Insects are generally small in size and possess segmented bodies supported by an exoskeleton made mostly of chitin. The body is divided into a head, a thorax, and an abdomen. The head supports a pair of sensory antennae, a pair of compound eyes as well as the mouth; the thorax has six legs (one pair per segment) and wings (if present in the species). The abdomen has excretory and reproductive structures.
Insects have a complete digestive system. That is, their digestive system consists of a tube that runs from mouth to anus, contrasting the incomplete digestive system found in simpler invertebrates. The excretory system consists of Malpighian tubules for the removal of nitrogenous wastes and the hindgut for osmoregulation. At the end of their hindguts, insects are able to reabsorb water along with k+ and Na+. Therefore, insects don't usually excrete water along with their feces, which allows them to store water in their bodies. This process of reabsorbtion enable them to withstand hot and deserted environments.
As noted above, most insects have two pairs of wings located on the second and third thoracic segments. Insects are the only invertebrate group to have developed flight, and this has played an important part in their success of reproduction. 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 structure.
More advanced flyers, which make up the Neoptera, generally have wings that can be folded over their back, keeping them out of the way when not in use. In these, the wings are powered mainly by indirect flight muscles that move them by stressing the thorax wall. 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 terrestrial vertebrates 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 hemolymph seeps back into the heart.
Insects hatch from eggs, and undergo a series of moults as they develop and grow in size. In most types of insects, the young, called nymphs, are basically similar in form to the adults (grasshopper), 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.
Social insects, such as the ant or the bee, are the most familiar species of eusocial animal. They live together in large well-organized colonies that are so tightly integrated and genetically similar the colonies are sometimes considered superorganisms.
Many insects possess very refined organs of perception; in some cases, their senses can be more capable than humans. For example, bees can see in the ultraviolet spectrum, and male moths have a specialized sense of smell that enables them to detect the pheromones of female moths over many kilometers.
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