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


Antennal sensilla of Metasyrphus venablesi (Cn.) and Eupeodes volucris 0.S. (Diptera: Syrphidae) were studied by scanning and transmission electron microscopy. Males and females both had four types of sensilla. Three of these, two multiporous perforated (MPP) sensilla (one round-tipped and one pointed), and a grooved peg, multiporous sen-sillum, were also confirmed by SEM on the following species: Syrphus torvus (♂, ♀) Scaeva pyrastri (♂, ♀), Dasysyrphus amalopsis (♀), Xantho- grarnma flavipes (♀), Brachyopa perplexa (♂), Pipiza sp. (♀), Xylota sp. (♂). The fourth MPP sensillum had thicker walls and fewer pores. All four types were located among dense non-innervated setae on the antennal bulb and appeared to be olfactory. EAG study of the antennae of female M. venablesi and E. volucris showed that both species responded to: common green plant volatiles, trans- and cis-2-hexen-1-ol, trans- and cis-3-hexen-1-ol, cis-3-hexenylacetate, and hexanol; other volatile plant substances, methylsal-icylate and amylacetate; crushed carnation petals and crushed aphids. There was no response to honeydew or some of its components (e.g. tryptophan, indolealdehyde or indoleacetaldehyde) nor was there a response to water vapour. A gustatory sensillum on the ovipositor of these two species was studied by scanning and transmission electron microscopy and by neuro-physiological methods. One mechanosensitive and four chemosensitive neurons innervate each hair. The chemosensitive neurons are exposed to the exterior by a terminal pore, and respond to honeydew, tryptophan, indoleacetaldehyde, alanine, sucrose, and water. Labellar hairs are also sensitive to sucrose. Olfactometer study of M. venablesi and E. volucris showed that olfactory stimulation by flowers would induce searching by both sexes. A mixture of tryptophan and indoleacetaldehyde induced mated females to search for and locate the stimulus. Other components of the oviposition 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 oviposition stimulus were presented on green glass rods to mated females. The attractive elements included fresh dead aphids, honeydew, crushed bean, tryptophan and indoleacetaldehyde, and clusters of black spots similar in size to aphids. Males were attracted only to honeydew and crushed bean. Both sexes responded to potential food sources, such as honeydew, but only females responded to aphids and attractants that characterized oviposition sites. A stimulus-response sequence is proposed for these aphidophagous syrphids that involves dual and/or multiple stimulus combinations.

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