Insects In General
The farmer who has fought "bugs" on crop after crop needs no argument to
convince him that insects are serious enemies to agriculture. Yet even
he may be surprised to learn that the damage done by them, as estimated
by good authority, amounts to millions and millions of dollars yearly in
the United States and Canada.
Every one thinks he knows what an insect is. If, however, we are willing
in this matter to make our notion agree with that of the people who have
studied insects most and know them best, we must include among the true
insects only such air-breathing animals as have six legs, no more, and
have the body divided into three parts--head, thorax, and abdomen. These
parts are clearly shown in Fig. 136, which represents the ant, a true
insect. All insects do not show the divisions of the body so clearly as
this figure shows them, but on careful examination you can usually make
them out. The head bears one pair of feelers, and these in many insects
serve also as organs of smell and sometimes of hearing. Less prominent
feelers are to be found in the region of the mouth. These serve as
organs of taste.
The eyes of insects are especially noticeable. Close examination shows
them to be made up of a thousand or more simple eyes. Such an eye is
called a _compound eye_. An enlarged view of one of these is shown in
Attached to the thorax are the legs and also the wings, if the insect
has wings. The rear portion is the abdomen, and this, like the other
parts, is composed of parts known as segments. The insect breathes
through openings in the abdomen and thorax called _spiracles_ (see Fig.
An examination of spiders, mites, and ticks shows eight legs; therefore
these do not belong to the true insects, nor do the thousand-legged
worms and their relatives.
_a_, egg; _b_, larva, or maggot; _c_, pupa; _d_, adult male. (All
The chief classes of insects are as follows: the flies, with two wings
only; the bees, wasps, and ants, with four delicate wings; the beetles,
with four wings--two hard, horny ones covering the two more delicate
ones. When the beetle is at rest its two hard wings meet in a straight
line down the back. This peculiarity distinguishes it from the true bug,
which has four wings. The two outer wings are partly horny, and in
folding lap over each other. Butterflies and moths are much alike in
appearance but differ in habit. The butterfly works by day and the moth
by night. Note the knob on the end of the butterfly's feeler (Fig. 143).
The moth has no such knob.
It is important to know how insects take their food, for by knowing this
we are often able to destroy insect pests. Some are provided with mouth
parts for chewing their food; others have a long tube with which they
pierce 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.
_a_, adult; _b_, side view of sucking mouth-part Both _a_ and _b_ are
_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.
Some insects, the grasshopper for example, do not completely change
their form. Fig. 147 represents some young grasshoppers, which very
closely resemble their parents.
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 day,
and so continues for two years, probably laying no less than 40,000,000
eggs. In one summer the bluebottle fly could have 500,000,000
descendants if they all lived. The plant louse, at the end of the fifth
brood, has laid in a single year enough eggs to produce 300,000,000
young. Of course every one knows that, owing to enemies and diseases
(for the insects have enemies which prey on them just as they prey on
plants) comparatively few of the insects hatched from these eggs live
till they are grown.
Note outline of the butterfly]
The number of insects which are hurtful to crops, gardens, flowers, and
forests seems to be increasing each season. Therefore farm boys and
girls should learn to recognize these harmful insects and to know how
they live and how they may be destroyed. Those who know the forms and
habits of these enemies of plants and trees are far better prepared to
fight them than are those who strike in the dark. Moreover such
knowledge is always a source of interest and pleasure. If you begin to
study insects, you will soon find your love for the study growing.
Collect cocoons and pupae of insects and hatch them in a
breeding-cage similar to the one illustrated in Fig. 149. Make
several cages of this kind. Collect larvae of several kinds; supply
them with food from plants upon which you found them. Find out the
time it takes them to change into another stage. Write a
description of this process.
The plant louse could produce in its twelfth brood
10,000,000,000,000,000,000,000 offspring. Each louse is about one
tenth of an inch long. If all should live and be arranged in single
file, how many miles long would such a procession be?
Flower-pot, lamp-chimney, and cloth]
Next: Orchard Insects
Previous: Some Special Plant Diseases