Insect wing
Insect wings are exoskeletal outgrowths enabling flight.
Insect wings are outgrowths of the exoskeleton that appear in adult insects and allow them to fly. These structures are located on the mesothorax and metathorax—the second and third segments of the thorax—and the two sets are typically called forewings and hindwings, though some insects lack hindwings entirely, even as remnants. A network of longitudinal veins strengthens the wings, and these veins often connect via cross-veins to create closed cells within the membrane; dragonflies and lacewings show extreme examples of this. The patterns formed by vein fusion and cross-connections frequently serve as identifying features for different evolutionary groups, allowing classification down to the family or genus level in many insect orders.
Some insects move their flight muscles directly, while others do so indirectly. In direct-flight insects, the wing muscles attach straight to the wing base, so a small downward motion of that base lifts the wing upward. In indirect-flight insects, muscles attach to and distort the thorax, which in turn moves the wings. Wings appear in only one sex—often the male—in groups like velvet ants and Strepsiptera, or are lost in the worker castes of social insects such as ants and termites. Rarely, as in fig wasps, the female is winged while the male is not. In some cases, wings appear only during certain life stages, such as the dispersal phase of aphids. Wing structure and coloration can vary with morphs, seen in aphids, migratory locust phases, and polymorphic butterflies. At rest, wings may lie flat or fold along specific patterns; most often the hindwings fold, but in groups like vespid wasps, the forewings fold instead.
The evolutionary origin of insect wings remains debated. In the 19th century, two main positions emerged: one held that wings evolved from pre-existing structures, the other that they were entirely new formations. The "novel" hypothesis proposed that wings did not arise from ancestral appendages but as outgrowths of the body wall. Since then, research has built on the pre-existing-structures position. Recent literature points to several ancestral structures as key to wing origins, including gills, respiratory leg appendages, and lateral (paranotal) and posterolateral thoracic projections. More current candidates include gill-like structures, the paranotal lobe, and the crustacean tergal plate—the latter supported by genetic research showing insects are pan-crustacean arthropods with a direct crustacean ancestor and shared limb-development mechanisms. Other theories—the paranotal lobe theory, the gill theory, and the dual theory—suggest wings developed from extensions of the thoracic terga, from movable abdominal gills like those on mayfly naiads, or from the fusion of pre-existing endite and exite structures that already had articulation and tracheation.
**Morphology**
**Internal**
Each wing has a thin membrane supported by a system of veins. The membrane consists of two closely apposed integument layers, while veins form where these layers stay separate; sometimes the lower cuticle is thicker and more heavily sclerotized under a vein. Inside each major vein runs a nerve and a trachea, and because the vein cavities connect to the hemocoel, hemolymph can flow into the wings. During wing development, the dorsal and ventral integument layers become closely pressed together over most of their area, creating the wing membrane, while the remaining areas form channels—the future veins—where nerves and tracheae may occur. The cuticle around the veins thickens and becomes more sclerotized for strength and rigidity. Two types of hair can appear on wings: microtrichia, which are small and irregularly scattered, and macrotrichia, which are larger, have sockets, and may be limited to veins. The scales of Lepidoptera and Trichoptera are highly modified macrotrichia.
**Venation**
In very small insects, venation may be greatly reduced; for example, in chalcidoid wasps only the subcosta and part of the radius remain. Conversely, venation can increase through branching of existing veins to form accessory veins or through development of additional intercalary veins between the originals, as seen in Orthoptera (grasshoppers and crickets). Large numbers of cross-veins occur in some insects, forming a reticulum in Odonata (dragonflies and damselflies) and at the base of the forewings in Tettigonioidea and Acridoidea (katydids and grasshoppers). The archedictyon is a hypothetical wing-venation scheme proposed for the first winged insect, based on speculation and fossil data. Because all winged insects are thought to share a common ancestor, the archedictyon represents a template modified (and streamlined) by natural selection over 200 million years. According to the current hypothesis, the archedictyon contained 6–8 longitudinal veins, named using the Comstock–Needham system: Costa (C) is the leading edge, Subcosta (Sc) is the second longitudinal vein behind the costa, and so on.
- field
- Entomology, Evolutionary Biology
- known_for
- Flight-enabling outgrowths of the insect exoskeleton, with complex venation patterns used for identification
Lore & Background
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. The 'novel' hypothesis suggested that insect wings did not form from pre-existing ancestral appendages but rather as outgrowths from the insect body wall. Long since, research on insect wing origins has built on the 'pre-existing structures' position that was originally proposed in the 19th century. Recent literature has pointed to several ancestral structures as being important to the origin of insect wings. Among these include: gills, respiratory appendages of legs, and lateral (paranotal) and posterolateral projections of the thorax to name a few. According to more current literature, possible candidates include gill-like structures, the paranotal lobe, and the crustacean tergal plate. The latter is based on recent insect genetic research which indicates that insects are pan-crustacean arthropods with a direct crustacean ancestor and shared genetic mechanisms of limb development. Other theories of the origin of insect wings are the paranotal lobe theory, the gill theory and the dual theory of insect wing evolution. These theories postulate that wings either developed from paranotal lobes, extensions of the thoracic terga; that they are modifications of movable abdominal gills as found on aquatic naiads of mayflies; or that insect wings arose from the fusion of pre-existing endite and exite structures each with pre-existing articulation and tracheation.
Reader's Guide
Insect wings are a defining feature of the class Insecta, enabling flight and playing a critical role in dispersal, mating, and survival. Their structure—a thin membrane supported by a system of veins—varies widely across orders, with venation patterns serving as key diagnostic traits for identification to family or genus level. The wings are present on the mesothorax and metathorax, and their musculature can be either direct (muscles attach to wing base) or indirect (muscles deform the thorax). Wing presence and form can be sex-limited, as in velvet ants and Strepsiptera where only males are winged, or lost in worker castes of social insects. The evolutionary origin of insect wings remains debated, with competing theories including development from paranotal lobes, gill-like structures, or crustacean tergal plates, reflecting ongoing research into insect ancestry as pan-crustacean arthropods. The hypothetical ancestral wing venation, the archedictyon, provides a template for understanding the modification and streamlining of wing veins over evolutionary time. The Comstock–Needham system names the longitudinal veins (Costa, Subcosta, Radius, Media, Cubitus, Anal veins) and their branches, which are used to describe wing venation across insect orders. Overall, insect wings are a complex and diverse adaptation central to insect biology and evolution.
Did You Know?
- Insect wings are found on the second and third thoracic segments (mesothorax and metathorax).
- In some groups such as velvet ants and Strepsiptera, wings are present only in the male.
- The evolutionary origin of insect wings is debated, with theories including paranotal lobes, gill-like structures, and crustacean tergal plates.
- The archedictyon is a hypothetical scheme of wing venation proposed for the very first winged insect.
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