INSECTA. 157 
CHAPBER XVII: 
INSECTA. 
Crass IV. INseEctA.—The true Insects are distinguished from 
the preceding classes of articulate animals by the fact that fhe 
three divisions of the body—namely, the head, thorax, and abdo- 
men—are always distinct from one another; there are never 
more than three pairs of legs in the adult, and these are borne 
upon the thorax; the abdomen ts destitute of locomotive append- 
ages. Respiration is effected by means of air-tubes or trachee, 
and in most insects two pairs of wings are developed from the 
back of the second and third segments of the thorax. 
The integument in insects is more or less hardened by the 
deposition of chitine in it, and the body is deeply cut into seg- 
ments (hence the name /ysect, from the Latin zusectus, cut 
into). The head in insects (fig. 77, 2) is composed of several 
segments amalgamated together, and carries a pair of jointed 
feelers or antenne, a pair of eyes, usually compound, and the 
appendages of the mouth. The thorax in insects (4, ¢, d) is 
composed of three segments, which are amalgamated together, 
but are generally pretty easily recognised. Each of these seg- 
ments of the thorax carries, in perfect insects, a single pair of 
jointed legs, so that there are three pairs in all. To the back of 
the two hinder segments of the thorax, in most insects, there 
are also attached two pairs of wings. In their typical form the 
wings are membranous expansions, supported by more or less 
numerous hollow tubes, known as the “nervures.” One or 
both pairs of wings may be wanting, and when all are present 
the anterior pair may be much ‘modified by the deposition of 
chitine in it. These modifications will be treated of in speak- 
ing of the orders of insects. The abdomen in insects (e) is 
properly composed of nine segments, which are usually more 
or less freely movable upon one another, and which never carry 
