External morphology of sensilla in the sacculus of an antennal flagellum of the fruit fly Drosophila melanogaster Meigen (Diptera: Drosophilidae)

Sensilla in the sacculus of an antennal 3rd segment, a funiculus, of the fruit fly, Drosophila melanogaster (Diptera : Drosophilidae) have been examined by scanning electron microscopy. The sacculus was divided into 3 cavities in its interior. A morphologically distinct group of sensilla was present in each cavity. Grooved sensillum (GS), found in the largest cavity, was further subclassified on the basis of the side wall sculpture into 3 subgroups: GS Ia, GS Ib and GS II. GS Ia was 4 μm long and had 10-12 grooves (0.25 μm wide) and GS Ib was 3.8 μm long and had 6-9 grooves (0.25-0.4 μm wide). GS 1a and GS Ib were inferred to be olfactory and thermoreceptive, and olfactory, respectively. GS II was 3.2 μm long, and had 4-5 grooves. Basiconic sensillum (BS), found in the smallest cavity, was 4.5 μm long and had an irregularly sculptured side wall, suggesting the presence of numerous irregular-shaped olfactory pores. Blunt-tipped sensillum (BTS), found in the middle-sized cavity, was 1.9 μm long and had a smooth-surfaced side wall and a button-like structure on its apex. These features suggested that BTS was hygro- and thermoreceptive.