268 



MANUAL OF ZOOLOGY. 



with jointed appendages, and the nervous and circulatory 

 systems are constructed upon essentially the same plan as in 

 the Crustacea, Arachnida, and Myriapoda. The head, thorax, 

 and abdomen are distinct (fig. 104), and the total number of 

 somites in the body never exceeds twenty. " Of these, five 

 certainly, and six probably, constitute the head, which pos- 

 sesses a pair of antennae, a pair of mandibles, and two pairs of 

 maxillae, the hinder pair of which are coalescent, and form the 

 'labium.' Three, or perhaps, in some cases, more, somites 

 unite and become specially modified to form the thorax, to 

 which the three pairs of locomotive limbs, characteristic of 

 perfect Insects, are attached. Two additional pairs of loco- 



motive organs, the 



are developed, in most insects, 



from the tergal walls of the second and third thoracic somites. 



No locomotive limbs are ever developed from the abdomen of 



the adult insect, but the 

 ventral portions of the 

 abdominal somites, from 

 the eighth backwards, are 

 often metamorphosed in- 

 to apparatuses ancillary 

 to the generative func- 

 tion " (Huxley). 



The integument of the 

 Insecta, in the mature 

 condition, is more or less 

 hardened by the deposi- 

 tion of chitine, and usu- 

 ally forms a resisting exo- 

 skeleton, to which the 

 muscles are attached. The 

 segments of the head are 

 amalgamated into a single 

 piece, which bears a pair 

 of jointed feelers or an- 

 tennae, a pair of eyes, 

 usually compound, and 



Fig. 104. Diagram of Insect, a Head, carrying the appendages of the 

 the eyes and antennae ; b - Prothorax, carrying mO uth. The SCffmentS of 



t-Vip. fii-sf- nail- nf >><: r MpcntVini-Q v. <-ar-rTrincr r^O 



the first pair of legs ; c Mesothorax, carrying 



legs a 

 b Metathorax, with the 



the thorax are aiSO 



the second pair of legs and first pair of wings ; 



third pair of legs and o-e>mnt-p>r1 infr\ 



the second pair of wings ;* Abdomen, without grated mtO 



limbs, but having terminal appendages subser- piece I but thlS, never- 



vient to reproduction. ^^ ^^ Qf sepa _ 



ration into its constituent three somites (fig. 104). These 

 are termed respectively, from before backwards, the "pro- 



