8; 



CHAPTER XXXII. 



A R THR O POD A — continued. 

 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 o?i the 

 thorax. Respiration is effected by trachea. 



The integument of the Insecta, in the mature condition, is more 

 or less hardened by the deposition of chitine, and usually forms a 

 resisting exoskeleton, 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 antennae, a pair of eyes, usually 

 compound, and the appendages of the mouth. The segments of 

 the thorax are also amalgamated into a single piece ; but this, 

 nevertheless, admits of separation into its constituent three somites 

 (fig. 441). These are termed respectively, from before backwards, 

 the "prothorax," " mesothorax," and " metathorax," and each bears 

 a pair of jointed legs. In the great majority of Insects, the dorsal 

 arches of the mesothorax and metathorax give origin each to a pair 

 of wings. 



Each leg consists of from six to nine joints. The first of these, 

 which is attached to the sternal surface of the thorax, is called the 

 " coxa," and is succeeded by a short joint, termed the " trochanter." 

 The trochanter is followed by a joint, often of large size, called the 

 " femur," succeeded by the so-called " tibia," and this has articulated 

 to it the " tarsus," which may be composed of from one to five 

 joints. 



The wings of Insects are expansions of the sides of the meso- and 

 meta-thorax, these expansions being supported by slender but firm 

 tubes, known as the "nervures." Each nervure consists of a cen- 

 tral trachea or air-tube, running in the centre of a larger blood-tube ; 



