ARTHROPODS. CLASS INSECTS 115 



The head usually carries the eyes, a pair of feelers (an- 

 tennae), and three pairs of mouth-parts which may be fash- 

 ioned into a long, slender tube to be used in sucking, and 

 frequently as a piercing organ ; or they may be constructed 

 for cutting and biting. The thorax bears three pairs of 

 legs and often one or two pairs of wings. The appendages 

 of the abdomen are usually small and few in number, or 

 even absent. 



113. Internal anatomy. The restless activity of insects 

 is proverbial. Some appear to be incessantly moving about, 

 either on the wing or afoot, and are endowed with com- 

 paratively great strength. Ants and beetles lift many times 

 their own weight. Numerous insects are able to leap many 

 times their own length, and others perform different kinds 

 of work with a vigor and rapidity unsurpassed by any other 

 class of animals. As is to be expected, the muscular sys- 

 tem is well developed, and exhibits a surprising degree of 

 complexity. Over five hundred muscles are required for 

 the various movements of our own bodies, but in some of 

 the insects more than seven times this number exist. The 

 amount of food necessary to supply this relatively immense 

 system with the required nourishment is correspondingly 

 large. Many insects, especially in an immature or larval 

 condition, devour several times their own weight each day. 

 Their food may consist of the juices of animals or plants, 

 which they suck out, or of the firmer tissues, which are 

 bitten or gnawed off. 



Not only do the mouth-parts stand in direct relation to 

 the habits of the animal and to its food, but, as we have 

 often noticed before, the internal organization is also 

 adapted for the digestion and distribution of the nutritive 

 substances in the most economical way. For this reason 

 we find the alimentary canal differing widely in the various 

 forms of insects. In each case it extends from the mouth 

 to the opposite end of the animal, and ordinarily consists 

 of a number of different parts. In the insect shown in 



