General features ( insects )
In numbers of species and individuals and in adaptability and wide distribution, insects are perhaps the most eminently successful of all animals. They dominate the present-day land fauna; about 800,000 species have been described, representing about three-fourths of known animal life the actual number of living species could range from 5,000,000 to 10,000,000, entomologists estimate. 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 six millimetres (0.2 inch) long. The range in size, however, is wide. Some are almost microscopic, as certain of the feather-winged beetles and parasitic wasps; while some tropical forms attain considerable size, up to 16 centimetres in the hercules beetles, African goliath beetles, certain Australian stick insects, and some Asian and South American moths. 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 as adults. Among the social insects, queen termites may live for up to 50 years; some adult mayflies live less than two hours. 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. 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 dytiscid 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 of which (tropical termites and ants) may reach populations of millions of inhabitants.
Distribution and abundance
No scientist familiar with insects has attempted to estimate individual numbers beyond areas of a few acres or a few square miles in extent. Figures soon become so large as to be incomprehensible. The large populations and great variety of insects are related to their small size, high rates of reproduction, and abundance of suitable food supplies. Insects abound in the tropics, both in numbers of different kinds and in numbers of individuals. If the insects (including the young and adults of all forms from microscopic young springtails [Collembola] to all large adults) are counted on a square yard of rich moist surface soil, 500 are found easily and 2,000 are not unusual in soil samples in the north temperate zone.
This amounts to roughly 4,000,000 insects on one moist acre. In such an area only an occasional butterfly, bumblebee, or large beetle supergiants among insects probably would be noticed. Only a few thousand species, those that attack man’s crops, herds, and products and those that carry disease, interfere with human life seriously enough to require control measures. When they do appear as enemies of man, however, they can build up enormous populations with speed. Insects are adapted to every land and freshwater habitat where food is available, from deserts to jungles, from glacial fields and cold mountain streams to stagnant, lowland ponds and hot springs. Many live in brackish water up to 1/10 the salinity of seawater, a few live in seawater, and some fly larvae can live in pools of crude petroleum, where they eat other insects that fall in.
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