Johnston's organ of the black fly Simulium vittatum Zett.

Abstract

Ultrastructure of the Johnston's organ was similar in both sexes of the black fly Simulium vittatum. Three types of scolopidia were found: types A, B, and C. Type A and B scolopidia are amphinematic and type C scolopidia are mononematic. Males had slightly wider pedicels than females and bore about 30 type A, 150 type B, and 4 type C scolopidia. Females had about 20 type A, 105 type B, and 5 type C scolopidia. The basic structure and location of each type are similar to, although less well organized than, these types in the adult mosquito. The number of type A and B scolopidia, while much lower than that found in mosquitoes, is similar to the number in other insects. Each amphinematic scolopidium has either two (type A) or three (type B) sensory cells, as well as three sheath cells and a long cuticular cap. A mononematic type C scolopidium has two sensory cells, a sheath cell, a short cap, and a microtubular cap cell. The structure of the Johnston's organ complements previous behavioural observations indicating that sound detection is absent or minimal in the black fly, even in the mate-seeking males. The probable function of each scolopidial type is discussed as it may relate to flying and wind detection.