194 
INSECTS 
in many cases a single species numbers myriads of individuals. 
Certain kinds, as the grasshoppers, sometimes travel in such 
swarms as to darken the sky. 
Their Rapid Increase. — Insects multiply at an enormous 
rate, and it is thought that they might often become so nu- 
A CRAYFISH 
Its shell is the only skele- 
ton it has. 
merous as to devour every green 
thing in sight if they were not kept 
in check. The principal agency that 
nature has provided for keeping the 
numbers within limits is the birds 
that feed upon them. Our northern 
winters are also very unfavorable to 
insect life. 
Wuat AN Insect Is 
To help us distinguish insects from 
other small animals let us note care- 
fully their chief characteristics as 
given in the following paragraphs. 
External Skeleton. — In insects 
and their relatives, the skeleton is external; that is, it is on 
the outside of the body instead of lying 
within and being covered with flesh, as in 
the case of higher animals. This arrange- 
ment of the skeleton may be more easily 
seen in the common crayfish and the lobster, 
whose body is formed on the same plan as 
an insect’s. 
Parts of the Body.— The body of an in- Wasp 
sect consists of three distinct parts: head, 
thorax, and abdomen. 
With head, thorax, 
and abdomen sep- 
In the wasp these arated; ¢, thorax; 
parts are seen very clearly. Observe how 4 abdomen. 
