358 PHYSIOLOGY OF THE INVERTEBRATA. 



ganglion, passes to the region occnpiecl 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 the 

 ganglion passes along the groove to the ' stem,' and ends in 

 a ganglion in its dilatation. From this ganglion certain fine 

 filaments proceed. In the Aclididcc and LocustWIa', the tibiae 

 of the fore-legs present similar tympaniform membranes, 

 which are easily seen in the common cricket, but, in other 

 forms, become hidden by the development over them of folds 

 of the cuticle of the adjacent region of the limb. Two 

 spacious tracheal sacs occupy the greater part of the cavity of 

 the tibia, and a large nerve ends in a ganglion in the remain- 

 ing space. Upon this ganglion a series of peculiar short rod- 

 like bodies are set." 



For a tolerably full resume concerning the auditory organ 

 in the Insecta the reader is referred to Mr. A. H. Swinton's 

 Insect Variety : Its Fropagation and Distrihtdion, pp. 230- 

 252. 



As a general rule, the Insecta have a pair of compound eyes, 

 which are sessile and are situated upon the sides of the head. 

 The compound eye is literally an agglomeration of simple 

 eyes, having each a cornea, a vitreous humour of conical form, 

 a pigmented layer, and a nervous filament. In some insects 

 a compound eye contains upwards of twenty-five thousand of 

 these simple eyes. All the small cornea? are hexagonal, and 

 unite together so as to form a kind of common cornea, whose 

 surface presents a vast number of facets. The retina of 

 such eyes has a hemispherical form, the convex surface being 

 directed outwards, and consists of large compound nerve-rods 

 and retinula3, which ai-e separated from each other by pig- 

 mented sheaths. In front of these rods are placed the strongly 

 refractile crystalline cones, and in front of these again the 

 lens-shaped corneal facets. The compound eye is enclosed 

 by a firm chitinous layer, which following the sheath of the 



