GRASSHOPPER ABDOMEN SNODGRASS 



simple oval depression of the tergum (fig. 2 A), the margin of which 

 is interrupted ventrally, and the tympanum is thus continuous through 

 a narrow cleft in its frame with the membranous body wall below the 

 tergum. The same is true but less evident in Dissosteira (fig. i). The 

 development of the organ in the nymph shows clearly that the tym- 

 panum is derived from a part of the laterodorsal membrane of the first 

 abdominal segment enclosed in a notch in the lower margin of the 

 tergum. In Dissosteira the tympanal depression is much deeper than 

 in Melanophis and forms a large capsule-like cavity expanded within 

 the outer opening (fig. 6 A, x). The rear wall of the capsule is deeper 



Fig. 6. — Tympanal organ of the first abdominal segment. Dissosteira Carolina. 



A, external view of left tympanal capsule and surrounding parts of body wall. 

 B, inner view of right tympanum and associated structures. C, tympanal sense 

 organ and its supports, inner view. 



a, sclerotic tubercle of tympanum with arms (b, c) supporting the sense 

 organ; CpCls, cap cells of sense organ; d, pyriform sclerite of tympanum; e, 

 muscle process of tympanal capsule; ISp, first abdominal spiracle; IT, tergum 

 of first abdominal segment; Nv, nerve of sense organ; sPh, part of third 

 phragma ; PN-.-., lateral part of postnotum of metathorax ; SCls, sense cells ; 

 Sco, sense rods, scolopes ; SO, scolopophorous sense organ ; Tm, tympanum ; 

 n, subtympanal lobe of metathorax; x, tympanal capsule; /^6. tensor muscle of 

 tympanum ; 14/, dilator muscle of spiracle ; 148, occlusor muscle of spiracle. 



than the front wall, and the plane of the tympanum is, therefore, 

 oblique, its outer surface being directed outward and posteriorly. The 

 first abdominal spiracle {ISp) is situated in the anterior wall of the 

 tympanal capsule, and a lobe (m) at the lower end of the metathoracic 

 epimeron (fig. i) forms the lower lip of the outer opening of the cap- 

 sule. In the Acridinae the tympanal capsule is much narrower than 

 in Oedipodinae and Cyrtacanthacrinae. 



The tympanum is a thin mcmljrane stretched tightly between the 

 inner margins of the tympanal depression or capsule (fig. 6B). A 

 small apodemal process (e) projects ventrally from the lower margin 

 of the latter and gives insertion to a muscle {146) arising ventrally 

 from a point in the membrane behind the base of the metacoxa laterad 



