Ultrastructure of the femoral chordotonal organs and their novel synaptic organization in the legs of Drosophila melanogaster Meigen (Diptera : Drosophilidae)

Abstract The chordotonal or scolopophorous organs located in the femoral segment of all the legs of both male and female Drosophila melanogaster (Diptera : Drosophilidae) were examined by light and transmission electron microscopes. The femoral chordotonal organs (FCO) are arranged in 3 groups: one large group consists of about 32 aligned scolopes whose distal ends extend and terminate at the distal epicuticular surface of the femur; the other 2 central groups together contain about 42 scattered scolopes, and distally they terminate into the membranes connecting the muscles of the femur. No sexual dimorphism is evident either in the numbers of scolopidial groups or the total number of scolopidia in both sexes. Each scolopidium is innervated by 2 neurons of which one is less electron-dense than the other. The dendrites of these neurons bear sensory cilia. The fine structure of these chordotonal sensilla suggests that they probably respond to stretch or flexion. Proximally in the femur, the axons from FCO form a novel glomerular organization. These axons show lateral extensions and form different morphological types of synapses among themselves. From the presence of a large number of putative chemical synapses in the legs, it is evident that some degree of information processing is taking place in D. melanogaster at the periphery before being relayed to the central nervous system.