In numbers of species and individuals and in adaptability and wide distribution, insects are perhaps the most eminently successful group of all animals. They dominate the present-day land fauna with about 1 million described species. This represents about three-fourths of all described animal species. Entomologists estimate the actual number of living insect species could be as high as 5 million to 10 million. The orders that contain the greatest numbers of species are Coleoptera (beetles), Lepidoptera (butterflies and moths), Hymenoptera (ants, bees, wasps), and Diptera (true flies).

Appearance and habits

The majority of insects are small, usually less than 6 mm (0.2 inch) long, although the range in size is wide. Some of the feather-winged beetles and parasitic wasps are almost microscopic, while some tropical forms such as the hercules beetles, African goliath beetles, certain Australian stick insects, and the wingspan of the hercules moth can be as large as 27 cm (10.6 inches).

African goliath beetle (Goliathus giganteus). Appel Color Photography

Walkingstick (Phasmatidae). Lilyan Simmons/Encyclopædia Britannica, Inc.

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In many species the difference in body structure between the sexes is pronounced, and knowledge of one sex may give few clues to the appearance of the other sex. In some, such as the twisted-wing insects (Strepsiptera), the female is a mere inactive bag of eggs, and the winged male is one of the most active insects known. Modes of reproduction are quite diverse, and reproductive capacity is generally high. Some insects, such as the mayflies, feed only in the immature or larval stage and go without food during an extremely short adult life. Among social insects, queen termites may live for up to 50 years, whereas some adult mayflies live less than two hours.

Female mayfly (Ephemera danica). G.E. Hyde—NHPA/Encyclopædia Britannica, Inc.

Some insects advertise their presence to the other sex by flashing lights, and many imitate other insects in colour and form and thus avoid or minimize attack by predators that feed by day and find their prey visually, as do birds, lizards, and other insects.

North American firefly (Photinus). Werner W. Schulz

Behaviour is diverse, from the almost inert parasitic forms, whose larvae lie in the nutrient bloodstreams of their hosts and feed by absorption, to dragonflies that pursue victims in the air, tiger beetles that outrun prey on land, and predaceous water beetles that outswim prey in water.

In some cases the adult insects make elaborate preparations for the young, in others the mother alone defends or feeds her young, and in still others the young are supported by complex insect societies. Some colonies of social insects, such as tropical termites and ants, may reach populations of millions of inhabitants.