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

Fig. 138.



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.

137).



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

enlarged)]



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

much enlarged]




_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.








=EXERCISE=



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]





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