220 THE CAUSES WHICH PROPAGATE 



mainly oecupied by a large centrally-constricted sub-oesophagal 

 air-bladder (Plate III., Fig-. 5a, m,m), presenting an instance of the 

 coalescing of the metathoracic and anterior abdominal vesicles, 

 which extends from the anterior of the metathorax to the 

 tennination of the second abdominal segment in the female, 

 sixth in the male; anteriorly, this bladder is closed by two sym- 

 metrical chitineous partitions forming the posterior termination 

 of the mesothorax; posteriorly, it is lined by an oblique 

 membranous diaphragm inclined from below forwards, and 

 above covering the viscera ; it is also divided longitudinally 

 into two symmetrical parts by a membranous partition [d) . 

 In this bladder are situated the organs of music and hearing 

 placed respectively in the first and second abdominal segments, 

 which are abnormal and constricted, and here they form local 

 boundaries, ap2)areutly but indurations and attenuations in the 

 texture of its membranous lining, with which they seem con- 

 tinuous. The organs of music usually take develojjment in the 

 male sex, and are only indicated and impotent in the females. 

 They consist in tAAO outwardly convex parchment-like mem- 

 branes (t. Figs. 5a, 5b), situated at either side of the dorsal arcs 

 of the first abdominal ring, that exhibit transverse chitineous 

 indurations, calculated to counteract atmospheric pressure, and 

 disposed so as to retain the symmetry of the structures. Among 

 phanerotympanums, Tibicen Jicematodes, Latr. (Plate VI., Fig. 4), 

 has six long oar-shaped ridges on its tymbals, and nine short 

 central callosities, bent forward and directed in a line from 

 within outwards ; but in Tettigonia oriii, Fabr., where the 

 musical organs are slightly covered posteriorly by a projection of 

 the second dorsal arc, there are only five long and three short 

 ones wider interspaced. Among the cryptotympanous kind, in 

 Tettigonia plelieja, Rossi, where the tymbals are totally covered 

 superiorly by a prolongation of the second dorsal arc [c], there 

 are but three imperfectly indurated ridges, as shown in Fig. 5a, 

 Plate III. 



Beneath this fluted structure the membrane has a lenticular 

 thickening, and at its anterior angle is a semi-pellucid spot [p) , 

 where is inserted the tendon [t) of a special muscle that performs 

 the function of motor. To form a firm point for the insertion of 

 the opposite extremity of these large muscles, which run upwards 



