Archaeognatha (jumping bristletails)
Monura - extinct
Thysanura (common bristletails)
Palaeodictyoptera - extinct
Odonata (dragonflies and damselflies)
Orthoptera (grasshoppers, cricketss, katydids)
Phasmatodea (walking sticks, timemas)
Psocoptera (booklice, barklice)
Hemiptera (true bugs)
Miomoptera - extinct
Megaloptera (alderflies, etc.)
Neuroptera (net-veined insects)
Strepsiptera (twisted-winged parasites)
Mecoptera (scorpionflies, etc.)
Diptera (true flies)
Lepidoptera (butterflies, moths)
Hymenoptera (ants, bees, wasps, etc.)
The scientific study of insects is entomology. More than 800,000 species of insects have been described. There are 5,000 dragonfly species, 20,000 orthopteran, 170,000 lepidopteran, 82,000 hemipteran, 350,000 coleopteran, and 110,000 hymenopteran species.
Insects have segmented bodies, divided into a head, thorax, and abdomen. The head supports a pair of sensory antennae, a pair of compound eyes as well as the mouth, the thorax has six legs, and the abdomen has excretory and reproductive structures. The insect body is supported by an exoskeleton made mostly of chitin. A few small groups with similar body plans, such as springtails (Collembola), are united with the insects as the Hexapoda. The true insects are distinguished from other forms in part by having ectognathous, or exposed, mouthparts.
Most insects also have two pairs of wings, located on the second and third thoracic segments. They are the only invertebrate group 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 do not breathe and do not have lungs; a system of airways called trachea allows oxygen to diffuse directly from the air into the tissues. The circulatory system of insects, like that of other arthropods, is open: the heart pumps the hemolymph through arteries to open spaces surrounding the organs; when the heart relaxes, the haemolymph seeps back into the heart.
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 successful 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.
Many insects are considered pests, because they transmit diseases (mosquitos, flies), damage structures (termites) or destroy agricultural goods (locusts). Others are useful to humans because they pollinate flowering plants (wasps, bees, butterflies) or produce substances such as honey wax or silk. Unique in the animal kingdom are the social insects such as ants or bees that live together in large well-organized colonies, so tightly integrated and genetically identical that they are often considered superorganisms.
For a more complete list of the species of insects that are covered in Wikipedia, see: List of insects.
Animals that feed on insects are said to be 'insectivorous' and are called 'insectivores'.