NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 



59 



(SB) first discovered by Miiller (1826), and sometimes known 

 as the organ of Miiller, which consists of a mass of scolopophorous 

 sensilla. Some are attached by short thick cap cells to a peg-like 

 invagination of the tympanum (c), others by long cap cells to a 

 pear-shaped thickening of the membrane (a). The nerve of the 

 organ (Nv) goes to the ventral ganglion in the third thoracic seg- 

 ment. Three large air sacs are applied to the inner surface of the 

 tympanum. The large membranous tympanum, set in a cavity of 

 the body wall, suggests an ear drum, and on this suggestion, rather 

 than on concrete evidence, is based the persistent idea that this 



EW. 



Tm 



Fig. 27. — The '"ear" of a grasshopper (Dissostcira Carolina). 



A, external view of tympanum (Tm) on side of first abdominal 

 tergum (IT) : a, pear-shaped thickening of tympanum ; b, external pit 

 forming peg on inner surface (B, c) ; d, dorsal supporting arm of in- 

 ternal peg; EW, wall of external tympanal cavity; IT, lateral part of 

 first abdominal tergum ; Sp, first abdominal spiracle ; Tm, membranous 

 tympanum ; v, ventral arm of internal peg. 



B, inner view of tympanum and wall of tympanal cavity ; c, hollow 

 chitinous peg projecting from tympanum and supporting the sensory 

 body (SB) ; Mel, tensor muscle of tympanum; Nz', chordotonal nerve; 

 SB, chordotonal sensory body, a branch of which goes to the pear- 

 shaped thickening (a). Other letters as on A. 



chordotonal organ of the grasshopper is an organ of hearing, and, 

 by inference the idea that all chordotonal organs have an auditory 

 function. 



In the fore tibiae of the Tettigoniidae (Locustidai) and the Gryl- 

 lidse, there are chordotonal organs also associated with tympana, but 

 not attached to them. The two tympanal areas at the upper end of 

 each tibia lie at the sides of two divisions of the principal leg trachea 

 (fig. 28, aTr, pTr), one anterior, the other posterior (oriented thus 

 with the leg extended at right angles to the body). In the Tetti- 

 goniidae (figs. 28, 29) the tympana (aTm, pTm) are covered by 

 folds of the leg wall, forming tympanal cavities {TC, TC) opening 

 through slits toward the outer surface of the leg (fig. 29, e, e). In 



