160 INVERTEBRATE AKIMALS. 



CHATTEE XVI. 

 INSEC'TA. 



Class IV. Insecta. 



The true Insects are distinguished from the preceding cliisses of 

 Articulate animals by the fact that the three dicisiong of the body — 

 nainely, the head, thorax, and abdomen — are always distinct from one 

 another ; there are nercr more than three pairs of legs in the adult, 

 and these are borne upon the thorax; the abdomen is destitute of 

 locomotive appendages. Respiration is effected by means of air-tubes 

 or trachese, and in most Insects two pairs of wings are developed 

 from the back of the second and third segments of the thorax. 



The integument in insects is more or less hardened by the de- 

 position of chitine in it, and the body is deeply cut into segments 

 (hence the name Insect, from the Latin insectus, cut into). The 

 head in insects (fig. 108, a) is composed of several segments amal- 

 gamated together, and carries a pair of jointed feelers or antennis, 

 a pair of eyes, usually compound, and the ajjpendages of the mouth. 

 The thorax in insects {b, c, d) is composed of three segments, which 

 are amalgamated together, but are generally prettily easily recog- 

 nised. Each of the segments of the thorax carries, in perfect 

 insects, a single pair of jointed legs, so that there are three pairs 

 in all. To the back of the two hinder segments of the thorax, in 

 most insects, there aie also attached two pairs of wings. In their 

 typical form the wings are membranous expansions, supported by 

 more or less numerous hollow tubes, known as the " nervures." 

 One or both pairs of wings may be wanting, and when both are 

 present the anterior pair may be much modified by the deposition 

 of chitine in it. These modifications will be treated of in speaking 

 of the orders of Insects. The abdomen in insects (c) is properly 

 composed of eleven segments, which are usually more or less freely 

 movable upon one another, and which never carry locomotive limbs, 

 as is so commonly the case in the Crustacea. The extremity of the 

 abdomen is, however, often furnished with appendages which are 



