378 THE ANATOMY OF INVERTEBRATED ANIMALS. 



of the stigmata. Their inner ends unite into a plexus, which 

 lies over the interval between two of the ganglia of the cen- 

 tral nervous cord, and they are connected by longitudinal 

 cords with one another, and with these ganglia. 



In insects, as in other arthropods, the branches of the 

 nerves which are distributed to the integument, and especially 

 those which pass to the bases of the larger or smaller seta3 

 with which the integument is provided, frequently end in 

 minute ganglia. Hensen has shown that in the Crustacea 

 similar setae in all probability have an auditory function ; and 

 Leydig, Hicks, Lespes, Landois, and others, have ascribed 

 functions of special sensation to these structures in insects. 

 But whether these seta?, on the antennae or elsewhere, sub- 

 serve either hearing or smell, is still very doubtful ; and the 

 only organs which can safely be regarded as auditory in in- 

 sects are those which occur in Grasshoppers (Aerididce)* 

 Crickets (Aehetidce), and Locusts (Locitstidce), and which 

 were first accurately described by Von Siebold. 1 Recently, 

 they have been studied by Leydig, Hensen, Ranke, 2 and Os- 

 car Schmidt, 3 but it must be confessed that much obscurity 

 still hangs over their minute structure. 



In the Acrididw, the chitinous cuticula of the metathorax 

 presents on each side, above the articulation of the last pair 

 of legs, a thin tympaniform membranous space surrounded 

 by a raised rim. On its inner face, the cuticular layer of the 

 tympaniform membrane is produced into two processes, one 

 of which is a slender stem ending in a hollow triangular dila- 

 tation. A large tracheal vesicle lies over the tympanic mem- 

 brane, and between its wall and the latter, a nerve derived 

 from the metathoracic ganglion, passes to the region occupied 

 by the processes, and there enlarges into a ganglion, the outer 

 face of which, beset with numerous glassy rods arranged side 

 by side, is in contact with the tympaniform membrane. A 

 nerve arising from this ganglion passes along a groove to the 

 " stem " and ends in a ganglion in its dilatation. From this 

 ganglion certain fine filaments proceed. 



In the Achetidce and Locustidw the tibiae of the fore-legs 

 present similar tympaniform membranes which are easily seen 

 in the common Cricket, but, in other forms, become hidden 



1 " Archiv fur Naturgeschichte," 1864. 



2 "Beitrage zu der Lehre von den Uebergangs-Sinnesorganen." (Zeit- 

 schriftfiir wiss. Zoologie, 1875.) 



3 Schmidt, "Die Gehdrorgane der Heuschreeken." ("Archiv fur mikr. 

 Anatomie," 1875.) 



