122 INSECTA. 



vidence affords even to the smallest of its creatures ; of the 

 means it employs for perpetuating them, and of that great 

 arrangement of nature by which one set of living beings find 

 subsistence by devouring another, and by which life is con- 

 tinued through every part of the creation without a pause. 



Insects have a dorsal vessel analogous to a heart, but no 

 vessels for circulation ; respiration is performed by means of 

 tracheae extending parallel to each other throughout the body, 

 communicating at intervals with stigmata or external open- 

 ings, which admit air. The nervous system of most Insects 

 consists of two principal and twelve inferior ganglions, each 

 of which distribute nerves to the neighbouring parts. The 

 digestive apparatus is various according to their aliment. 



The head bears the antennae, eyes and mouth ; the eyes 

 are either simple or compound ; the latter have a cornea with 

 many small facets, and more or less convex, but in the former 

 it is smooth ; the eyes are generally three in number, seated 

 on the top of the head in a triangular form ; the mouth con- 

 sists of six parts, the two upper of which are called mandi- 

 bles, the rest maxilla ox jaws ; these latter are furnished with 

 articulated filaments called palpi. 



The trunk of insects is called thorax or corselet] it is com- 

 posed of three segments, affording points of attachment for 

 the legs and wings. These are denominated the prothorax, 

 the mesothorax and the metathorax. Sometimes, as in the 

 Coleoptera, the anterior, much the largest and separated from 

 the succeeding by an articulation, appears to form the whole 

 trunk, and is called thorax or corselet. 



The wings are membranous, dry and elastic organs, com- 

 monly translucent and attached to the sides of the back of the 

 thorax ; when they are four the first pair is joined to the 

 second, and the other to the third segment of the thorax, 

 when only two to the second segment. They are formed of 

 two membranes, traversed by numerous nervures, which are 

 tracheal tubes. The wings are either two or four in number; 

 when only two, an opaque species of scale (elytra), more or 

 less solid, and which opens and shuts, takes the place of the 

 anterior wings in many Insects, and beneath this the wings 

 are folded when at rest. 



