INSECTS. 



[ 430 ] 



INSECTS. 



a branch of the optic nerve. The length of 

 the lens is variable, in the Diptera being 

 very short, whilst in the Coleoptera and 

 Lepidoptera it is five or six times as long 



as broad, and in Libellula it even exceeds 

 this length. The compound cone, consist- 

 ing of the lens and vitreous humour, is sur- 

 rounded by a sheath of pigment, forming a 



Fig. 361. 



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



a the head, with the eyes 6 and the antennae c; d, the thorax, consisting of e the prothorax, to which the first pair 

 of legs^are attached ; g, the mesothorar, to which the first pair of wings h, and the second pair of legs t are attached ; 

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

 femur ; o, tibia with its spines, and p tarsus with its claws. 



the submentum (fig. 3m); at others this is 

 consolidated with the occiput (fig.27w). The 

 most anterior portion is the ligula (figs. 2, 

 3 t), which is frequently notched or lobed, 

 and is sometimes furnished with two lateral 

 portions called paraglossae (fig. 2*). Between 

 the ligula and the mentum or submentum 

 are the palpigers, one on each side (fig. 2 i) 

 these are sometimes united, and to them 

 the labial palpi (figs. 2, 3 k) are attached. 

 Below the labrum are the mandibles, one on 

 each side, forming two strong curved jaws, 

 and frequently furnished with powerful 

 teeth (figs. 3/, 22 6); these are the proper 

 organs of manducation. Below the mandi- 

 bles are two other lateral organs, the maxillae 

 (figs.^1, 2, 3g-, fig. 22 c) ; they are usually 

 less firm than the mandibles, and serve to 

 hold and convey the food to the back of the 

 mouth. Each maxilla is furnished with a 



choroid membrane (PI. 33. fig. 6r), in which 

 numerous tracheae ramity j this extends 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. 



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. 33. figs. le,3e, 22 a), forming the 

 upper boundary of the mouth, and articu- 

 lated at its base with the clypeua. A lower 

 single piece, forming its lower margin, called 

 the labium or lower lip (PL 33. fig. 2 t, /, m). 

 This consists of several parts : the most 

 posterior is the mentum (fig. 3 I), which is 

 articulated posteriorly with the gula (fig. 3 M). 

 Sometimes an intermediate portion occurs 



