Insect wing

Insect wings are 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. Insect wings do not constitute appendages in technical parlance, as insects only have one pair of appendages per segment. 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.

Fully functional wings are present only in the adult stage, after the last moult. The one exception is the order Ephemeroptera, in which the penultimate instar (also called the subimago) possesses well-developed and functional wings, which are shed at the final moult. Wings are only present in the subclass Pterygota, with members of the archaic Apterygota being wingless. Wings may also be lost in some pterygote clades, such as the fleas and lice.

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. Two main theories on the origins of insect flight are that wings developed from paranotal lobes, extensions of the thoracic terga; and that they are modifications of movable abdominal gills as found on aquatic naiads of mayflies.^[1]

Structure

Veins

A hypothetical primitive wing venation pattern is often used as a basis for describing the pattern in extant forms. This primitive archetype is termed the archedictyon.

File:Delias eucharis venation.png

Wing venation of a butterfly (Order Lepidoptera)

The veins are hollow structures formed from the coupling of the upper and lower walls of the wing. The bigger ones may contain nerves, tracheae and hemolymph. The smaller ones can be mainly chitinous rigidifying structures. The pattern is extremely variable and can be very useful for identification.

The veins provide both rigidity and flexiblility enabling the use of the insect wings for flight. Most importantly, the wing membrane between the veins is commonly corrugated, distorted or angled (or pleated) in order to give further strength to the wing, as well as changing the effective cross-section of the wing from a flat sheet into something more closely approximating an airfoil.

Cells

The cells are spaces formed by the cross-connecting of the veins. They can be open (extending to the margin of the wing) or closed (delimited only by veins).

Naming system

The veins and cells are named after the Comstock-Needham system (1898).

Flight

Insect flight can be extremely fast, manoeuverable and versatile. This flight is possible due to the changing shape, extraordinary control and variable motion of the insect wing. Insect orders use different flight mechanisms, for example, the flight of a butterfly can be explained using steady-state, non-transitory aerodynamics and thin aerofoil theory. For a more detailed description, see insect flight.

Adaptations

Several orders of insects have specially-adapted wings.

For orientation

In the Diptera (true flies), the posterior pair of wings are reduced to halteres, which help the fly to sense its orientation and movement, as well as to improve balance by acting similar to gyroscopes.
In the Strepsiptera, it is the anterior wings of the males that are reduced to form halteres. The females are wingless.

For protection

Cockchafer (Order Coleoptera) opening its elytra and spreading its wings

In Coleoptera (beetles), the front pair of wings are sclerotised (hardened) to form elytra and they protect the delicate hindwings which are folded beneath.
In Hemiptera (true bugs), the forewings may be hardened, though to a lesser extent than in the beetles. For example, the anterior part of the front wings of stink bugs is hardened, while the posterior part is membranous. They are called hemelytron (pl. hemelytra). They are only found in the suborder Heteroptera; the wings of the Homoptera, such as the cicada, are typically entirely membranous.
Other orders such as the Dermaptera (earwigs), Orthoptera (grasshoppers, crickets), Mantodea (praying mantis) and Blattodea (cockroaches) have rigid leathery forewings that aren't used for flying, sometimes called tegmen (pl. tegmina), elytra, or pseudoelytron.
In a number of other orders, the forewings may occasionally be modified for protection, and this usually occurs in conjunction with the loss or reduction of the hindwings (i.e., in flightless insects). Similarly, flightless members of the preceding orders often entirely lack hindwings.

Other adaptations

Some orders may use their wings for communication. For example, the elaborate colours on butterfly wings are sometimes a warning for predators (aposematism), as is the case in toxic species such as the monarch butterfly. Many insects can see in the ultraviolet range of light and some species have UV reflective patches on their wing, which act as indicators of fitness used in mate selection (see sexual selection).
In the Dipteran subsection Calyptratae, the very hindmost portion of the wings are modified into somewhat thickened flaps called calypters which cover the halteres.
In a number of Diptera, especially in the superfamily Tephritoidea (various "picture-winged" flies), the wings are used in elaborate courtship displays by the males, though not in flight; the wings are lifted, flipped, and rotated in various ways (often left and right independently) while the male walks or dances near the female he is courting.
Males in a few groups of Lepidoptera have specially-modified sets of wing scales that are associated with pheromone glands in the wings themselves, and structured in such a way as to facilitate the evaporation and dispersal of the pheromones. Perhaps the most well-known species of this type is the Monarch butterfly, in which the modified scales form a small black bulge along one of the hindwing veins.
In the Mecoptera, males of the family Boreidae ("snow scorpionflies") have the wings reduced to bristles, which they use to help grasp the females during mating.
In the order Orthoptera wings are modified to help in sound production. In the Ensifera this is achieved by rubbing the edges of the wings, which have minute rasp like structures, against each other while the hind femora are rubbed against the wings in the Caelifera (see also stridulation). In a few grasshoppers, the sound-producing structures function only when the wings are flapping, with the forewings and hindwings hitting one another, and in some Lepidoptera (e.g., Cracker butterflies), sound is produced by the forewings striking one another at the peak of the upstroke.
Aquatic beetles such as the diving beetle Dytiscus use the space between the elytra and the abdomen to hold air.
Some species use the wings for thermoregulation. Many alpine butterflies have black patches on their wing which help absorb solar radiation and thermoregulate by changing the posture of the wings.
Some species of Tenebrionid beetles in the Namib desert have elytra which act as surfaces for fog to condense and have a ridge to divert the water towards their mouth.

Notes

^ Grimaldi, David (2005). Evolution of the Insects. New York, NY: Cambridge University Press. {{cite book}}: Unknown parameter |additionalauthors= ignored (help)

References

Triplehorn, Charles A. (2005). Borror and DeLong's introduction to the study of insects (7th edition ed.). Thomson Brooks/Cole. ISBN 0-03-096835-6. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

External links

North-Carolina state University course on insect wings

[1] Grimaldi, David (2005). Evolution of the Insects. New York, NY: Cambridge University Press. {{cite book}}: Unknown parameter |additionalauthors= ignored (help)

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