VIII] Introduction to Insects 85 
characteristics distinguish insects from other animals: 
the bodies of the adults are divided into three parts, 
the front known as the Head, the middle as the Thorax, 
and the hind portion as the Abdomen. 
The head bears : (1) The eyes, which may be (a) simple 
or (6) compound eyes, composed of what look like a 
large number of hexagonal cells. (2) The antennae or 
feelers, which vary considerably in different insects, 
and may be simple pointed rods or may possess knob- 
like or feathery endings. (3) The mouth, which is an 
opening below or in front of the head around which are 
grouped the jaws. These are fashioned according to 
the functions which they are called upon to perform. 
In biting insects they work horizontally and not up 
and down as in the case of higher animals. 
The thorax or middle portion of the body is divided 
up into three rings or segments, each bearing a pair of 
jointed legs. Both the second and third segments 
usually bear a pair of wings, a marked feature of the 
group. In flies however only the first pair are present 
on the middle segment, the third bearing the remains 
of the second pair which are used as balancing organs 
(see Fig. 45). In some species, e.g. certain ants and 
springtails, no wings are present, but other characters 
resemble those of other members of the group. It is 
therefore correct to classify them as insects. 
The abdomen, usually the largest portion of the 
body, is composed of a number of segments which 
closely resemble one another. They have no append- 
ages except in certain cases at the hind end of the body. 
Unlike the higher animals insects have no internal 
skeletons to support and protect their bodies, but the 
outside is covered with a horny substance known as 
