ARTHROPODS. CLASS INSECTS 115 
The head usually carries the eyes, a pair of feelers (an- 
tenn), 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 
