e plants or animals and, like the mosquito, suck their food from
the inside. Insects of this latter class cannot of course be harmed by
poison on the surface of the leaves on which they feed.

[Illustration: FIG. 140. A TYPICAL BUG
_a_, adult; _b_, side view of sucking mouth-part Both _a_ and _b_ are
much enlarged]

[Illustration: Fig. 141. BEETLE
_a_, larva; _b_, pupa; _c_, adult; _d_, burrow]

Many insects change their form from youth to old age so much that you
can scarcely recognize them as the same creatures. First comes the egg.
The egg hatches into a worm-like animal known as a grub, maggot, or
caterpillar, or, as scientists call it, a _larva_. This creature feeds
and grows until finally it settles down and spins a home of silk, called
a _cocoon_ (Fig. 145). If we open the cocoon we shall find that the
animal is now covered with a hard outside skeleton, that it cannot move
freely, and that it cannot eat at all. The animal in this state is known
as the _pupa_ (Figs. 145 and 146). Sometimes, however, the pupa is not
covered by a cocoon, sometimes it is soft, and sometimes it has some
power of motion (Fig. 141). After a rest in the pupa stage the animal
comes out a mature insect (Figs. 142 and 143).

From this you can see that it is especially important to know all you
can about the life of injurious insects, since it is often easier to
kill these pests at one stage of their life than at another. Often it is
better to aim at destroying the seemingly harmless beetle or butterfly
than to try to destroy the larvae that hatch from its eggs, although, as
you must remember, it is generally the larvae that do the most harm.
Larvae grow very rapidly; therefore the food supply must be great to meet
the needs of the insect.

[Illustration: FIG. 142. MOTH AND COCOON]

Some insects, the grasshopper for example, do not completely change
their form. Fig. 147 represents some young grasshoppers, which very
closely resemble their parents.

[Illustration: FIG. 143. BUTTERFLY]

[Illustration: FIG. 144. STRUCTURE OF THE CATERPILLAR]

[Illustration: FIG. 145. MOTH PUPA IN COCOON]

Insects lay many eggs and reproduce with remarkable rapidity. Their
number therefore makes them a foe to be much dreaded. The queen honeybee
often lays as many as 4000 eggs in twenty-four hours. A single house fly
lays between 100 and 150 eggs in one day. The mosquito lays eggs in
quantities of from 200 to 400. The white ant often lays 80,000 in a