86 Introduction to Insects [CH. 
chitin, which serves the same purpose. Insects do 
not breathe in the same way as the higher animals, 
that is to say they have no lungs into which they draw 
air through a special opening. They have, running 
throughout their bodies, a number of air-tubes known 
as tracheae, which give off numerous branches serving 
the same functions as the lungs, their exterior openings, 
at the sides of the segments, being known as spiracles. 
Differences in such parts as wings, mouth parts, 
antennae, legs, and eyes, etc., help us to distinguish 
one insect from another. 
The above characters also enable us to decide 
whether the animals under observation are to be 
classed as insects. Examination of a butterfly shows 
that it possesses all the above characteristics and 
therefore must be classified as an insect. If the animal 
we are examining is a spider we find that its body is 
not divided into three parts, that the adult form has 
four pairs of legs and that it possesses neither wings 
nor feelers. These and other characteristics have 
caused spiders to be put into a different group. 
An insect differs very considerably from most of 
the higher animals in its structure and appearance at 
the time of its birth as compared with its adult stage. 
When a calf is born or a young chick hatched each is 
very similar in appearance to its adult form, only very 
much smaller. In the case of most insects this is not 
so. On hatching from its egg it has few of the adult 
characteristics. It passes through several changes 
during its life, its appearance at one stage being very 
different from that of the other stages. A description 
of the life of a typical insect will help us to understand 
these changes. 
