I.

Introduction

Insect, small, air-breathing animal characterized by a segmented body with three main parts—head, thorax, and abdomen. In their adult forms, insects typically have three pairs of legs, one pair of antennae, and in most instances, two pairs of wings.

For sheer variety and abundance, insects rank among the most successful animals on Earth. About one million species of insects have been identified so far, which is about half of all the animals known to science. Insects live in almost every habitat on land. For example, distant relatives of crickets called rock crawlers survive in the peaks of the Himalayas by producing a kind of antifreeze that prevents their body fluids from freezing solid. At the other extreme are worker ants that forage for food in the Sahara Desert at temperatures above 50° C (122° F). Insects consume an enormous variety of food. In the wild, many eat leaves, wood, nectar, or other small animals, but indoors some survive on a diet of wool clothes, glue, and even soap. As a group, insects have only one important limitation: although many species live in fresh water—particularly when they are young—only a few can survive in the salty water of the oceans.

Insects are often regarded as pests because some bite, sting, spread diseases, or compete with humans for crop plants. Nevertheless, without insects to pollinate flowers, the human race would soon run out of food because many of the crop plants that we rely on would not be able to reproduce. Insects themselves are valued as food in most of the world, except among Western societies (see Entomophagy). They help to recycle organic matter by feeding on wastes and on dead plants and animals. In addition, insects are of aesthetic importance—some insects, such as dragonflies, beetles, and butterflies, are widely thought to be among the most beautiful of all animals.

Insects are invertebrates, animals without backbones. They belong to a category of invertebrates called arthropods, which all have jointed legs, segmented bodies, and a hard outer covering called an exoskeleton. Two other well-known groups of arthropods are crustaceans, which include crayfish and crabs, and arachnids, which include spiders, ticks, mites, and scorpions. Many types of arthropods are commonly called bugs, but not every “bug” is an insect. Spiders, for example, are not insects, because they have eight legs and only two main body segments.