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Insects are the predominant group of arthropods (more than 800,000 species discovered), and of the animals in general. They may be found in nearly all environments except for the oceans. Insects have segmented bodies, divided into a head, thorax, and abdomen. The head supports a pair of sensory antennae as well as the mouth, the thorax has six legs, and the abdomen has excetory and reproductive structures. A few small groups with similar body plans are united with the insects as the Hexapoda. The true insects are distinguished from other forms in part by having ectognathous, or exposed, mouthparts.
There are 5,000 dragonfly species, 20,000 orthopteran, 170,000 lepidopteran, 82,000 hemipteran, 350,000 coleopteran, and 110,000 hymenopteran.
Most insects also have two pairs of wings, located on the second and third thoracic segments. They are the only group other than the vertebrates to have developed flight, and this has played an important part in their success. The winged insects, and their secondarily wingless relatives, make up the Pterygota[?]. Insect flight is not very well understood, relying heavily on turbulent atmospheric effects. In primitive insects it tends to rely heavily on direct flight muscles, which act upon the wing.
More advanced flyers, which make up the Neoptera[?], generally have wings which can fold over their back, keeping them out of the way when not in use. In these, the wings are powered mainly by indirect flight muscles, which move them by stressing the thorax. These muscles have adapted to contract when stretched without nervous impulses, allowing the wings to beat much faster than would be otherwise possible.
Insects hatch from eggs, and undergo a series of moults as they develop. In most groups the young, called nymphs, are basically similar in form to the adults, though the wings are not yet developed. This is called incomplete metamorphosis. Complete metamorphosis distinguishes the Endopterygota[?], which include many of the most succesful insect groups. In these, the egg hatches to produce a larva, which is generally worm-like in form and may be fairly helpless. This in turn becomes a pupa, which is often sealed within a cocoon or chrysalis, and undergoes considerable change in form before emerging as an adult.
See also : insectivorous