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insect wing
Insect wings are adult outgrowths of the insect exoskeleton that enable insects to fly. They are found on the second and third thoracic segments (the mesothorax and metathorax), and the two pairs are often referred to as the forewings and hindwings, respectively, though a few insects lack hindwings, even rudiments. The wings are strengthened by a number of longitudinal veins, which often have cross-connections that form closed "cells" in the membrane (extreme examples include Odonata and Neuroptera). The patterns resulting from the fusion and cross-connection of the wing veins are often diagnostic for different evolutionary lineages and can be used for identification to the family or even genus level in many orders of insects.
The physical dynamics of flight are composed of direct and indirect flight. Those species that employ direct flight have wing muscles directly attached to the wing base, so that a small downward movement of the wing base lifts the wing itself upward. However, insects with indirect flight have muscles that attach to the thorax and deform it; since the wings are extensions of the thoracic exoskeleton, the deformations of the thorax cause the wings to move as well.
The wings may be present in only one sex (often the male) in some groups such as velvet ants and Strepsiptera, or selectively lost in "workers" of social insects such as ants and termites. Rarely, the female is winged but the male not, as in fig wasps. In some cases, wings are produced only at particular times in the life cycle, such as in the dispersal phase of aphids. Beyond the mere presence/absence of wings, the structure and colouration will often vary with morphs, such as in the aphids, migratory phases of locusts and in polymorphic butterflies.
At rest, the wings may be held flat, or folded a number of times along specific patterns; most typically, it is the hindwings which are folded, but in a very few groups such as vespid wasps, it is the forewings. How and why insect wings evolved is not well understood. Three main theories on the origins of insect flight are that wings developed from paranotal lobes, extensions of the thoracic terga; that they are modifications of movable abdominal gills as found on aquatic naiads of mayflies; and that insect wings arose from the fusion of pre-existing endite and exite structures each with pre-existing articulation and tracheation.
==Morphology==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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