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Scientific classification

Subclass Apterygota (wingless insects)
  Archaeognatha (jumping bristletails)
  Monura - extinct
  Thysanura (common bristletails)
  Collembola (springtails)
  Diplura (two-pronged bristle tails)
  Protura (proturans)
Subclass Pterygota
 Infraclass Paleoptera
  Palaeodictyoptera - extinct
  Ephemeroptera (mayflies)
  Odonata (dragonflies and damselflies)
 Infraclass Neoptera
  Blattodea (cockroaches)
  Mantodea (mantids)
  Isoptera (termites)
  Dermaptera (earwigs)
  Plecoptera (stoneflies)
  Orthoptera (grasshoppers, cricketss, katydids)
  Phasmatodea (walking sticks, timemas)
  Embioptera (webspinners)
Superorder Hemipterodea
  Psocoptera (booklice, barklice)
  Phthiraptera (lice)
  Hemiptera (true bugs)
  Thysanoptera (thrips)
Superorder Endopterygota
  Miomoptera - extinct
  Megaloptera (alderflies, etc.)
  Raphidioptera (snakeflies)
  Neuroptera (net-veined insects)
  Coleoptera (beetles)
  Strepsiptera (twisted-winged parasites)
  Mecoptera (scorpionflies, etc.)
  Siphonaptera (fleas)
  Protodiptera extinct
  Diptera (true flies)
  Trichoptera (caddisflies)
  Lepidoptera (butterflies, moths)
  Hymenoptera (ants, bees, wasps, etc.)
Insects are the predominant group of arthropods and of the animals in general. They may be found in nearly all environments except for the oceans.

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.

Table of contents
1 Insect biology
2 Insect roles in the environment
3 Fossil insects
4 External link

Insect biology

Insects are usually very small and 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 excretory system directly effects the osmoregulation of insects. 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. Insects have a complete digestive system.

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 using lungs as humans do; instead they use tracheal respiration in order to transport oxygen through their bodies. Insects have openings on the surface of their bodies called spiracles that lead to their tracheal systems. The air goes into the tracheal tubes and passes through the system of branching trachea. 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.

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.

Insects often have much more refined sense organs than humans. For example bees can see ultraviolet light, and male moths can follow the odor of female moths for many kilometers. A variety of odors, called pheromones are produced by insects in order to communicate with other insects.

Insect roles in the environment

Many insects are considered pests, because they transmit diseases (mosquitos, flies), damage structures (termites) or destroy agricultural goods (locusts, boll weevils). Many entomologists are involved in various forms of pest control, often using insecticides, but more and more are learning methods of biocontrol.

Although pest insects get the most attention, more insects are beneficial to the environment and to humans. Some pollinate flowering plants (wasps, bees, butterflies). According to the theory of evolution, insects have co-evolved in step with them. Pollination is a trade between plants which need to reproduce, and pollinators which receive rewards of nectar and pollen.A serious environmental problem today is the decline of populations of pollinator insects, and a number of species of insects are now cultured primarily for pollination management in order to have sufficient pollinators in the field, orchard or greenhouse at bloom time.

Insects also produce useful substances such as honey wax or silk. Honeybee, (pictured above]] have been cultured by humans for thousands of years for honey, although contracting for crop pollination is becoming more significant for beekeepers. The silkworm has greatly affected human history as silk-driven trade established relationships between China and the rest of the world.

In some parts of the world, insects are used for human food. This minor use could be developed, if prejudices could be overcome, to provide a major source of protein in human nutrition. Since it is impossible to entirely eliminate pest insects from the human food chain, insects already are present in many foods, especially grains. Most people do not realize that food laws do not prohibit insect parts in food, but rather limit the quantity.

Many insects, especially beetles, are scavengers, feeding on dead animals and fallen trees, recycling the biological materials into useful forms for other organisms .

Although mostly unnoticed by most humans, arguably the most useful of all insects are 'insectivores', those that feed on other insects. Many insects, such as grasshoppers can potentially reproduce so fast that they could literally bury the earth in a single season. However there are hundreds of other insect species that feed on grasshopper eggs, and some that feed on grasshopper adults. This role in ecology is usually assumed to be primarily one of birds, but insects, though less glamorous, are much more significant. For any pest insect one can name, there is a species of wasp that is either a parasitoid or predator upon that pest, and forms a significant role in controlling it.

Human attempts to control pests by insecticides may backfire, because important, but unrecognized insects already helping to control the pest populations, are also killed by the insecticides, leading to later population explosions of the pests.

Fossil insects

The study of fossilized insects is Paleoentomology.

For a more complete list of the species of insects that are covered in Wikipedia, see: List of insects.

Arthropods often confused with insects include centipedes, millipedes, scorpions and spiders.

External link