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 the dragonflies and lacewings.
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. Physically, some insects move their flight muscles directly, others indirectly. In insects with direct flight, the wing muscles directly attach to the wing base, so that a small downward movement of the wing base lifts the wing itself upward.
Those insects with indirect flight have muscles that attach to and deform the thorax, causing the wings to move as well. The wings are present in only one sex often the male in some groups such as velvet ants and Strepsiptera , or are 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. Wing structure and colouration often vary with morphs , such as in the aphids , migratory phases of locusts and 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 few groups such as the vespid wasps , it is the forewings.
The evolutionary origin of the insect wing is debated. During the 19th century, the question of insect wing evolution originally rested on two main positions. One position postulated insect wings evolved from pre-existing structures, while the second proposed insect wings were entirely novel formations. Among these include: gills, respiratory appendages of legs, and lateral paranotal and posterolateral projections of the thorax to name a few.
