210 MANUAL OF ZOOLOGY. 



mouth. The segments of the thorax are also amalgamated 

 into a single piece ; bat this, nevertheless, admits of separation 

 into its constituent three somites (fig. 61). These are termed 

 respectively, from before backwards, the ' pro thorax,' ' meso- 

 thorax,' 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,' and this has articulated to it 

 the ' tarsus,' which may be composed of from two to five joints. 



The wings of Insects are membranous, ' flattened vesicles, 

 sustained by slender, but firm, hollow tubes, called "nervures," 

 along which branches of the trachea? and channels of the cir- 

 culation are continued.' (Owen.) 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 Dlptera the 

 posterior pair of wings are rudimentary, and are converted 

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

 In the Strep&i/ptera the anterior pair of wings are rudimentary, 

 and are converted into twisted filaments. 



The abdomen in Insects is normally composed of nine 

 somites, which are usually more or less freely movable upon 

 one another, and never carry locomotive limbs. The extremity 

 of the abdomen is, however, not infrequently furnished with 

 appendages, which are connected with the generative function, 

 and not infrequently serve as offensive and defensive weapons. 

 Of this nature are the ovipositors of Ichneumons and other 

 Insects, and the sting of Bees and Wasps. In the Earwig 

 (Forficula) these caudal appendages form a pair of forceps, 

 whilst in many Insects they are in the form of bristles, by 

 which powerful leaps can be effected, as is seen in the Spring, 

 tails (Podurce). 



The organs about the mouth in Insects are collectively 

 termed the ' trophi,' or ' instrumenta cibaria.' Two principal 

 types require consideration, namely, the masticatory and the 

 suctorial, both types being sometimes modified, and occasion- 

 ally combined. 



