160 IXVKUTKBKATE ANIMALS. 



C HATTER XVI. 



INSECTA. 



Class IV. Insei.ta. 



The true Insects mx- (listiiigiiisUed from the preceding classes of 

 Articulate aninials by the fact that t/c three dii-isinns t>f the hodij — 

 nuiiiejii^ till' hciid, tlioro.x, mul ahdinneii — are always distinct from one 

 another ; tliere are ncrer more than three pairs of legs in the aduli, 

 and these are home npo'n the thorax; the alxlomen is destitute of 

 loi'oniotire aj>jiendai/es. Respiration is effected by means of air-tubes 

 or tracheae, anil in most Insects two ])airs of wings are developed 

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



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

 position of cliitine in it, and the body is ilceply cut into .segments 

 (hence the name Insert, from the Latin inseetns, cut into). The 

 head in insects (tig. lOS, a) is coni])osed of several segments amal- 

 gamated together, .-ind carries a, pair of jointed feelers or antennie, 

 a pair of eyes, usually coni)Miund, .and the .appendages of the mouth. 

 The thorax in insects (/), c, d) is composed of three segments, which 

 are amalgamated together, but are gener.-illy prettily easily recog- 

 niseil. Eai-h of the segments of the thor.ax carries, in perfect 

 insects, a single jiair of jointed legs, so that tliei'e are three pairs 

 in all. To the b.-irk of the two hinder segments of the thorax, in 

 most insects, theiv arc also .-ittached two jiairs of wings. In their 

 typical form the wings .■iro meml.i'anous exjiansions, supported bv 

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

 <_)ne or both ]iairs of wings m.-iy lie wanting, and when both are 

 present the .-interior pair may be much moditied by the deposition 

 of chitine in it. Tliese modifie.-itioiis will be treated of in speaking 

 of the orders of Insects. 'I'he abdomen in insects (e) is properly 

 comjiosed of ek-ven segments, \\hirh are usually moi'e or less freely 

 movable upon oiio .■iiiollior, and wliich he\er r.arry locomotive lindis, 

 .IS is ,so rommoiily the r;isi. in the ('riislaeea. 'Jlie extremity of the 

 a.liiloiiieii is, howeyei-, often furnished -with .-ippendages which are 



