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Insect wings are adult outgrowths of the insect exoskeleton that enable them to fly. With their wings, insects are the only group of invertebrates known to have evolved flight. They are found on the second and third thoracic segments, 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. The patterns resulting from the fusion and cross-connection of the wing veins are often diagnostic for different evolutionary lineages.

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.

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. (see more...)