Insects are invertebrates.

How Insects are Alike

            All over the world there are insects. There are huge insects and tiny ones, in every shape and color you can think of. There are insects with wings and without wings. There are insects that swim, that crawl, that fly, and that hop. But they are all insects.
            Most adult insects are alike in these ways: (1) they have 6 jointed legs; (2) they have 3 body parts; (3) they have a hard cover. This is called an outside skeleton.
            The 3 body parts are the head, thorax, and abdomen. On some insects you cannot easily see three separate parts, but they are there just the same.
            Scientists say that the antennae are the insect's main sense organs. With their antennae, insects sense their food, sense each other, and sense direction.

Antennae

            Long or short, thick or thin, all antennae help the insect to sense the world that is around it.

Mouths

            Some insects cut their food, some chew it, some suck it.

Eyes

            Insects can see almost all the way around them. A fly has two large eyes and three smaller ones. Between them, the eyes of the fly can see up, down, and all the way around.

Thorax

            Every adult insect has a thorax. The muscles that drive the insects' wings and legs are attached to the thorax. Most insects have wings and legs. But insects differ in the shape of their legs and in the kinds of wings they have.

Abdomen

            The abdomen contains important parts of the body. Here food is digested. Insects breathe through this part of their bodies, and it is here that they produce their eggs.
            Head, thorax, and abdomen. All adult insects have these three parts. Insects are alike in still another way. They have outside skeletons. Their outside skeletons hold them up and give them their shapes.
            An outside skeleton, three body parts, and six legs...these are the ways in which most adult insects are alike.

How Insects Grow

            Insects change very much as they grow. Many insects change four times before they become full-grown adults. Most insects take several weeks or longer. Some insects take several years. But there is one little insect, called a fruit fly, that goes through its four stages in about eight or ten days! Scientists use fruit flies in many experiments because they go through their life stages so quickly.

Growing in Four Stages

            Many insects go through four stages as they grow.
            1. Eggs are laid.
            2. The eggs hatch into small, wormlike larvae. Some larvae are called caterpillars. Others are called maggots. Larvae eat and grow. When they grow too big for their skins, they shed them, or molt. They grow a new skin. Some kinds of larvae molt four or five times.
            3. Now the larvae become pupae. They stop eating and moving about. In this stage many insects build a little case around themselves. The case that a butterfly pupa has is called a chrysalis. Moth pupae have cases called cocoons.
            4. The pupa becomes an adult insect. It crawls out and spreads its wings. The adult female lays eggs.

Growing in Three Stages

            1. Some insects go through only three stages of growth. The young that are hatched from the eggs of this kind of insect are called nymphs. Nymphs look much like their parents. But nymphs are smaller and have no wings.
            2. The nymphs eat and grow, molting their skins when they outgrow them. In some grasshoppers, this molting happens five times.
            3. At last the adult stage is reached. The female lays eggs.

Insects Survive

            Most insects have many enemies, and most insects are quite small. Yet more kinds of insects survive than all the bigger animals put together. To survive means to stay alive.
            One reason why insects survive is that they lay so many eggs. Another is that they can lay the eggs in many different places: In grain, on roots, in running water or in still water, on other insects, on other animals, on rocks, under floors, on birds' feathers, on old carpets, in red pepper, in dry cocoa, in garbage, and in manure. Insect eggs can be laid almost anywhere in the world.

Insects Find Each Other

            Another reason insects survive is that they have many ways of finding each other. One way that insects find each other is by the sense of smell. They can smell things from a distance. Some insects give off a substance with a special smell. When scientists put a tiny bit of this special smell substance from a female gypsy moth into a jar, they found that male moths flew to the jar from far away.
            Insects can hear each other. Some insects say "Here I am" by making a noise. You have probably heard the call of the male katydid. It sounds like "Katy did, Katy did" over and over again, until the female katydid finds the male katydid. Male katydids have two special parts for making the katydid sound. One is like a nail file and the other is a scraper that scrapes the file.
            Some insects glow at night. Nighttime is a good time for insects to be out because most insect-eaters are asleep. Some insects that stay up at night can glow. In this way they can find each other. They give off light. A firefly is such an insect. Fireflies have two special substances in their bodies. When the substances are made to come together, they give off a glow.
            Still another way that insects find each other is by color, shape, and design. Red, purple, orange, yellow – there are insects of every color of the rainbow. Some are striped; some have dots. Some insect colors and designs are very fancy.

Color that Protects

            Color works in other ways for insects. Insects are the favorite food of many birds, lizards, and other animals. Some insects have a color or shape which is hard to see. Insects with special ways to keep from being seen have a better chance to survive.
            Many insects have bright colors that warn their enemies. Some insects may have a bad taste. Some are poisonous. Some have stingers. These bad-tasting, stinging insects warn their enemies. Birds and other animals soon learn not to eat these brightly colored insects. Color protects them from being eaten.
            Some insects are called mimics, because they look like other insects which birds will not eat because of their bad taste. Birds expect these mimic insects to taste bad too, and will not eat them.

Insects that Work Together

            Some insects live together and share the work. Such insects are called social insects. Ants, hornets, and honeybees are social insects.

Insect Pests

            The boll weevil will eat the buds, flowers, and bolls of cotton plants. Another insect that does much damage is called a corn borer. The adult corn borers are moths and do not eat corn or anything at all. But the female moth lays eggs – hundreds of them – on the young parts of the corn plant. The eggs soon hatch into larvae with healthy appetites. The larvae eat their way into the cornstalk, where they eat and grow, eat and grow. Soon the larvae become pupae. Then the pupae become adult moths which are able to lay more eggs. Corn borers do a great deal of damage to crops. To people, these insects are pests.
            There are thousands of insect pests all over the world that do damage. Some spoil crops. Some carry disease. Some spoil stored food.

Problems for scientists

            Farmers have called in scientists to help them fight insect pests. The scientists have tried many kinds of poison powders and liquids. Poisons are pumped from tank trucks and sprayed from airplanes and helicopters. These poisons kill insect pests. The farmers' crops are saved. But other problems are raised.
            1. Many birds eat insects.
            2. Poisons kill useful insects too.
            3. Many fish feed on insects.
            4. The poisons that are sprayed on insects also are sprayed on the plants. Some of these poisons stay on the fruits and vegetables. These poisons are not good for insects. Neither are they good for people.