CHAPTER XVn. 



INSECTA. 



Class IV. Istsecta. — The true Insects are distinguished 

 from the preceding classes of articulate animals by the fact 

 that the three divisions of the body, namely, the head, thorax, 

 and abdomen, are always distinct from one another ; tliere 

 are never more than three pairs of legs in the adult, and these 

 are borne upon the thorax / the abdomen is destitute of loco- 

 motive appendages. Respiration is effected by means of air- 

 tubes or trachea, 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 deposition 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. 63, a) is composed of several 

 segments amalgamated together, and carries a pair of jointed 

 feelers or antennae, a pair of eyes, usually compound, and the 

 appendages of the mouth. The thorax in insects {b, c, d) is 

 composed of three segments, which are amalgamated together, 

 but are generally pretty easily recognized. Each of these 

 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 are 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 " nerv- 

 ures." One or both pairs of wings may be wanting, and 

 when all 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 abdo- 

 men in insects (e) is properly composed of nine segments, 



