ANNULOSA : IN SECT A. 



333 



CHAPTER XXXVIII. 



INSECT A. 



GENERAL CHARACTERS OF THE INSECTA. 



CLASS IV. INSECTA. The Insecta are defined as articulate 

 animals in which the head, thorax, and abdomen are distinct ; 

 there are three pairs of legs borne on the thorax ; the abdomen 

 is destitute of legs ; a single pair of antenna is present ; mostly, 

 there are two pairs of 

 wings on the thorax. 

 Respiration is effected 

 by trachece. 



In the Insecta the 

 body is divided into 

 a variable number 

 of definite segments, 

 or somites, some of 

 which are furnished 

 with jointed append- 

 ages, and the nervous 

 and circulatory sys- 

 tems are constructed 

 upon essentially the 

 same plan as in the 

 Crustacea, ' Arachni- 

 da, and Myriapoda. 

 The head, thorax, and 

 abdomen are distinct 

 (figs. 174, 175), and 

 the total number of 

 somites in the body 

 never exceeds twenty. 

 " Of these, five cer- 

 tainly, and six prob- 

 ably" (according to 



\, f thorax, with the third pair of legs and the second pair ot 



SOme authorities, lOUr w i ngs ; e Abdomen, without limbs, but carrying ter- 



nnlv^ u rrmstiriitp the minal appendages concerned in reproduction ; f Femur ; 



Only;, i jnSUUUC UJC * Tibia ;*<* Tarsus. 



head, which possesses 



a pair of antennae, a pair of mandibles, and two pairs of max- 

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

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



Fig. 174. Diagram of the external anatomy of an insect. 

 a Head carrying the eyes (o) and antennae (an) ; b First 

 segment of the thorax, with the first pair of legs ; c 

 Second segment of the thorax, with the second pair of 

 egs and the first pair of wings ; d Third segment of the 



