ANNULOSA: INSECTA. 335 



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 separa- 

 tion into its constituent three somites (figs. 174, 175). These 

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

 rax," " 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 (see fig. 178). 

 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 nerv- 

 ure consists of a central trachea or air-tube, running in the 

 centre of a larger blood-tube ; so that the wings not only act 

 as organs of flight, but at the same time assist in the process 

 of respiration. Normally, two pairs of wings are present, but 

 one or other may be wanting. In the Coleoptera (Beetles) the 

 anterior pair of wings become hardened by the deposition 

 of chitine, so as to form two protective cases for the hinder 

 membranous wings. In this condition the anterior wings are 

 known as the "elytra," or "wing-cases." In some of the 

 Hemiptera this change only affects the inner portions of the 

 anterior wings, the apices of which remain membranous, and 

 to these the term " hemelytra " is applied. In the Diptera the 

 posterior pair of wings are rudimentary, and are converted 

 into two capitate filaments, called "halteres" or "balancers." 

 In the Strepsiptera the anterior pair of wings are rudimentary, 

 and are converted into twisted filaments. 



The typical number of somites in the abdomen of the In- 

 secta is eleven, and this number can often be recognised in the 

 Neuroptera and in some other forms. In the Hymenoptera 

 and Lepidoptera not more than ten can be recognised, and in 

 other cases even fewer can be made out. The abdominal 

 somites are usually more or less freely movable upon one an- 

 other, and never carry locomotive limbs. The extremity of 

 the abdomen is, however, not infrequently furnished with ap- 

 pendages, which are connected with the generative function, 

 and not infrequently serve as offensive and defensive weapons. 



