94 



TJie Fly of CJiironomus 



Auditory 

 organ 

 in the 

 antenna. 



no large liairs or setae ; it is mucli narrowed at its inser- 

 tion into the second joint. 



The second joint in Chironomidae and Culicidae 

 (especially in the males) exhibits a peculiar structure, 

 which is believed to serve for the perception of sound 

 (figs. 64-69). This joint swells out into a nearly globular 

 capsule, four or five times as wide as any of the succeed- 

 ing joints. On its upper surface ^ is the deep socket 

 for the third joint, which is incompletely divided into 

 an upper and a lower cavity by a horizontal, circular 

 shelf. The chitinous roof of the lower cavit}^, which 



we shall call the striated 

 plate, is convex upwards, 

 and perforated by a central 

 hole for the base of the third 

 joint, from which radiate 

 many close-set striae. 



The internal structure of 

 the second joint can only be 

 investigated by thin sections 

 and other delicate methods. 

 The striated plate is con- 

 tinued into the cavity of the 

 joint as a thin sheet, stiffened by very numerous radi- 

 ating fibres (the peripheral fibres), which, like the sheet 

 which unites them, are of chitinous substance. In Chiro- 

 nomus the sheet curves downwards from the socket of 

 the third joint, and forms a kind of dome (fig. 66). In the 

 gnat (figs. 67, 68) it curves upwards from the socket, and 

 forms a kind of basin. Each fibre is exactly in line with 

 one of the radiating striae on the striated plate. Outside 

 the peripheral fibres (i. e. between them and the outer 

 wall of the second joint), and to a less extent on the 



Fig. 66. — Vertical section ot' en- 

 largeil second joint of antenna ot 

 male Cliirononius-fly. x 150. 



' The antenna in this description is supposed to stand upright, with 

 the attached base downwards. 



