Insect

Insects are invertebrates belonging to the class Insecta, the biggest group in the arthropod phylum. They are the most varied animal group, with over a million known species—more than half of all animal species. Their bodies have an exoskeleton and are split into three parts: head, thorax, and abdomen. They also have three pairs of jointed legs, compound eyes, and one pair of antennae. Insects live on every continent and in nearly every land habitat, with far more species in tropical areas than in temperate ones.

The word "insect" comes from Latin *insectum*, meaning "cut up," because their bodies look divided into three sections. Pliny the Elder introduced this Latin term, basing it on the Ancient Greek *éntomon* (also meaning "cut in pieces"), which Aristotle used for this group. The English word first appeared in 1601.

In everyday language, insects and other land arthropods are often called bugs or creepy crawlies. Entomologists, however, reserve "bugs" for true bugs—the order Hemiptera, which includes cicadas and shield bugs. Other land arthropods like centipedes, millipedes, woodlice, spiders, mites, and scorpions are sometimes mistaken for insects because they also have jointed exoskeletons. Adult insects are the only arthropods that ever have wings, with up to two pairs on the thorax. Whether winged or not, adult insects can be told apart by their three-part body and three pairs of legs on the thorax.

Estimates suggest there may be around 5.5 million insect species in existence, of which about one million have been described and named. That makes up roughly half of all eukaryote species, including animals, plants, and fungi. The most diverse insect orders are Hemiptera (true bugs), Lepidoptera (butterflies and moths), Diptera (true flies), Hymenoptera (wasps, ants, and bees), and Coleoptera (beetles), each with over 100,000 described species.

Insects have a nervous system made up of a brain and a ventral nerve cord. They breathe through paired openings along their sides, connected to small tubes that deliver air directly to tissues; their blood does not carry oxygen. Blood is only partly contained in vessels, with some circulating in an open hemocoel. Their vision relies mainly on compound eyes, plus small ocelli. Many insects can hear using tympanal organs, and they smell through receptors, usually on the antennae and mouthparts.

Most insects reproduce by laying eggs. Growth is limited by their inelastic exoskeleton, so development involves a series of molts. Immature stages often differ from adults in structure, habits, and habitat. Groups that go through four-stage metamorphosis usually have a nearly immobile pupa. Adult insects typically move by walking and flying; some can swim. Insects are the only invertebrates capable of sustained powered flight, which evolved just once. Many insects are at least partly aquatic, with larvae that have gills; in some species, adults are also aquatic. Some, like water striders, can walk on water. Most insects are solitary, but some—such as bees, ants, and termites—are social and live in large, well-organized colonies. Others, like earwigs, provide maternal care by guarding eggs and young. Insects communicate in various ways: male moths sense female pheromones over great distances; crickets stridulate (rub their wings together) to attract mates and repel rivals; lampyrid beetles use light signals.

Insects that undergo three-stage metamorphosis lack a pupa, developing through increasingly adult-like nymphal stages. Fossilized insects from the Paleozoic era include giant dragonfly-like forms with wingspans of 55 to 70 cm. The most diverse insect groups appear to have coevolved with flowering plants. Humans see many insects as pests, especially those that damage crops, and try to control them with insecticides and other methods. Others are parasitic and can spread diseases. Insect pollinators are essential for the reproduction of many flowering plants and their ecosystems. Many insects are ecologically beneficial as predators of pest insects, while a few provide direct economic benefits. Two species were domesticated centuries ago: silkworms for silk and honey bees for honey. Insects are intentionally eaten as food in 80% of the world’s nations, by people in roughly 3,000 ethnic groups. Human activities are seriously affecting insect biodiversity.

Insects are found on every continent and in nearly every land habitat, with many more species in the tropics, especially rainforests, than in temperate zones. Different regions have received varying attention from entomologists. The British Isles have been thoroughly surveyed, so the total of about 22,500 species there is likely within 5% of the actual number. Canada’s list of 30,000 described species is probably over half of the real total, while the 3,000 species in the American Arctic are thought to be broadly accurate. In contrast, most insect species in the tropics and southern hemisphere are likely undescribed. Some 30,000 to 40,000 species live in freshwater, but very few—perhaps a hundred—are marine. Insects like snow scorpionflies thrive in cold habitats.