NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 6l 



tion, gives off a sensory branch (fig. 28, SgNv) that innervates the 

 upper part of the siibgenual organ. The tympanal nerve (TmNv) 

 consists entirely of sensory fibers. It follows the anterior wall of 

 the leg and, in the upper part of the tibia, gives off branches to the 

 lower part of the subgenual organ (SgO) and to the intermediate 

 organ (ImO), while its main trunk ends in fibers to the sense cells 

 of the crest (TmO). 



The subgenual organ (SgO) has the usual chordotonal structure, 

 and is probably the homologue of the chordotonal organ that occurs 

 in a similar position in the legs of some other insects. It is attached 

 by the converging ends of its elongate cap cells to the posterior wall 

 of the leg just above the posterior tympanum. Its sensilla radiate 

 inward and upward, like the ribs of a fan, in cross-section. 



The intermediate organ (ImO) consists of an irregular mass 

 of sensilla lying at the upper end of the crest on the outer surface 

 of* the anterior trachea. Its distal end is attached to the outer wall 

 of the leg by a strand of fibrous and cellular tissue (not shown in 

 the figure). 



The tympanal organ (TmO) is the organ of particular interest 

 in the Tettigoniid leg, because its sensilla are arranged serially and 

 their outer ends, containing the scolopalse, form a crest-like ridge 

 (Cr) along the outer surface of the anterior trachea (aTr), in which 

 the cap cells and scolopalse decrease in size from above downward. 

 This arrangement, suggesting that the receptive elements are graded 

 to respond to different wave lengths of sound, has strengthened 

 the general belief that the organ has an auditory function. 



The sense cells of the tympanal organ lie along the anterior 

 margin of the trachea, in the angle between the latter and the anterior 

 tympanum (figs. 28, 29 A, B, Scl) where each receives a fiber from 

 the tympanal nerve trunk (fig. 28, TmNv). The distal processes 

 of the sense cells, and the basal parts of the enveloping cells extend 

 posteriorly on the surface of the trachea to its middle where they 

 turn abruptly outward in the crest (Cr). The ridge of the crest is 

 formed by the cap cells (fig. 29 B, CCl). The crest is covered by a 

 " crest mass" (a), which Schwabe (1906) says consists of a fibrous 

 matrix in the meshes of which there is a clear substance and a varying 

 number of scattered cells. Finally, both the crest and the intermediate 

 organ are ensheathed in a delicate membrane (Mb) reflected on one 

 side from the inner wall of the anterior tympanum and on the other 

 from the wall of the trachea. 



The tympanal chordotonal organ of the Tettigoniidas appears, at 

 first sight, to differ from all other sense organs in having its cap cells 



