971. A s u r f i e t of s t i m u l i : a paucity of r e c e p t o r s . Am. S c i . 59: 706-715. De t h i e r , V.G. 1972. S e n s i t i v i t y of the contact chemoreceptors of the b l o w f l y t o vapours. Proc. N a t l . Acad. S c i . USA 69: 2189-2192. Deth i e r , V.G. and F.E. Hanson. 1968. E l e c t r o p h y s i o l o g i c a l responses of the chemoreceptors of the b l o w f l y to sodium s a l t s of f a t t y a c i d s . Prod. N a t l . Acad. S c i . 60: 1296-1303. Deth i e r , V.G. and J.M. Kuch. 1971. E l e c t r o p h y s i o l o g i c a l studies of l e p i d o p t e r o u s l a r v a e . I. Comparative s e n s i t i v i t y to sugars, amino acids and g l y c o s i d e s . Z. V e r g l . P h y s i o l . 72: 343-363. Deth i e r , V.G., J.R. Larsen, and J.R. Adams. 1963. The f i n e s t r u c t u r e of the o l f a c t o r y receptors of the b l o w f l y . Pages 105- 114. in. Y. Z o t t e r -man ed., O l f a c t i o n and Taste 1. Pergamon Press, Oxford. Dixon, J . J . 1959. S t u d i e s on the o v i p o s i t i o n behaviour of Syrphidae. Trans. Roy. Entomol. Soc. London 111: 57-80. Dusek, J. and P. Laska. 1966. Occurrence of syrphid l a r v a e on some aphids. Pages 37-38. i n I. Hodek, ed. Ecology of aphidophagous i n s e c t s . Academia Pub. House, Prague. E l i z a r o v , Y.A. and S.Y. Chaika. 1975. An e l e c t r o n microscopic study of the gustatory and o l f a c t o r y s e n s i l l a e of the b l a c k f l y Boophthora  erythrocephala S i m u l i i d a e , D i p t e r a . Vestn. Mosk. Univ. B i o l . 30: 1-7.(Abstr.) E l i z a r o v , Y.A. and S.Y. Chaika. 1977. U l t r a s t r u c t u r e of o l f a c t o r y sen-s i l l a of blood-sucking tabanids ( D i p t e r a : Tabanidae). Entomol. Obozr. 56: 283-291.(Abstr.) - 149 -van Emden, H.F. and K.S. Hagen. 1976. O l f a c t o r y r e a c t i o n s of the green lacewing, Chrysopa carnea, t o tryptophan and c e r t a i n breakdown pro-ducts. Env. Entomol. 5: 469-473. Evans, D.R. and de F M e l l o r . 1962. J . Gen. P h y s i o l . 45: 487-500. F e l t , B.T. and J.S. Vande Berg. 1976. U l t r a s t r u c t u r e of the b l o w f l y chemoreceptor s e n s i l l u m Phormia regina J . Mophol. 150: 763-783. F r a z e r , B.D. 1972. A simple and e f f i c i e n t method of r e a r i n g aphido-phagous h o v e r f l i e s ( D i p t e r a : Syrphidae). J. Entomol. Soc. B.C. 69: 23-2 4. von F r i s c h K. 1969. The foraging bee: How she f i n d s and e x p l o i t s sources of food. Bee World 50: 141-152. Gnatzy, W. and K.M. Weber. 1978. Tormogen c e l l and receptor-lymph space i n i n s e c t o l f a c t o r y s e n s i l l a . Fine s t r u c t u r e and h i s t o c h e m i c a l pro-p e r t i e s in C a l l i p h o r a . C e l l & Tissue Res. 189: 549-554. Greenberg, B. and N. Ash. 1972. S e t i f e r o u s plaques on antennal p e d i c e l s of muscoid D i p t e r a : appearance i n v a r i o u s s p e c i e s and t e s t s of func-t i o n . Ann. Entomol. Soc. Am. 65: 1340-1346. Guerin, P.M. and J.H. V i s s e r . 1980. Electroantennogram responses of the c a r r o t f l y P s i l a rosae to v o l a t i l e p l a n t components. P h y s i o l . Ento-mol. 5: 111-120. Hagen, K.S. , P. Greany, F.F. Sawall J r . and R.S. Tassan. 1976. Trypto-phan in a r t i f i c i a l honeydew as a source of an a t t r a c t a n t f o r adult Chrysopa carnea. Env. Entomol. 5: 458-468. Haggart, D.A. and E.E. Davis. 1980. Ammonia-sensitive neurons on the f i r s t t a r s i of the t i c k , Rhipicephalus sanguinieus. J. Insect Phy-s i o l . 26: 517-523. Holloway, B.A. 1976. P o l l e n feeding i n h o v e r - f l i e s ( D i p t e r a : S y r p h i -dae). N.Z.J. Zool. 3: 339-350. Hooper, R.L., C.W. P i t t s , and J.A. W e s t f a l l . 1972. Sense organs on the o v i p o s i t o r of the f a c e f l y , Musca autumnalis. Ann. Entomol. Soc. Am. 65: 577-586. Horridge, G.A., K. Mimura, and Y. Tsukara. 1975. F l y photoreceptors I I . S p e c t r a l and p o l a r i z e d l i g h t s e n s i t i v i t y i n the d r o n e f l y E r i s t a l i s . Proc. R. Soc. Lond. B. 190: 225-237. U s e , D. 1949. Colour d e s c r i m i n a t i o n i n the d r o n e f l y E r i s t a l i s tenax. Nature Lond. 163: 255-256. Jez, D.H. and S.B. Mclver. 1980. Fine s t u c t u r e of antennal s e n s i l l a o f l a r v a l Toxorhynchites b r e v i p a l p i s D i p t e r a : C u l i c i d a e . I n t . J. Insect Morphol. & Embryol. 9: 147-1 60. - 150 -Kaib, M. 1974. Die F l e i s c h - und Blumenduftrezeptoren auf der Antenne der Schmeibfliege Calliphora v i c i n a . [english summary] J. Comp. Phy-s i o l . 95: 105-121. Kellogg, F.E. 1970. Water vapour and carbon dioxide receptors in Aedes  aegypti (L.). J. Insect Physiol. 16: 99-108. Kennedy, J.S. and J.E. Moorhouse. 1969. Laboratory observations on locust responses to wind-borne grass odour. Ent. Expl. & Appl. 12: 487-503. Kugler, H. 1956. The o p t i c a l effect of flowers p o l l i n a t e d by Diptera on f l i e s . Deutsche Botan. Gesel. Berich. 69: 387-389. Kugler, H. 1970. Blutenokologie. Vol. 2 v b l l i g neu bearbeitete und erweiterte Auflage. Gustav Fischer Verlag., S t u t t g a r t . Lacher, V. 1967. Elektrophysiologischen Undersuchungen an Einzelnen Geruschsrezeptoren auf den Antennen Weiblicher Mosquitos (Aedes agypti L. ) [english summary]. J. Insect Physiol. 13: 1461-1 470. Lacher V. 1971. Arbeitshereiche von Geruchsrezeptoren auf der Moskito antenne (Aedes aegypti). J. Insect Physiol. 17: 507-517. Larsen, J.R. and V.G. Dethier. 1963. The f i n e structure of the l a b e l l a r and antennal chemoreceptors of the blowfly, Phormia regina. Proc. Int. Cong. Zool. 16th. 3: 81-83. Lewis, C T . 1971. S u p e r f i c i a l sense organs of the antennae of the f l y , Stomoxys c a l c i t r a n s . J. Insect P h y s i o l . 17: 449-461. Maier, C T . and G.P. Waldbauer. 1979a. Dual mate-seeking strategies i n male syrphid f l i e s (Diptera: Syrphidae). Ann. Entomol. Soc. Am. 72: 54-61. Maier, C T . and G.P. Waldbauer. 1979b. Diurnal a c t i v i t y patterns of flower f l i e s (Diptera: Syrphidae) i n an I l l i n o i s sand area. Ann. Entomol. Soc. Am. 72: 237-245. Matthews, R.W. and J.R. Matthews. 1978. Insect behaviour. John Wiley & Sons, N.Y. Mclver, S.B. 1972a. Fine structure of the s e n s i l l a chaetica on the antennae of Aedes aegypti (Diptera: C u l i c i d a e ) . Ann. Entomol. Soc. Am. 65: 1390-1397. Mclver, S.B. 1972b. Fine structure of pegs on the palps of female C u l i -cine mosquitoes . Can. J. Zool. 50: 571-576. Mclver, S.B. 1973. Fine structure of antennal s e n s i l l a coeloconica of c u l i c i n e mosquitoes. Tissue & C e l l 5: 105-112. - 151 -Mclver, S.B. 1974. Fine s t r u c t u r e of antennal grooved pegs of the mosquito, Aedes a e g y p t i . C e l l T i s s . Res. 153: 327-337. Mclver, S.B. 1975. S t r u c t u r e of c u t i c u l a r mechanoreceptors of a r t h r o -pods. Ann. Rev. Entomol. 20: 381-397. Mclver, S.B. 1978. S t r u c t u r e of s e n s i l l a t r i c h o d e a of female Aedes  aegypti w i t h comments on i n n e r v a t i o n of antennal s e n s i l l a . J. Insect P h y s i o l . 24: 383-390. Mclver, S. and A. Hudson. 1972. S e n s i l l a on the antennae and palps of s e l e c t e d Wyeomyia mosquitoes. J. Med. Entomol. 9: 337-345. Mclver, S.B. and R. S i e m i c k i . 1976. F i n e s t r u c t u r e of the antennal t i p of the crabhole mosquito D e i n o c e r i t e s cancer D i p t e r a C u l i c i d a e . I n t . J. I n s e c t Morphol. Embryol. 5: 319-334. Mclver, S. and R. S i e m i c k i . 1978. F i n e s t r u c t u r e of antennal s e n s i l l a c oeloconica o f adult Toxorhynchites b r e v i p a l p i s ( D i p t e r a : C u l i c i d a e ) . J . Med. Entomol. 14: 673-676. Mclver, S. and R. S i e m i c k i . 1979. F i n e s t r u c t u r e of antennal s e n s i l l a o f male Aedes aegypti ( L . ) . J. Insect P h y s i o l . 25: 21-28. Mclver, S.B., R. S i e m i c k i , and J . S u t c l i f f e . 1980. B i f u r c a t e s e n s i l l a on the t a r s i o f female b l a c k f l i e s Simulium venustum D i p t e r a : S i m u l i -idae, contact chemo s e n s i l l a adapted f o r o l f a c t i o n . J. Morphol. 165: 1-1 2. Mercer, K.L. and S.B. Mclver. 1973. S t u d i e s on the antennal s e n s i l l a of s e l e c t e d b l a c k f l i e s ( D i p t e r a : S i m u l i i d a e ) . Can. J . Zool. 51: 729-734. Moeck, H. 1968. E l e c t r o n microscopic s t u d i e s of antennal s e n s i l l a on the ambrosia b e e t l e Trypodendron lineatum ( O l i v e r ) ( S c o l y t i d a e ) . Can. J. Zool. 46: 521-556. Navai, S. and W.W. Wirth. 1978. Terminology of some antennal sensory organs of C u l i c o i d e s b i t i n g midges ( D i p t e r a : Ceratopogonidae). J. Med. Entomol. 15: 43-49. Neumeyer, C. 1980. Simultaneous colour c o n t r a s t i n the honeybee. J . Comp. P h y s i o l . 139: 165-176. Omand, E. and V.G. De t h i e r . 1969. An e l e c t r o p h y s i o l o g i c a l a n a l y s i s of the a c t i o n o f carbohydrates on the sugar receptor of the b l o w f l y . Proc. Nat. Acad. S c i . 62: 136-143. Osgood, C E . and R.H. Kempster. 1971. An a i r - f l o w olfactometer f o r d i s -t i n g u i s h i n g between o v i p o s i t i o n a t t r a c t a n t s and s t i m u l a n t s of mosqui-toes. J. Econ. Entomol. 5: 1109-1110. - 1 52 -Peschken, D. 1965. Untersuchungen zur Orientierung aphidophager Schweb-f l i e g e n ( D i p t e r a : Syrphidae) [ e n g l i s h summary], Z. angew. Ent. 55: 201^2 35. P h i l l i p s , C E . and J.S. Vande Berg. 1976. Mechanism f o r s e n s i l l u m f l u i d flow i n trichogen and tormogen c e l l s of Phormia regina ( D i p t e r a : C a l -l i p h o r i d a e ) . I n t . J. Insect Morphol. & Embryol. 5: 423-431. Rees, C.J.C. 1968. The e f f e c t of aqueous s o l u t i o n s of some 1:1 e l e c t r o -l y t e s on the e l e c t r i c a l response of the Type I ( s a l t ) chemoreceptor c e l l i n the l a b e l l a of Phormia. J. Insect P h y s i o l . 14: 1331-1 364. Rees, C.J.C. 1970. The primary process of r e c e p t i o n i n the Type 3 ('water') receptor of the f l y Phormia terranova. Proc. Roy. Soc. Lond. B. 174: 469-490. Ri c e , M.J. 1976. Contact chemoreceptors on the o v i p o s i t o r of L u c i l i a  cuprina the A u s t r a l i a n sheep b l o w f l y . Aust. J . Zool. 24: 353-360. Ruzicka, Z. 1976. Prey s e l e c t i o n by l a r v a e of Metasyrphus c o r o l l a e ( D i p t e r a : Syrphidae). Acta Entomol. Bohemoslov. 73: 305-311. Sanders, vonW. 1979. Das Eiablageverhalten der Schwebfliege Syrphus  c o r o l l a e Fabr. i n Abhangigkeit von der Grobe der B l a t t l a u s k o l o n i e [ e n g l i s h summary]. Z. angew. Zool. 66: 217-232. Sanders, von W. 1980. O v i p o s i t i o n behaviour of the syrphid Syrphus  c o r o l l a e depending on the s p a t i a l p o s i t i o n of the aphid colony [ i n German, E n g l i s h summary]. Z. angew. Zool. 67: 35-46. Seabrook, W.D. 1977. Insect chemosensory responses t o other i n s e c t s . Pages 15-43. i n H.H. Shorey & J . J . McKelvey J r . , eds. Chemical con-t r o l o f i n s e c t behaviour: theory and a p p l i c a t i o n . Wiley, N.Y. S c h a l l e r F. 1971. I n d i r e c t sperm t r a n s f e r by s o i l arthropods. Ann. Rev. Entomol. 16: 407-446. Schneider, D. 1957. E l e k t r o p h y s i o l o g i s c h e Untersuchungen von Chemo- und Mechanorezeptoren der Antenne dew Seidenspinners Bombyx mori L. Z. v e r g l . P h y s i o l . 40: 8-41. Schneider, D. 1963. E l e c t r o p h y s i o l o g i c a l i n v e s t i g a t i o n of i n s e c t o l f a c -t i o n , pp 85-1 03 i_n Y. Zotterman (ed.) O l f a c t i o n and Taste v o l . 1. Per-gamon Press, Schneider, F. 1969. Bionomics and physiology of aphidophagous S y r p h i -dae. Ann. Rev. Entomol. 14: 103-124. Schoonhoven, L.M. 1968. Chemosensory basis of host plant s e l e c t i o n . Ann. Rev. Entomol. 13: 115-136. - 153 -S i l b e r g l i e d , R.E. 1 9 7 9 . Communication i n the u l t r a v i o l e t . Ann. Rev. E c o l . & System. 10: 373-398. Simpson, R.E. 1976. Bioassay of pine o i l components as a t t r a c t a n t s f o r S i r e x n o c t i l i o (Hymenoptera: S i r i c i d a e ) using electroantennogram t e c -niques. Entomol. E x p l . & Appl. 19: 11-18. S l i f e r , E.H. 1970. The s t r u c t u r e of arthropod chemoreceptors. Ann. Rev. Entomol. 15: 121-142. S l i f e r , E.H. and S.S. Sekhon. 1964. F i n e s t r u c t u r e of the sense organs on the antennal f l a g e l l u m of a f l e s h f l y , Sarcophaga argyrostoma R. D. (D i p t e r a : Sarcophaga). J. Morphol. 114: 185-208. S t a d l e r , E. and F.E. Hanson. 1975. O l f a c t o r y c a p a b i l i t i e s of the "gus-t a t o r y " chemoreceptors of the tobacco hornworm la r v a e . J. Comp. Phy-s i o l . 104: 97-1 02. S t e i n b r e c h t , R.E. 1969. On the question o f nervous s y n c i t i a : lack o f axon f u s i o n i n two i n s e c t sensory nerves. J . C e l l . S c i . 4: 39-53. Steward, C.C. and C.E. Atwood. 1963. The sensory organs of the mosquito antenna. Can. J . Zool. 41: 5 7 7 - 5 9 4 . T h o r n h i l l R. 1976. Sexual s e l e c t i o n and n u p t i a l feeding behaviour i n B i t t a c u s a p i c a l i s ( I n s e c t a : Mecoptera). Am. Nat. 110: 529-548. V i s s e r , J.H. 1979. Electroantennogram responses of the Colorado potato be e t l e to plan t v o l a t i l e s . Entomol. E x p l . & Appl. 25: 86-97. V i s s e r , J.H. and D.A. Ave. 1978. General green l e a f v o l a t i l e s in o l f a c -tory o r i e n t a t i o n o f the Colorado potato b e e t l e . Entomol. E x p l . & Appl. 24: 7 3 8 - 7 4 9 . V i s s e r , J.H. and J.K. N i e l s e n . 1977. S p e c i f i c i t y i n the o l f a c t o r y o r i e n t a t i o n o f the Colorado potato b e e t l e , L e p t i n o t a r s a decemlineata. Entomol. E x p l . & Appl. 21: 14-22. V i s s e r , J.H. , A. van S t r a t e n , and H. Maarse. 1979. I s o l a t i o n and iden-t i f i c a t i o n o f v o l a t i l e s i n the f o l i a g e of potato, a host o f the Colorado potato b e e t l e . J . Chem. E c o l . 5: 12-23. Volk, S. 1964. Untersuchungen zur Eiablage von Syrphus c o r o l l a e Fabr. [ e n g l i s h summary]. Z. angew. Entomol. 54: 365-386. White, S. L. and D.E. Bay. 1980. Antennal o l f a c t o r y s e n s i l l a o f the h o r n f l y Haematobium i r r i t a n s i r r i t a n s (L.) (Dipt e r a : Muscidae). J. Kansas Entomol Soc. 53: 641-652. White R.A., U. Paim, and W.D. Seabrook. 1974. M a x i l l a r y a n d l a b i a l s i t e s of carbon d i o x i d e - s e n s i t i v e receptors of l a r v a l Orthosoma brunneum (F o r s t e r ) (Coleoptera: Cerambycidae). J. Comp. P h y s i o l . 88: 235-246. — 1 54 -Kakhontov, U.V. 1966. C o c i n e l l i d a e and S y r p h i d a e as p r e d a t o r s o f a p h i d s i n U z b e k i s t a n . Pages 267^269. i n I . Hodek, ed. E c o l o g y of a p h i d o -phagous i n s e c t s . Academia Pub. House, Prague. Zacharuk, K.Y. 1980. U l t r a s t r u c t u r e and f u n c t i o n o f i n s e c t chemosen-s i l l a . Ann. Rev. E n t o m o l . 25: 27-47. - 155 -APPENDIX 1 - 156 -M a t e r i a l s and Methods Preference Experiments with N a t u r a l Flowers Six v a r i e t i e s of garden flowers grown i n a p l o t protected from i n s e c t i c i d e s , were presented i n cages t o lab-reared M. venablesi and E_. v o l u c r i s . I used the f o l l o w i n g v a r i e t i e s of f l o w e r s : p e t u n i a , s a l p i -g l o s s i s , phlox, alyssum, z i n n i a , and 4 o'clocks. One v a r i e t y a t a time was presented i n a l l a v a i l a b l e c o l o u r s . During any one t r i a l , e i t h e r s i n g l e flowers or c l u s t e r s of comparable s i z e were presented and the number, sex, and a c t i v i t y o f i n s e c t s at each flower were recorded at f i v e minute i n t e r v a l s . Observation periods ranged from 0.5 to 2 hours, but the m a j o r i t y were 1 hour. Time of day, and the s p e c i e s , age, and the number of i n s e c t s i n the cage were recorded f o r each observation p e r i o d . As in experiments w i t h a r t i f i c i a l f l o w e r s , t o t a l o b s e r v a t i o n a l i n t e r v a l s were combined f o r each flower type a f t e r f i r s t comparing s p e c i e s , and time o f day d i f f e r e n c e s . T o t a l frequencies of v i s i t s by males, females and the sexes combined, t o each flower colour were com-pared w i t h Chi-squared t e s t s . Frequencies of v i s i t s devoted t o feeding, r e s t i n g or other a c t i v i t i e s were also compared i n the same s t a t i s t i c a l t e s t s . Because numbers of i n d i v i d u a l s i n each cage d i f f e r e d , frequencies among flower types were not d i r e c t l y compared. Instead, an index of a t t r a c t i v e n e s s to compare r e s u l t s from d i f f e r e n t flowers was used: - 1 5 7 -# v i s i t s to one flower colour # i n s e c t s exposed / t o t a l time ( i n hours) of observation - 1 58 -F i g u r e 20. Spectroradiometer P l o t s of R e f l e c t e d L i g h t From A r t i f i c i a l Coloured Flowers. Spectroradiometer values at 10 nm i n t e r v a l s from 200 t o 900 nm f o r l i g h t r e f l e c t e d from coloured paper sheets on the f l o o r of a syrphid cage. The c o l o u r s of paper are A) blue, B) green, C) y e l l o w , and D) red. - 1 59 -Experiments with A r t i f i c i a l Flowers Four, 10 cm diameter paper flowers (blue, green, y e l l o w , red) each w i t h a sugar cube at i t s center were placed i n a cage o f adult syrphids ( w i t h 11-13 members of each sex unless otherwise noted). At five-minute i n t e r v a l s , the number of i n d i v i d u a l s of each sex and t h e i r r e s p e c t i v e a c t i v i t i e s were recorded f o r each f l o w e r . There were no other food sources i n the cage. Spectroradiometer determinations of energy a t 10 nm i n t e r v a l s between 200 and 900 nm were taken from l i g h t r e f l e c t e d by each c o l o u r . Graphs of the s p e c t r a l compositions are presented i n F i g -ure 20. R e f l e c t e d l i g h t was measured from a l a r g e sheet of each of the coloured papers placed a t a 45° angle to the cage f l o o r , 4-5 cm from the sensor. F i r s t - and second-generation lab-reared M. venablesi and E. v o l u c r i s were used i n a l l but one of these experiments Syrphus o p i n a t o r , from the f i r s t l a b - r e a r e d generation were used during one experiment. The t e s t s r e q u i r e d 0.5-2.5 h to complete. At the end of one ob s e r v a t i o n , t o t a l number of i n t e r v a l s were summed and added to those f o r s i m i l a r experiments on the same s p e c i e s ; e.g. species time # of i n t e r v a l s of observation M. venablesi AM 27 PM E. v o l u c r i s AM PM 63 Syrphus AM 18 M. venablesi (males only) AM 13 - 160 -PM 19 To f a c i l i t a t e comparisons, each group w i t h l e s s than 63 i n t e r v a l s was m u l t i p l i e d by a f a c t o r that brought i t up to 63. Then the frequency of v i s i t s to each f l o w e r w i t h i n such groups was also m u l t i p l i e d by t h i s f a c t o r . Chi-squared t e s t s were used t o compare spontaneous preferences f o r flower colour w i t h i n and between sexes, morning and afternoon v i s i t s , and between s p e c i e s . Results Real Flower Preference Results A) Petunias Six colours of petunia were used; p u r p l e , mauve, r e d , pink, magenta and variegated p u r p l e . A s i n g l e flower o f each was placed i n a g l a s s v i a l i n the syrphid cage. T h i r t y male and 32 female M. venablesi (4-5 weeks old) were observed w i t h these flowers f o r a t o t a l o f 7 hours; 3.7 i n the morning, and 3.3 i n the afternoon. These i n s e c t s had been lab reared f o r 10 generations. As the number of i n s e c t s used was the same f o r each observation p e r i o d , the mean number of i n s e c t s per hour wa s 62. P r o p o r t i o n of v i s i t s spent i n v a r i o u s a c t i v i t i e s were as f o l l o w s : Feeding Resting Others (walk, hover, preen) m f both prop. 0 .29 .26 f r e q . 0 36 36 m f both .92 .44 .49 12 56 68 m f both .08 .27 .25 1 34 35 Males spent s i g n i f i c a n t l y more v i s i t s r e s t i n g while females showed no d i f f e r e n c e s in amounts of time i n each a c t i v i t y . Morning and afternoon v i s i t s d i f f e r e d s i g n i f i c a n t l y . A t o t a l o f 96 v i s i t s (88f, 8m) occurred i n the morning while only 44 (37f, 7m) occurred i n the afternoon. Females v i s i t e d flowers s i g n i f i c a n t l y more - 1 62 -ofte n than males in both morning and afternoon. In morning observations males p r e f e r r e d the mauve flo w e r (P< 0.01) Females v i s i t e d a l l flowers but the purple and red ( t h i s was s i g n i f i c a n t at P< 0.001). In afternoon observations, males v i s i t e d only the purple flower but the numbers of v i s i t s were too low to be r e l i a b l e (7). Females showed a s i g n i f i c a n t preference f o r the mauve and variegated f 1 owe rs . When a l l v i s i t s were combined, males showed no s i g n i f i c a n t p r e f e r -ence f o r any colour while females showed a s i g n i f i c a n t avoidance of pur-p l e . The a t t r a c t i v e n e s s i n d i c e s f o r these flowers are: male female mauve 0. 20 1.09 pur p i e 0.23 0.03 pink 0.03 0.94 red 0.03 0. 13 magenta 0.00 0.44 variegated 0.00 1 .28 The order of preference f o r males i s that l i s t e d , but females pre-f e r r e d i n order; variegated p u r p l e , mauve, pink, magenta, red then pur-p l e . - 163 -Phlox F i v e colours of phlox were presented together i n syrphid cages; peach magenta, p u r p l e , pink (w i t h a dark c e n t e r ) , and white. Small c l u s t e r s of flowers were used and c l u s t e r s were the same s i z e . Cages w i t h between 38 and 40 E_. v o l u c r i s of each sex were observed f o r 3.5 afternoon hours while M. venablesi (31 males, 30 females) were observed f o r 2 afternoon hours. Morning observations c o n s i s t e d of 1 E. v o l u c r i s hour, and 3 hours of females o n l y , both species (25 M., 8 E.) f o r a t o t a l of 9.5 hours. A l l i n s e c t s were f i r s t and second generation l a b -reared and up to one week i n age. Proportions and frequencies of v i s i t s spent i n v a r i o u s a c t i v i t i e s were as f o l l o w s : Feeding Resting Others (walk, hover, preen) m f both m prop. .05 .17 .13 .77 f r e q . 6 36 42 90 ( t o t a l s f o r males and females and both t i o n s ) Both males and females spent sign r e s t i n g than i n any other a c t i v i t y a t t r a c t i v e nectar or p o l l e n sources f o r male v i s i t s r e f l e c t e d i n p a r t , the th t i o n . f both m f both .64 .67 .18 .21 .20 131 221 21 45 66 were used f o r r e s p e c t i v e propor-i f i c a n t l y more v i s i t s (P< .001) These flowers may not have had syrphids. The lower number of ree hours of female only observa-1 64 -P r o p o r t i o n s and frequency of v i s i t s to each flower were as f o l -lows : magenta peach purple m f b m f ' b m f b f r e q . 40 47 87 21 54 75 22 58 80 prop. .34 .22 .26 .18 .25 .23 .19 .27 .24 pink white* m f b m f b f r e q . 24 39 63 9 18 27 prop. .21 .18 .19 .08 .08 0 * white was used i n fewer experiments so was not compared i n Chi squared t es ts . Only males showed a s l i g h t preference f o r one colour (magenta) and only i n morning v i s i t s (P< 0.01). Female v i s i t s and afternoon male v i s i t s were as expected by chance. The o v e r a l l number of v i s i t s to each flower showed there were no s i g n i f i c a n t preferences. When species were considered s e p a r a t e l y f o r t h e i r r e s p e c t i v e spon-taneous preferences, E_. v o l u c r i s males showed a s i g n i f i c a n t (P< 0.001) d i f f e r e n c e from the expected. They v i s i t e d pink and magenta flowers most o f t e n while showing l e a s t i n t e r e s t i n the purple flower. The a t t r a c t i v e n e s s i n d i c e s f o r these flowers were: male female magenta 1.38 1.38 peach 0.73 2.01 - 165 -purpie 0.76 2. 16 pink 0.83 1.45 white 0.26 0.51 The sexes were c o n s i s t e n t only i n f i n d i n g white l e a s t a t t r a c t i v e . S a l p i g l o s s i s Three afternoon hours (2 Meta. 1 Eup.) were spent observing 12 males and 8-12 females, f i r s t generation l a b - r e a r e d , 2-3 week o l d a d u l t s . S ix d i f f e r e n t coloured s i n g l e s a l p i g l o s s i s flowers were i n t r o -duced simultaneously. S a l p i g l o s s i s can be hig h l y variegated w i t h yellow or p l a i n e r i n colour to the human eye. Three of each type were presented: variegated mauve w i t h y e l l o w , burgundy with y e l l o w , l i g h t purple w i t h y e l l o w , p l a i n red, pink, and purple. Mean numbers of i n s e c t s per observed hour was 22.6. Proportions and frequencies of v i s i t s spent i n va r i o u s a c t i v i t i e s were as f o l l o w s : Feeding Resting Others (walk, hover, preen) m f both m f both m f both prop. .10 .05 .07 .80 .74 .77 .10 .21 .16 f r e q . 3 2 5 24 29 53 3 8 1 1 As w i t h Phlox, s i g n i f i c a n t l y more v i s i t s (P< 0.001) were concerned w i t h r e s t i n g rather than feeding. - 1 66 -Frequency and proportion of v i s i t s t o each flower colour were as f o l l o w s : mauve/ burgundy/ I t . p u r p l e / yellow yellow yellow m f b m f b m f b prop. .05 .17 .11 .03 .49 .25 .07 .15 .1 1 f r e q . 3 9 12 2 26 28 4 8 12 red pink purple m f b m f b m f b prop. .52 .04 .29 .26 .02 .14 .07 .13 .10 f r e q . 30 2 32 15 1 16 4 7 1 1 In M. venablesi v i s i t s , males spent a s i g n i f i c a n t l y greater (P< 0.001) number of v i s i t s on the red flower w i t h r e l a t i v e l y few v i s i t s t o other c o l o u r s . Females showed a s l i g h t e r preference f o r burgundy/yellow (P< 0.01). E_. v o l u c r i s males and females showed the same preferences; males f o r the red flower and females f o r the burgundy/yellow (P< 0.001). These sex d i f f e r e n c e s remained when species v i s i t s were combined. The a t t r a c t i v e n e s s i n d i c e s f o r these flowers were: males f emales mauve/yellow 0.38 1 . 35 burgundy/yellow 0.25 3. 90 I t purple/yellow 0.50 1. 00 red 3.75 0. 30 pink 1.88 0. 15 pur p l e 0.50 1. 05 - 1 6 7 -Females seemed to be more a t t r a c t e d to the variegated flowers while males seemed a t t r a c t e d more to the p l a i n e r coloured f l o w e r s . Alyssum Four colours of alyssum; y e l l o w , white, purple and pink were presented i n e q u a l l y s i z e d c l u s t e r s t o cages of syrp h i d s . One e x p e r i -mental cage had both sexes of both species (Meta. 9m 12f, Eup. 15m 18f). A 1.5 hour morning and 3 hours of afternoon observation were done w i t h t h i s cage. Other morning experiments t o t a l e d 3 hours (Meta. =2, Eup. =1). A l l i n s e c t s were f i r s t o r second generation lab-reared and up to 2 weeks i n age. Cages (except f o r the mixed species cage) had means of 3 3 males (+ or - 8.7) and 36 females (+ or - 9.6) i n them. Frequency and proportion of v i s i t s spent i n v a r i o u s a c t i v i t i e s was as f o l l o w s : Feeding Resting Others (walk, hover, preen) m f both m f both m f both prop. .84 .96 .92 .13 .03 .06 .03 .01 .02 f r e q . 427 892 131 9 67 26 93 16 8 24 The frequency of feeding v i s i t s was s i g n i f i c a n t l y greater i n both sexes (P< 0.001) when tested by the Chi squared s t a t i s t i c . T his d i f f e r e d from the other f l o w e r s , where v i s i t s were more f r e q u e n t l y not f o r feed-i n g . Alyssum must be an a t t r a c t i v e nectar and/or p o l l e n source f o r both sexes. Frequency and proportion of v i s i t s to each flower colour were as f o l l o w s : - 1 6 8 -yellow white m f b m f b prop. .39 .64 .55 .22 .20 .21 f r e q . 200 590 790 112 188 300 purple pink m f b m f b prop. .14 .07 .10 .25 .09 .15 f r e q . 70 67 137 128 81 209 O v e r a l l v i s i t s by males were compared with the Chi squared s t a t i s -t i c and found to s i g n i f i c a n t l y (P< 0.001) favour y e l l o w , as d i d the females. White was the colour of second choice f o r both, followed by pink and purp l e . In morning experiments, both sexes of both species showed a s i g n i -f i c a n t (P< 0.001) preference f o r yellow except f o r E. v o l u c r i s males. In afternoon experiments the same p a t t e r n e x i s t e d w i t h s i g n i f i c a n t l y more v i s i t s to the yellow f l o w e r except f o r E_. v o l u c r i s males. These males p r e f e r r e d showed no preference i n the morning while favouring the purple flower in the afternoon (P< 0.001). Females had a s i g n i f i c a n t l y greater frequency of feeding v i s i t s than males while males rested s i g n i f i c a n t l y more often than females (P< 0.001 f o r both). N e i t h e r sex p a r t i c i p a t e d i n "other" a c t i v i t i e s t o a greater degree than the other. - 1 69 -The a t t r a c t i v e n e s s i n d i c e s f o r each flower colour f o r each sex we re: male female yellow 7.59 20. 1 1 white 4.25 6.41 pink 4.86 2.76 pur p i e 2.66 2.28 Both sexes had the same order of a t t r a c t i o n to flower c o l o u r s . The higher i n d i c e s of these flowers compared to the others i s also r e f l e c t e d i n the r e l a t i v e l y h i g h proportion o f v i s i t s t o number of i n s e c t s exposed. These flowers were c l e a r l y the most a t t r a c t i v e o f a l l t es te d. Z i n n i a s Five colours of z i n n i a ; l i g h t and dark orange, l i g h t and dark pink, and green, were presented i n cages of E. v o l u c r i s or M. venablesi w i t h a mean of 34 males (+ or - 9.7) and 39 females (+ or - 12.6). Three morning hours (Meta.= 2, Eup. = 1) and two afternoon hours (Meta.= 1, Eup.= 1) were observed. A l l i n s e c t s were second generation l a b -reared and up to one week of age. Proportion and frequency of v i s i t s spent i n various a c t i v i t i e s were as f o l l o w s : Feeding Resting Others (walk, hover, preen) m f both m f both m f both - 170 -prop. .08 .28 .19 .77 .63 .69 .15 .09 .12 f r e q . 8 33 41 75 74 149 15 10 25 Both males and females spent s i g n i f i c a n t l y more (P< 0.001) v i s i t s r e s t i n g than i n other a c t i v i t i e s . When compared to males though, females spent a greater proportion of t h e i r v i s i t s feeding than males. Proportions and actu a l frequencies of v i s i t s to each colour of flower were as f o l l o w s : l i g h t dark l i g h t orange orange pink m f b m f b m f b prop. .14 .16 .15 .14 .22 .19 .23 .16 .20 f r e q . 14 19 33 14 26 40 23 19 42 dark pink green m f b m f b prop. .33 .21 .27 .15 .24 .20 f r e q . 32 25 57 15 28 43 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 colour preference f o r o v e r a l l r e s u l t s of males or females or r e s u l t s combined. When morning and afternoon r e s u l t s were separated, morning showed no preferences while i n the afternoon there was a s l i g h t avoidance of the dark orange flower (P< 0.01). - 1 7 1 -A t t r a c t i v e n e s s i n d i c e s f o r these flowers f o r each sex were as f o l -lows : males females dark orange 0.41 0.48 l i g h t orange 0.41 0.66 dark pink 0.67 0.48 l i g h t pink 0.93 0.64 green 0.44 0.71 As was expected, a l l i n d i c e s were low i n value and s i m i l a r i n both sexes. 4 0'clocks Only one hour of observation w i t h second generation M. venablesi up to one week i n age w i t h four flower c o l o u r s was recorded; r e d d i s h p u r p l e , variegated pink w i t h white, white, and yellow. The experiment was done i n the morning w i t h 32 males and 31 females. P r o p o r t i o n of v i s i t s spent i n v a r i o u s a c t i v i t i e s was as f o l l o w s : Feeding Resting Others (walk, hover, preen) m f both m f both m f both prop. .40 .92 .83 .20 .02 .05 .40 .06 .12 f r e q . 4 45 49 2 1 3 4 3 7 - 172 -Males showed no preference f o r any a c t i v i t y , while females fed s i g n i f i c a n t l y more o f t e n ( P < 0.001), almost to the e x c l u s i o n o f other a c t i v i t i e s . Frequency and proportion of v i s i t s to each flower colour were as f o l l o w s : red/purple var. pink/white prop, f r e q . m 0 0 f b .10 .0c 5 5 white i f b .90 .57 .63 28 37 ye1low m f b m f b prop. 06 .05 . 10 .27 .24 f r e q . 0 3 3 1 13 14 Chi squared a n a l y s i s of the frequency of v i s i t s to each colour showed s i g n i f i c a n t l y more (P< 0.001) male and female v i s i t s to the variegated pink/ white flower (though male frequencies were too low t o be r e l i a b l e ) . The yellow f l o w e r received the second greatest p r o p o r t i o n of v i s i t s by both sexes. The a t t r a c t i v e n e s s i n d i c e s f o r these flowers were: male female red/purple 0.00 0.16 var. pink/white 0.28 0.90 ye 1 low 0.03 0.42 white 0.00 0. 10 - 173 -The in d i c e s r e f l e c t the s i g n i f i c a n t preference recorded f o r the variegated pink and white f l o w e r , followed by the yellow f l o w e r as second choice. Unfortunately only one hour was observed w i t h t h i s flower as at l e a s t f o r females, i t seemed to be an a t t r a c t i v e feeding source s i m i l a r to alyssum. Only two of the flowers presented t o i n s e c t s represented favour-able food sources, alyssum and 4 o'clocks. In these, preferences by males and females were f o r the same flower colour but the same colour was not p r e f e r r e d i n each type o f flower. For the other f l o w e r s , v i s i t s were mainly concerned w i t h r e s t i n g , and each v a r i e t y showed d i f f e r e n t species preferences. There was no o v e r a l l spontaneous a t t r a c t i o n f o r any one common co l o u r , a t l e a s t as discerned by the human eye. Most flowers used i n experiments were placed e i t h e r under UV l i g h t or photographed w i t h a UV f i l t e r . Their v a r i o u s fluorescences or l a c k of i t was noted. Of the pet u n i a s , only the pink showed enough UV r e f l e c t a n c e to d i s t i n g u i s h the flowers from the f o l i a g e . Alyssum flowers a l l appeared f l a t and dark l i k e the f o l i a g e when photographed i n the garden. When examined i n a UV chamber however, the white flower p e t a l s r e f l e c t e d s l i g h t l y , c o n t r a s t i n g w i t h a darker center. Of the z i n n i a s placed i n the UV chamber, (green, pink and dark orange) only exposed p o l l e n f l u o r e s c e d and was q u i t e apparent against the f l a t dark background o f the f l o w e r s . Only the pink phlox w i t h the red center was examined under UV l i g h t . I t showed none o f the contrast apparent under white l i g h t , i t appeared f l a t and black. - 174 -The presence of r e f l e c t e d UV l i g h t then seemed to have l i t t l e b earing on the colour choices where noted. Paper Flower Experiments The number of v i s i t s per flower (adjusted f i g u r e s , see m a t e r i a l s and methods) f o r each category of experiment are given below: Red Blue species M. venablesi time m f b m f b AM 68 65 133 61 89 1 49 PM 56 52 108 60 62 122 E. v o l u c r i s AM 32 10 42 35 3 38 PM 38 20 52 19 19 38 Green Yellow species time m f b m f b M. venablesi AM 61 68 128 72 33 105 PM 47 66 1 13 62 49 111 E. v o l u c r i s AM 52 4 56 18 39 57 PM 42 6 48 29 26 55 M. venablesi and E_. v o l u c r i s were compared e x t e n s i v e l y as most experiments involved these two sp e c i e s . M. venablesi females showed a spontaneous preference f o r the blue flower (P< 0.001) i n the morning experiments while the yellow f l o w e r received the fewest v i s i t s . There was no preference i n afternoon experiments. Males showed no preference at any time. E. v o l u c r i s females showed a preference i n the morning f o r - 1 7 5 -the yellow f l o w e r (P<0.001), while males showed a preference f o r the green flower (P< 0.001). Females devoted more v i s i t s to the yellow f l o w e r in the afternoon but not at the P=0.001 l e v e l . There was a s i g -n i f i c a n t r e j e c t i o n of the green flower f o r feeding (by females). There was no s i g n i f i c a n t d i f f e r e n c e between number of v i s i t s i n morning and afternoon f o r each species.. When i n d i v i d u a l c o l o u r s were compared between morning and afternoon, M. venablesi v i s i t s showed no d i f f e r e n c e in preference, not even in female v i s i t s to blue. When morn-ing and afternoon M. venablesi v i s i t s are combined, the female p r e f e r -ence f o r blue i s s t i l l s i g n i f i c a n t (P<0.001). When v i s i t s t o i n d i v i d u a l c o l o u r s are compared between morning and afternoon f o r E_. v o l u c r i s o n l y , females spent s i g n i f i c a n t l y l e s s time v i s i t i n g the blue flower in the morning over the afternoon. Combined morning and afternoon E. v o l u c r i s v i s i t s s t i l l showed a preference f o r green by males and f o r yellow by females. Females also v i s i t e d green l e s s o f t e n than any of the other colour s. A comparison of t o t a l v i s i t s by each species shows that M. vena- b l e s i made a s i g n i f i c a n t l y greater number of v i s i t s than did E. v o l u c r i s . The same held true f o r number of v i s i t s to each c o l o u r . Syrphus opinator experiments were paired w i t h an i d e n t i c a l e x p e r i -ment w i t h E_. v o l u c i s so that numbers of observation i n t e r v a l s were s i m i -l a r (18 Syrphus, 21 Eupeopdes) and i n d i v i d u a l s i n the respec t i v e cages were almost equal (40m 30f Syrphus, 39m 41f Eupeodes). A l l Syrphus r e s u l t s were m u l t i p l i e d by a f a c t o r of 1.17 t o compare them w i t h Eupeodes. Eupeodes experiments were a l l afternoon while Syrphus were a l l morning but as t h i s had caused no d i f f e r e n c e s in past experiments, - 1 7 6 -they were considered equivalent. Resultant v i s i t s to each f l o w e r colour by each sex of each s p e c i e s were as f o l l o w s : Red Blue Green Yellow species m f both m f both m f both m f both Syrphus 21 0 21 0 1 1 9 0 9 8 1 1 19 Eupeodes 6 10 16 6 7 13 20 0 20 9 5 14 S. opinator males showed a s i g n i f i c a n t preference f o r red w i t h a r e j e c t i o n of blue by Chi squared a n a l y s i s . S. opinator females showed a s i g n i f i c a n t preference f o r yel l o w . In f a c t , they fed almost e x c l u s i v e l y on the yellow f l o w e r . E_. v o l u c r i s males, as expected from the e a r l i e r experiments, s i g n i f i c a n t l y p r e f e r r e d (P< 0.01) the green flower. Females did not feed at a l l on the green and they avoided i t i n previous afternoon experiments. - 177 -D i s c u s s i o n Real Flower Spontaneous Colour Preferences There was nothing of c o n s i s t e n t a t t r a c t i o n between flower species e i t h e r i n v i s i b l e r e f l e c t e d l i g h t or UV r e f l e c t e d l i g h t . The a t t r a c t i o n of the y e l l o w alyssum may have had more to do w i t h " v i s i b l e " l i g h t o r the i n s e c t s s p e c i a l s e n s i t i v i t y to yellow l i g h t . With the 4 o'clocks, the patterned pink and white flower was more a t t r a c t i v e than the y e l l o w . This may have had to do w i t h an a t t r a c t i o n to p a t t e r n s . Further e v i -dence came from the s a l p i g l o s s i s observation that females found the variegated patterned flowers most a t t r a c t i v e (while males found the reverse) even though there was no a p p r e c i a b l e amount of feeding on them. The other variegated flower p r e f e r r e d by females was the variegated pur-p l e petunia. The yellow 4 o'clocks d i d not appear as b r i g h t to the human eye as the yellow alyssum, and t h i s may have also been the case f o r s y r p h i d s . That syrphids can probably d i s t i n g u i s h wavelengths as long as y e l -low has been shown by c l a s s i c a l t r a i n i n g experiments f o r E r i s t a l i s tenax by U s e (1949) and f o r M. venablesi by Smith (1981 pers. comm.). In the l a t t e r experiments, l a b reared male and female M. venablesi were t r a i n e d to y e l l o w and blue but were not able to recognize ( i . e . t r a i n to) red food sources. This suggests they are s e n s i t i v e t o wavelengths up to approximately 59U nm, but not to those above. Bishop (1974) has shown with r e t i n a l c e l l r ecordings, that E.tenax a l s o has f u n c t i o n a l UV receptors though these have not been shown to operate at the behavioural l e v e l yet. More - 1 78 -r e c e n t l y , other photo rec e p t i v e c e l l s have been found i n the compound eye of E. tenax. Besides a UV r e c e p t o r w i t h peak s e n s i t i v i t y at 340-360 nm, Horridge et a_l (1975) showed the existence of a UV and blue s e n s i t i v e c e l l w i t h peaks at 350 and 450, and a " v i s i b l e " l i g h t receptor c e l l w i t h a l a r g e peak at 520 nm (green) and smaller peaks up i n t o the yellow r ange. I t seems p l a u s i b l e that other syrphids, e s p e c i a l l y those feeding i n the same h a b i t a t s as E. tenax ( i . e . f l o w e r s ) possess the same v i s u a l s e n s i t i v i t y . The spontaneous preferences f o r paper flowers and r e a l flowers shown here are evidence f o r colour d i s c r i m i n a t i o n . Some other d i p t e r a tested f o r spontaneous responses to colour and showing no preferences, are L u c i l i a spp. (Tachinidae), C a l l i p h o r a spp. ( C a l l i p h o r i -dae) and Sarcophaga spp. (Sarcophagidae) (Kugler 1956). Spontaneous Colour Feeding Preferences : A r t i f i c i a l Flowers Outdoor observations suggested t h a t there might be d i f f e r e n c e s i n feeding preferences between morning and afternoon. Although there was no d i f f e r e n c e in t o t a l v i s i t s between the two periods, the three p r e f e r -ences that were d i s p l a y e d a l l appeared during the morning experiments. In f a c t , most outdoor feeding was observed during the morning hours. I t therefore seemed p o s s i b l e that the o v e r a l l greater number of v i s i t s by M. venablesi and the females preference f o r blue might be associated w i t h t h e i r outdoor feeding h a b i t s . M. venablesi required f o r indoor c o l o n i e s c o u l d be caught most o f t e n near or on f l o w e r s , whereas II. v o l u c r i s was found mainly along grassy verges i . e . where uncut grass had gone to f l o w e r . Perhaps t h i s attachment t o grassy edges may help to - 1 7 9 -e x p l a i n the preference f o r green that E. v o l u c r i s males e x h i b i t e d , and may also e x p l a i n observed i n t e r s p e c i f i c d i f f e r e n c e s i n female p r e f e r -ences . The actual c o l o u r s that species p r e f e r r e d may be more apparent than r e a l . The four colours were chosen by human colour perception. Even the information from the spectroradiometer i s o n l y of l i m i t e d value i n making such s e l e c t i o n s . T h i s instrument g i v e s q u a l i t a t i v e informa-t i o n about s p e c t r a l composition, not about an i n s e c t s ' perceptions of that composition. I t may be more important t o note that each coloured paper r e f l e c t e d the whole spectrum of wavelengths, though i n d i f f e r i n g amounts, th a t were r e l a t e d t o the co l o u r . Since a l l wavelengths were present i n d i f f e r i n g amounts, the i n s e c t eye need not have been respond-ing t o the one colour the human eye perceived i n any one paper fl o w e r . I n t e n s i t y v a r i e d between f l o w e r s , as did c o n t r a s t w i t h the white cage f l o o r . Brightness a l s o was a f a c t o r that could not be e a s i l y con-t r o l l e d , s i n c e i t i s a s u b j e c t i v e q u a l i t y dependent on the s e n s i t i v i t y of a p a r t i c u l a r species t o a p a r t i c u l a r stimulus. The s u b j e c t i v e nature of b r i g h t n e s s has not often been defined i n i n s e c t colour v i s i o n e x p e r i -ments (Neumeyer 1980), f o r i t depends on the d i f f e r i n g s e n s i t i v i t y o f a p a r t i c u l a r i n s e c t eye to v a r i o u s wavelengths. S e n s i t i v i t y i n t u r n depends on how many elements or c e l l s i n the eye are s e n s i t i v e to a par-t i c u l a r q u a l i t y o f l i g h t . For i n s t a n c e , i f the eye i s very s e n s i t i v e to yel l o w , the amount of yellow i n any paper flower w i l l c o n t r i b u t e to i t s apparent b r i g h t n e s s . Electroretinogrammes o f f e r a more o b j e c t i v e method of determining an i n d i v i d u a l s ' perception of brig h t n e s s . - 180 -Nevertheless, i n these experiments with coloured papers, even though brightness was not known, nor intensity controlled, i t i s worth noting the', fact that different species had different spontaneous preferences. I f brightness or intensity were the only factors involved, a l l species and both sexes should have dislayed a narrow range of responses. That they didn't suggests that hue was also important. 

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