Abstract
The olfactory sensing system of the syrphid fly Eupeodes corollae is essential in pollination and prey localization, but little is known about the ultrastructural organization of their olfactory organs. In this study, the morphology, distribution, and ultrastructural organization of antennal sensilla of E. corollae in both sexes were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Neuronal responses of a subtype of sensilla basiconica to floral scent compounds were recorded by single sensillum recording (SSR). Ten morphological types, including Böhm bristles, sensilla chaetica, microtrichiae, sensilla trichodea, sensilla basiconica, sensilla clavate, sensilla coeloconica, sensilla styloconica, sensilla placodea, and sensory pit, were identified. Except for Böhm bristles and sensilla chaetica, which were distributed on the scape and pedicel of E. corollae antennae, innervated sensilla were densely distributed on the flagellum, a vital sensory organ. Further, observing ultrastructural organization showed that the sensilla trichodea, basiconica, and clavate are single-walled with multiple nanoscale pores perforating the cuticle. Sensilla coeloconica are double-walled and have no wall pores, but instead, have longitudinal grooves along with the pegs. Sensilla chaetica, Böhm bristles, and microtrichiae did not have wall pores on the cuticle or sensory cells at the base. The SSR results indicated that neuron B housed in the subtype of sensilla basiconica I (SBI) mainly responded to methyl eugenol and other aromatic compounds. Overall, our results provide valuable information to understand the morphology and ultrastructure of antennal sensilla from E. corollae. These findings are beneficial for the studies of the neuronal function map of olfactory sensilla and for determining evolutionary relationships in Diptera.
Highlights
The insect olfactory system allows sensitive detection and precise discrimination of relevant odor cues from natural surroundings (Bruce et al, 2005; Renou and Anton, 2020)
Scanning electron microscopy observation revealed that the antennae of both sexes of E. corollae were composed of three segments, scape (Sc), pedicel (Pe), and flagellum (Fl)
long SC (LSC) was observed in the distal region of the Sc and Pe interspersed by numerous bristles, whereas short SC (SSC) was only found on the Pe
Summary
The insect olfactory system allows sensitive detection and precise discrimination of relevant odor cues from natural surroundings (Bruce et al, 2005; Renou and Anton, 2020). These cues include semiochemicals released by food sources, oviposition sites, predators, or competitors and pheromones emitted from conspecifics (Fleischer et al, 2018; Renou and Anton, 2020). Various volatile organic compounds are detected by odorant receptor neurons (ORNs) housed in the sensilla, hairlike structures that extend from the insect cuticle with multiple cuticular pores on the antennae and maxillary palps (Steinbrecht, 1997; Keil, 1999). The higher olfactory center reintegrates and processes this information and causes the insect to produce corresponding behavioral responses (Bates et al, 2020)
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