INSECTS. 



[ 356 ] 



INSECTS. 



drangular or hexagonal in form, and in im- 

 mediate contact laterally. Hence their com- 

 pound cornea, when viewed from before 

 behind, presents the appearance of a 



or 



membrane with numerous, beautifully regu- 

 lar six- or four-sided facets (PI. 26. figs. 

 5 a, b). The facets are very variable 

 in number, but often many thousands are 



Fig. 365. 



c 



h 



m 



P 



a 



g 



' d 



n 



Diagram showing the principal parts of the cutaneous skeleton of a grasshopper. 



a the head, with the eyes b and the antennae c ; d, the thorax, consisting of e the prothorax, to which the first pair 

 of legs/ is attached ; g, the mesothorax, to which the first pair of wings h, and the second pair of legs i are attached ; 

 k, the metathorax, to which the second pair of wings /, and the third pair of legs m are attached; n, abdomen; 

 m, femur ; o, tibia with its spines, audp tarsus with its claws. 



present. The facets are sometimes broader 

 in front than behind, and they are some- 

 times doubly convex (as in the Lepidoptera), 

 at others concavo-convex (in Libellula, PI. 

 26. fig. 6 c), but usually the surfaces are 

 parallel. The cornea possesses a laminated 

 structure. 



Behind each cornea is a transparent cone 

 (PI. 26. fig. 6/), representing a crystalline lens, 

 theapex of which is imbedded in a transparent 

 mass corresponding to a vitreous humour ; 

 and this is surrounded by a cup-shaped 

 expansion of a branch of the optic nerve. 

 The length of the lens is variable, in the 

 Diptera being very short, whilst in the Coleo- 

 ptera and Lepidoptera it is five or six times 

 as long as broad ; and in Libellula it even 

 exceeds this length. The compound cone, 



consisting of the lens and vitreous humour, is 

 surrounded by a sheath of pigment, forming 

 a choroid membrane (PI. 26. fig. 6g), in 

 which numerous tracheae ramify; this ex- 

 tends over the front of the base of the cone, 

 leaving, however, a small pupillary space or 

 pupil, which is separated from the back of 

 the cornea by an anterior chamber (fig. 

 6d). 



The antennce are noticed under Antenna. 



The trophi or organs of the mouth vary 

 in structure in the different orders, but the 

 following form the typical parts : an upper 

 central single piece, the labrum or upper lip 

 (PI. 26. figs. \ e, 3 e, 22 a), forming the 

 upper boundary of the mouth, and articu- 

 lated at its base with the clypeus. A lower 

 single piece, forming its lower margin, called 



