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Insect, member of the class Insecta, phylum Arthropoda, or invertebrate animals with jointed legs. There are about 750,000 known species of insects, and more are being discovered every year.


The body of an insect, like that of other arthropods, is covered by a hard, waterproof “shell” that forms an external skeleton to which the muscles are attached. The head bears jaws and other structures for dealing with food. It also carries several sense organs, including the compound eyes made up of clusters of separate units. The antennae, or feelers, are organs of touch, smell, and occasionally of hearing. Sense organs are also found on other parts of the body: Grasshoppers and crickets have ears on the legs or thorax, and houseflies have tastebuds on their feet. The middle section, or thorax, bears the limbs. It consists of 3 parts, each of which carries a pair of jointed legs ending in hooks that allow the insect to hang on walls or ceilings. Two pairs of wings sprout from the back of the thorax. Some insects have no wings, and the flies have turned one pair into balancing organs. The abdomen is the largest section of the body and has no visible external organs except those concerned with reproduction and occasionally a pair of short sense organs, such as antennae called cerci (for example, in earwigs). Insects do not have a closed system of blood vessels. The heart simply pumps blood through the arteries to the extremities, where it washes around the organs and slowly drains back to the heart. Nor do insects have lungs. Air is carried into the body down minute tubes, called tracheae, which run from openings in the skin into each organ.

Life cycles

Most insects lay eggs, but some give birth to live young. The mother usually abandons her eggs, but the social insects, including bees and wasps, have elaborate systems of caring for their young. The eggs hatch into larvae, such as the maggots of flies, the caterpillars of butterflies and moths, and the nymphs of dragonflies. The development into an adult insect is accomplished in one of two ways. (1) Incomplete metamorphosis: In dragonflies, grasshoppers, and others, the newly hatched larva looks rather like the adult. It molts several times, shedding its skin each time and growing a little larger. Adult features such as wings become more apparent at each molt until, at the final molt, the adult insect crawls out of the old skin. (2) Complete metamorphosis: The larva is very different from the adult. After growing for some time, it changes into the adult in one drastic step. To do this it forms a pupa or chrysalis in which it can reorganize its structure. In this way a sausage-shaped caterpillar turns into a butterfly. A single insect is able to exploit 2 very different ways of life. Thus a caterpillar feeds on leaves and a butterfly drinks nectar.


Insects vary in size from almost microscopic wasps to the huge extinct dragonflies with wingspans of over 2 ft (61 cm). They live in many different environments, boring into wood, burrowing underground, living in other animals, and swimming underwater. They are found in the hottest deserts, on the coasts of Antarctica, in hot springs, and in the saltiest lakes (such as Salt Lake, Utah); the petroleum fly lives in pools of crude petroleum oil in California. They may also exist in huge numbers. Locust swarms may contain 1 billion individuals, and springtails form dense carpets on the ground.

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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Inert gas to Jaruzelski, Wojciech