Characterisation of mechanosensory structures on the antennal flagellum in Daphnis nerii

Abstract

The insect antenna is a multi-sensory organ, which senses odour, mechanical as well as thermal and humidity cues. The significance of mechanosensory feedback provided by antennae has been particularly highlighted in recent studies. Mechanosensory cues such as touch, sound, airflow, aerial rotations etc. are detected by the exquisitely sensitive Johnston’s organs situated in the basal antennal segments. Another set of mechanosensors, antennal hair plates or Böhm’s bristles, have been implicated in precise positioning of the antennae through proprioception, especially during locomotion. The mechanosensory cues transduced by these sensors are crucial for flight stability. Although the role of Johnston’s organs and Böhm’s bristles has received much attention, there are also several mechanosensors along the length of the antennal flagellum in moths, whose role is relatively unexplored. We know very little about the nature and distribution of mechanosensors along the flagellum, or their role in insect behaviour. In this thesis, we explore the morphology and function of flagellar mechanosensors in the study system of the Oleander hawkmoth (Daphnis nerii), using various techniques such as scanning electron micrographs, neural dye fills, confocal microscopy, behavioural assays and electroantennography. Scanning electron micrographs revealed the location of putative mechanosensors on every annulus, except the first few annuli. Fluorescent dye labelling of sensory neurons innervating the flagellum (whole flagellar fill) revealed central projections in the ipsilateral Antennal Lobe (AL), ipsilateral Antennal Mechanosensory and Motor Centre (AMMC), ipsilateral and contralateral Sub-Esophageal Zone (SEZ), and also into the mesothoracic ganglia. We hypothesized that flagellar mechanosensors mediate specific behaviors such as antennal grooming by insects, but our attempts to assay these behaviours were unsuccessful. We also tried to measure their response to mechanical stimuli such as wind, using electroantennography (EAG). Together, these data describe the nature and distribution of flagellar mechanosensors in Daphnis nerii.