Insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...
s are the only group of
invertebrate
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s that have evolved
wings
A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expresse ...
and
flight
Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...
. Insects first flew in the
Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
, some 350 to 400 million years ago, making them the first animals to evolve flight. Wings may have evolved from appendages on the sides of existing limbs, which already had nerves, joints, and muscles used for other purposes. These may initially have been used for sailing on water, or to slow the rate of descent when gliding.
Two insect groups, the
dragonflies
A dragonfly is a flying insect belonging to the infraorder Anisoptera below the order Odonata. About 3,000 extant species of true dragonfly are known. Most are tropical, with fewer species in temperate regions. Loss of wetland habitat threa ...
and
mayflies
Mayflies (also known as shadflies or fishflies in Canada and the upper Midwestern United States, as Canadian soldiers in the American Great Lakes region, and as up-winged flies in the United Kingdom) are aquatic insects belonging to the orde ...
, have flight muscles attached directly to the wings. In other winged insects, flight muscles attach to the thorax, which make it oscillate in order to induce the wings to beat. Of these insects, some (
flies
Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...
and some
beetle
Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...
s) achieve very high wingbeat frequencies through the evolution of an "asynchronous" nervous system, in which the thorax oscillates faster than the rate of nerve impulses.
Not all insects are capable of flight. A number of
apterous
Aptery is the anatomical condition of an animal completely lacking any kind of wings. An animal with this condition is said to be apterous.
Most animal species belong to and are phylogenetic descendants of apterous taxa. These groups are said to ...
insects have
secondarily lost their wings through evolution, while other more
basal insects like
silverfish
The silverfish (''Lepisma saccharinum'') is a species of small, primitive, wingless insect in the order Zygentoma (formerly Thysanura). Its common name derives from the insect's silvery light grey colour, combined with the fish-like appearance ...
never evolved wings. In some
eusocial
Eusociality (from Greek εὖ ''eu'' "good" and social), the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping generat ...
insects like
ant
Ants are eusocial insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from vespoid wasp ancestors in the Cretaceous period. More than 13,800 of an estimated total of 22 ...
s and
termite
Termites are small insects that live in colonies and have distinct castes (eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blattode ...
s, only the
alate
Alate (Latin ''ālātus'', from ''āla'' (“wing”)) is an adjective and noun used in entomology and botany to refer to something that has wings or winglike structures.
In entomology
In entomology, "alate" usually refers to the winged form o ...
reproductive castes develop wings during the
mating season
Seasonal breeders are animal species that successfully mate only during certain times of the year. These times of year allow for the optimization of survival of young due to factors such as ambient temperature, food and water availability, and cha ...
before shedding their wings after mating, while the members of other castes are wingless their entire lives.
Some very small insects make use not of steady-state
aerodynamics
Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...
, but of the
Weis-Fogh clap and fling mechanism, generating large lift forces at the expense of wear and tear on the wings. Many insects can hover, maintaining height and controlling their position. Some insects such as
moth
Moths are a paraphyletic group of insects that includes all members of the order Lepidoptera that are not butterflies, with moths making up the vast majority of the order. There are thought to be approximately 160,000 species of moth, many of w ...
s have the forewings
coupled
''Coupled'' is an American dating game show that aired on Fox Broadcasting Company, Fox from May 17 to August 2, 2016. It was hosted by television personality, Terrence J and created by Mark Burnett, of ''Survivor (U.S. TV series), Survivor'', ''T ...
to the hindwings so these can work in unison.
Mechanisms
Direct flight
Unlike other insects, the wing muscles of the
Ephemeroptera (mayflies) and
Odonata
Odonata is an order of flying insects that includes the dragonflies and damselflies. Members of the group first appeared during the Triassic, though members of their total group, Odonatoptera, first appeared in Late Carboniferous.
The two comm ...
(dragonflies and damselflies) insert directly at the wing bases, which are hinged so that a small downward movement of the wing base lifts the wing itself upward, much like
rowing
Rowing is the act of propelling a human-powered watercraft using the sweeping motions of oars to displace water and generate reactional propulsion. Rowing is functionally similar to paddling, but rowing requires oars to be mechanically atta ...
through the air.
Dragonflies
A dragonfly is a flying insect belonging to the infraorder Anisoptera below the order Odonata. About 3,000 extant species of true dragonfly are known. Most are tropical, with fewer species in temperate regions. Loss of wetland habitat threa ...
and
damselflies
Damselflies are flying insects of the suborder Zygoptera in the order Odonata. They are similar to dragonflies, which constitute the other odonatan suborder, Anisoptera, but are smaller and have slimmer bodies. Most species fold the wings along ...
have fore and hind wings similar in shape and size. Each operates independently, which gives a degree of fine control and mobility in terms of the abruptness with which they can change direction and speed, not seen in other flying insects. Odonates are all aerial predators, and they have always hunted other airborne insects.
File:Direct flight in insects.gif, Direct flight: muscles attached to wings. Large insects only
File:Australian Emperor in flight.jpg, The Odonata
Odonata is an order of flying insects that includes the dragonflies and damselflies. Members of the group first appeared during the Triassic, though members of their total group, Odonatoptera, first appeared in Late Carboniferous.
The two comm ...
(dragonflies and damselflies) have direct flight musculature, as do mayflies
Mayflies (also known as shadflies or fishflies in Canada and the upper Midwestern United States, as Canadian soldiers in the American Great Lakes region, and as up-winged flies in the United Kingdom) are aquatic insects belonging to the orde ...
.
File:Dragonfly in ultra slow motion.webm, Slow motion flight of dragonfly
Indirect flight
Other than the two orders with direct flight muscles, all other living winged insects fly using a different mechanism, involving indirect flight muscles. This mechanism evolved once and is the defining feature (
synapomorphy
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...
) for the infraclass
Neoptera
Neoptera (Ancient Greek ''néos'' (“new”) + ''pterón'' (“wing”)) is a classification group that includes most orders of the winged insects, specifically those that can flex their wings over their abdomens. This is in contrast with the mo ...
; it corresponds, probably not coincidentally, with the appearance of a wing-folding mechanism, which allows Neopteran insects to fold the wings back over the abdomen when at rest (though this ability has been lost secondarily in some groups, such as in the
butterflies
Butterflies are insects in the macrolepidopteran clade Rhopalocera from the Order (biology), order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The ...
).
What all Neoptera share, however, is the way the muscles in the
thorax
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the cre ...
work: these muscles, rather than attaching to the wings, attach to the thorax and deform it; since the wings are extensions of the thoracic
exoskeleton
An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...
, the deformations of the thorax cause the wings to move as well. A set of longitudinal muscles along the back compresses the thorax from front to back, causing the dorsal surface of the thorax (
notum
The notum (plural nota) is the dorsal portion of an insect's thoracic segment, or the dorsal surface of the body of nudibranch gastropods. The word "notum" is always applied to dorsal structures; in other words structures that are part of the back ...
) to bow upward, making the wings flip down. Another set of muscles from the
tergum
A ''tergum'' (Latin for "the back"; plural ''terga'', associated adjective tergal) is the Anatomical terms of location#Dorsal and ventral, dorsal ('upper') portion of an arthropod segment other than the head. The Anatomical terms of location#Ante ...
to the
sternum
The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sh ...
pulls the notum downward again, causing the wings to flip upward.
File:Motion of Insectwing.gif, Indirect flight: muscles make thorax oscillate in most insects
File:Butterfly landing.jpg, The Neoptera
Neoptera (Ancient Greek ''néos'' (“new”) + ''pterón'' (“wing”)) is a classification group that includes most orders of the winged insects, specifically those that can flex their wings over their abdomens. This is in contrast with the mo ...
, including butterflies
Butterflies are insects in the macrolepidopteran clade Rhopalocera from the Order (biology), order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The ...
and most other insects, have indirect flight musculature
Insects that beat their wings fewer than one hundred times a second use synchronous muscle. Synchronous muscle is a type of muscle that contracts once for every nerve impulse. This generally produces less power and is less efficient than asynchronous muscle, which accounts for the independent evolution of asynchronous flight muscles in several separate insect clades.
Insects that beat their wings more rapidly, such as the
bumblebee
A bumblebee (or bumble bee, bumble-bee, or humble-bee) is any of over 250 species in the genus ''Bombus'', part of Apidae, one of the bee families. This genus is the only extant group in the tribe Bombini, though a few extinct related gener ...
, use asynchronous muscle; this is a type of muscle that contracts more than once per nerve impulse. This is achieved by the muscle being stimulated to contract again by a release in tension in the muscle, which can happen more rapidly than through simple nerve stimulation alone. This allows the frequency of wing beats to exceed the rate at which the nervous system can send impulses. The asynchronous muscle is one of the final refinements that has appeared in some of the higher Neoptera (
Coleoptera
Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...
,
Diptera
Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...
, and
Hymenoptera
Hymenoptera is a large order (biology), order of insects, comprising the sawfly, sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are Par ...
). The overall effect is that many higher Neoptera can beat their wings much faster than insects with direct flight muscles.
Aerodynamics
There are two basic aerodynamic models of insect flight: creating a leading edge vortex, and using clap and fling.
Leading edge vortex
Most insects use a method that creates a spiralling
leading edge
The leading edge of an airfoil surface such as a wing is its foremost edge and is therefore the part which first meets the oncoming air.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 305. Aviation Supplies & Academics, ...
vortex
In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...
. These flapping wings move through two basic half-strokes. The downstroke starts up and back and is plunged downward and forward. Then the wing is quickly flipped over (
supination
Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...
) so that the leading edge is pointed backward. The upstroke then pushes the wing upward and backward. Then the wing is flipped again (
pronation
Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...
) and another downstroke can occur. The
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
range in insects with synchronous flight muscles typically is 5 to 200
hertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that on ...
(Hz). In those with asynchronous flight muscles, wing beat frequency may exceed 1000 Hz. When the insect is hovering, the two strokes take the same amount of time. A slower downstroke, however, provides
thrust
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
.
Identification of major forces is critical to understanding insect flight. The first attempts to understand flapping wings assumed a quasi-steady state. This means that the air flow over the wing at any given time was assumed to be the same as how the flow would be over a non-flapping, steady-state wing at the same angle of attack. By dividing the flapping wing into a large number of motionless positions and then analyzing each position, it would be possible to create a timeline of the instantaneous forces on the wing at every moment. The calculated lift was found to be too small by a factor of three, so researchers realized that there must be unsteady phenomena providing aerodynamic forces. There were several developing analytical models attempting to approximate flow close to a flapping wing. Some researchers predicted force peaks at supination. With a dynamically scaled model of a
fruit fly, these predicted forces later were confirmed. Others argued that the force peaks during supination and pronation are caused by an unknown rotational effect that fundamentally is different from the translational phenomena. There is some disagreement with this argument. Through
computational fluid dynamics
Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid flows. Computers are used to perform the calculations required to simulate th ...
, some researchers argue that there is no rotational effect. They claim that the high forces are caused by an interaction with the wake shed by the previous stroke.
Similar to the rotational effect mentioned above, the phenomena associated with flapping wings are not completely understood or agreed upon. Because every model is an approximation, different models leave out effects that are presumed to be negligible. For example, the
Wagner effect, as proposed by
Herbert A. Wagner in 1925, says that circulation rises slowly to its steady-state due to viscosity when an inclined wing is accelerated from rest. This phenomenon would explain a lift value that is less than what is predicted. Typically, the case has been to find sources for the added lift. It has been argued that this effect is negligible for flow with a
Reynolds number
In fluid mechanics, the Reynolds number () is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be domi ...
that is typical of insect flight. The Reynolds number is a measure of
turbulence
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
;
flow is laminar (smooth) when the Reynolds number is low, and turbulent when it is high. The Wagner effect was ignored, consciously, in at least one model.
One of the most important phenomena that occurs during insect flight is leading edge suction. This force is significant to the calculation of efficiency. The concept of leading edge suction first was put forth by D. G. Ellis and J. L. Stollery in 1988 to describe vortex lift on sharp-edged
delta wing
A delta wing is a wing shaped in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta (Δ).
Although long studied, it did not find significant applications until the Jet Age, when it proved suitabl ...
s. At high angles of attack, the flow separates over the leading edge, but reattaches before reaching the trailing edge. Within this bubble of separated flow is a vortex. Because the angle of attack is so high, a lot of momentum is transferred downward into the flow. These two features create a large amount of lift force as well as some additional drag. The important feature, however, is the lift. Because the flow has separated, yet it still provides large amounts of lift, this phenomenon is called
stall delay
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', p. 486. Aviation Supplies & Academics, 1997. This occurs when t ...
, first noticed on aircraft propellers by H. Himmelskamp in 1945. This effect was observed in flapping insect flight and it was proven to be capable of providing enough lift to account for the deficiency in the quasi-steady-state models. This effect is used by
canoeists
A canoe is a lightweight narrow water vessel, typically pointed at both ends and open on top, propelled by one or more seated or kneeling paddlers facing the direction of travel and using a single-bladed paddle.
In British English, the term ...
in a
sculling draw stroke.
All of the effects on a flapping wing may be reduced to three major sources of aerodynamic phenomena: the leading edge vortex, the steady-state aerodynamic forces on the wing, and the wing’s contact with its wake from previous strokes. The size of flying insects ranges from about 20 micrograms to about 3 grams. As insect body mass increases, wing area increases and wing beat frequency decreases. For larger insects, the Reynolds number (Re) may be as high as 10000, where flow is starting to become turbulent. For smaller insects, it may be as low as 10. This means that viscous effects are much more important to the smaller insects.
Another interesting feature of insect flight is the body tilt. As flight speed increases, the insect body tends to tilt nose-down and become more horizontal. This reduces the frontal area and therefore, the body drag. Since drag also increases as forward velocity increases, the insect is making its flight more efficient as this efficiency becomes more necessary. Additionally, by changing the geometric angle of attack on the downstroke, the insect is able to keep its flight at an optimal efficiency through as many manoeuvres as possible. The development of general thrust is relatively small compared with lift forces. Lift forces may be more than three times the insect's weight, while thrust at even the highest speeds may be as low as 20% of the weight. This force is developed primarily through the less powerful upstroke of the flapping motion.
Clap and fling
Clap and fling, or the Weis-Fogh mechanism, discovered by the Danish zoologist
Torkel Weis-Fogh
Torkel Weis-Fogh (25 March 1922 – 13 November 1975) was a Danish zoologist and Professor at the University of Cambridge and the University of Copenhagen. He is best known for his contributions to the understanding of insect flight, especially ...
, is a lift generation method utilized during small insect flight. As insect sizes become less than 1 mm,
viscous forces
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inter ...
become dominant and the efficacy of lift generation from an airfoil decreases drastically. Starting from the clap position, the two wings fling apart and rotate about the trailing edge. The wings then separate and sweep horizontally until the end of the downstroke. Next, the wings
pronate
Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...
and utilize the leading edge during an upstroke rowing motion. As the clap motion begins, the leading edges meet and rotate together until the gap vanishes. Initially, it was thought that the wings were touching, but several incidents indicate a gap between the wings and suggest it provides an aerodynamic benefit.
Lift generation from the clap and fling mechanism occurs during several processes throughout the motion. First, the mechanism relies on a wing-wing interaction, as a single wing motion does not produce sufficient lift.
As the wings rotate about the trailing edge in the flinging motion, air rushes into the created gap and generates a strong leading edge vortex, and a second one developing at the wingtips. A third, weaker, vortex develops on the trailing edge.
The strength of the developing vortices relies, in-part, on the initial gap of the inter-wing separation at the start of the flinging motion. With a decreased gap inter-wing gap indicating a larger lift generation, at the cost of larger drag forces. The implementation of a heaving motion during fling, flexible wings,
and a delayed stall mechanism were found to reinforce vortex stability and attachment.
Finally, to compensate the overall lower lift production during low Reynolds number flight (with
laminar flow
In fluid dynamics, laminar flow is characterized by fluid particles following smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing. At low velocities, the fluid tends to flow without lateral mi ...
), tiny insects often have a higher stroke frequency to generate wing-tip velocities that are comparable to larger insects.
The overall largest expected drag forces occur during the dorsal fling motion, as the wings need to separate and rotate.
The attenuation of the large drag forces occur through several mechanisms. Flexible wings were found to decrease the drag in flinging motion by up to 50% and further reduce the overall drag through the entire wing stroke when compared to rigid wings.
Bristles on the wing edges, as seen in ''
Encarsia formosa
''Encarsia formosa'' is a species of chalcidoid wasp and a well known parasitoid of greenhouse whitefly, one of the first to be used commercially for biological pest control, from the 1920s. They can use at least 15 species of whitefly as a ho ...
'', cause a porosity in the flow which augments and reduces the drag forces, at the cost of lower lift generation. Further, the inter-wing separation before fling plays an important role in the overall effect of drag. As the distance increases between the wings, the overall drag decreases.
The clap and fling mechanism is also employed by the marine
mollusc
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...
''
Limacina helicina
''Limacina helicina'' is a species of small swimming planktonic sea snail in the family Limacinidae, which belong to the group commonly known as sea butterflies (Thecosomata).
''Limacina helicina'' is a keystone species of mesozooplankton in Ar ...
'', a sea butterfly. Some insects, such as the vegetable leaf miner ''
Liriomyza sativae
''Liriomyza sativae'', commonly known as the vegetable leaf miner, is a species of insect, a fly in the family Agromyzidae. The larvae of this fly mine the leaves of a range of vegetables and weeds, but seem to favour plants in the families Cucur ...
'' (a fly), exploit a partial clap and fling, using the mechanism only on the outer part of the wing to increase lift by some 7% when hovering.
File:Clap and Fling 1- clap 1.svg, Clap 1: wings close over back
File:Clap and Fling 2- clap 2.svg, Clap 2: leading edges touch, wing rotates around leading edge
The leading edge of an airfoil surface such as a wing is its foremost edge and is therefore the part which first meets the oncoming air.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 305. Aviation Supplies & Academics, ...
, vortices form
File:Clap and Fling 3 - clap 3.svg, Clap 3: trailing edges close, vortices shed, wings close giving thrust
File:Clap and Fling 4- fling 1.svg, Fling 1: wings rotate around trailing edge to fling apart
File:Clap and Fling 5- fling 2.svg, Fling 2: leading edge moves away, air rushes in, increasing lift
File:Clap and Fling 6- fling 3.svg, Fling 3: new vortex forms at leading edge, trailing edge vortices cancel each other, perhaps helping flow to grow faster ( Weis-Fogh 1973)
Governing equations
A wing moving in fluids experiences a
fluid force, which follows the conventions found in aerodynamics. The force component normal to the direction of the flow relative to the wing is called lift (''L''), and the force component in the opposite direction of the flow is drag (''D''). At the Reynolds numbers considered here, an appropriate force unit is 1/2(ρU
2S), where ρ is the density of the fluid, S the wing area, and ''U'' the wing speed. The dimensionless forces are called lift (''C
L'') and drag (''C
D'') coefficients, that is:
:
''C
L'' and ''C
D'' are constants only if the flow is steady. A special class of objects such as
airfoil
An airfoil (American English) or aerofoil (British English) is the cross-sectional shape of an object whose motion through a gas is capable of generating significant lift, such as a wing, a sail, or the blades of propeller, rotor, or turbine.
...
s may reach a steady state when it slices through the fluid at a small angle of attack. In this case, the
inviscid flow
In fluid dynamics, inviscid flow is the flow of an inviscid (zero-viscosity) fluid, also known as a superfluid. The Reynolds number of inviscid flow approaches infinity as the viscosity approaches zero. When viscous forces are neglected, such as ...
around an airfoil can be approximated by a potential flow satisfying the no-penetration boundary condition. The
Kutta-Joukowski theorem of a 2D airfoil further assumes that the flow leaves the sharp trailing edge smoothly, and this determines the total circulation around an airfoil. The corresponding lift is given by
Bernoulli's principle
In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy. The principle is named after the Swiss mathematici ...
(
Blasius theorem):
:
The flows around birds and insects can be considered
incompressible
In fluid mechanics or more generally continuum mechanics, incompressible flow ( isochoric flow) refers to a flow in which the material density is constant within a fluid parcel—an infinitesimal volume that moves with the flow velocity. An e ...
: The
Mach number
Mach number (M or Ma) (; ) is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a boundary to the local speed of sound.
It is named after the Moravian physicist and philosopher Ernst Mach.
: \mathrm = \frac ...
, or velocity relative to the speed of sound in air, is typically 1/300 and the wing frequency is about 10–103 Hz. Using the governing equation as the
Navier-Stokes equation being subject to the no-slip boundary condition, the equation is:
:
Where u(x, t) is the flow field, p the pressure, ρ the density of the fluid, ν the kinematic viscosity, u
bd the velocity at the boundary, and u
s the velocity of the solid. By choosing a length scale, L, and velocity scale, U, the equation can be expressed in nondimensional form containing the Reynolds number, R
e=uL/ν . There are two obvious differences between an
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 hindwin ...
and an airfoil: An insect wing is much smaller and it flaps. Using a dragonfly as an example, Its chord (c) is about , its wing length (l) about , and its wing frequency (f) about 40 Hz. The tip speed (u) is about , and the corresponding Reynolds number about 103. At the smaller end, a typical
chalcidoid wasp has a wing length of about and beats its wing at about 400 Hz. Its Reynolds number is about 25. The range of Reynolds number in insect flight is about 10 to 10
4, which lies in between the two limits that are convenient for theories: inviscid steady flows around an airfoil and Stokes flow experienced by a swimming bacterium. For this reason, this intermediate range is not well understood. On the other hand, it is perhaps the most ubiquitous regime among the things we see. Falling leaves and seeds, fishes, and birds all encounter unsteady flows similar to that seen around an insect.
The chordwise Reynolds number can be described by:
and
Where
is the average chord length,
is the speed of the wing tip,
is the stroke amplitude,
is the beat frequency,
is the radius of gyration,
is the wing area, and
is the length of wing, including the wing tip.
In addition to the Reynolds number, there are at least two other relevant dimensionless parameters. A wing has three velocity scales: the flapping velocity with respect to the body (''u''), the forward velocity of the body (''U''
0), and the pitching velocity (Ω''c''). The ratios of them form two dimensionless variables, ''U''
0/''u'' and Ωc/''u'', the former is often referred to as the advance ratio, and it is also related to the reduced frequency, ''fc''/''U''
0.
If an insect wing is rigid, for example, a ''
Drosophila
''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...
'' wing is approximately so, its motion relative to a fixed body can be described by three variables: the position of the tip in
spherical coordinates
In mathematics, a spherical coordinate system is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the ''radial distance'' of that point from a fixed origin, its ''polar angle'' measu ...
, (Θ(t),Φ(t)), and the
pitching angle ψ(t), about the axis connecting the root and the tip. To estimate the aerodynamic forces based on blade-element analysis, it is also necessary to determine the
angle of attack
In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is m ...
(α). The typical angle of attack at 70% wingspan ranges from 25° to 45° in hovering insects (15° in hummingbirds). Despite the wealth of data available for many insects, relatively few experiments report the time variation of α during a stroke. Among these are wind tunnel experiments of a tethered locust and a tethered fly, and free hovering flight of a fruit fly.
Because they are relatively easy to measure, the wing-tip trajectories have been reported more frequently. For example, selecting only flight sequences that produced enough lift to support a weight, will show that the wing tip follows an elliptical shape. Noncrossing shapes were also reported for other insects. Regardless of their exact shapes, the plugging-down motion indicates that insects may use aerodynamic drag in addition to lift to support its weight.
Hovering
![Hoverfly September 2007-7 (cropped)](https://upload.wikimedia.org/wikipedia/commons/d/d5/Hoverfly_September_2007-7_%28cropped%29.jpg)
Many insects can
hover, or stay in one spot in the air, doing so by beating their wings rapidly. Doing so requires sideways stabilization as well as the production of lift. The lifting force is mainly produced by the downstroke. As the wings push down on the surrounding air, the resulting reaction force of the air on the wings pushes the insect up. The wings of most insects are evolved so that, during the upward stroke, the force on the wing is small. Since the downbeat and return stroke force the insect up and down respectively, the insect oscillates and winds up staying in the same position.
The distance the insect falls between wingbeats depends on how rapidly its wings are beating: the slower it flaps, the longer the interval in which it falls, and the farther it falls between each wingbeat. One can calculate the wingbeat frequency necessary for the insect to maintain a given stability in its amplitude. To simplify the calculations, one must assume that the lifting force is at a finite constant value while the wings are moving down and that it is zero while the wings are moving up. During the time interval Δ''t'' of the upward wingbeat, the insect drops a distance ''h'' under the influence of gravity.
:
The upward stroke then restores the insect to its original position. Typically, it may be required that the vertical position of the insect changes by no more than 0.1 mm (i.e., h = 0.1 mm). The maximum allowable time for free fall is then
:
Since the up movements and the down movements of the wings are about equal in duration, the period T for a complete up-and-down wing is twice Δ''r'', that is,
:
The frequency of the beats, f, meaning the number of wingbeats per second, is represented by the equation:
:
In the examples used the frequency used is 110 beats/s, which is the typical frequency found in insects. Butterflies have a much slower frequency with about 10 beats/s, which means that they can't hover. Other insects may be able to produce a frequency of 1000 beats/s. To restore the insect to its original vertical position, the average upward force during the downward stroke, F
av, must be equal to twice the weight of the insect. Note that since the upward force on the insect body is applied only for half the time, the average upward force on the insect is simply its weight.
Power input
One can now compute the power required to maintain hovering by, considering again an insect with mass m 0.1 g, average force, F
av, applied by the two wings during the downward stroke is two times the weight. Because the pressure applied by the wings is uniformly distributed over the total wing area, that means one can assume the force generated by each wing acts through a single point at the midsection of the wings. During the downward stroke, the center of the wings traverses a vertical distance ''d''.
The total work done by the insect during each downward stroke is the product of force and distance; that is,
:
If the wings swing through the beat at an angle of 70°, then in the case presented for the insect with 1 cm long wings, d is 0.57 cm. Therefore, the work done during each stroke by the two wings is:
:
The energy is used to raise the insect against gravity. The energy ''E'' required to raise the mass of the insect 0.1 mm during each downstroke is:
:
This is a negligible fraction of the total energy expended which clearly, most of the energy is expended in other processes. A more detailed analysis of the problem shows that the work done by the wings is converted primarily into
kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...
of the air that is accelerated by the downward stroke of the wings. The power is the amount of work done in 1 s; in the insect used as an example, makes 110 downward strokes per second. Therefore, its power output P is, strokes per second, and that means its power output P is:
:
Power output
In the calculation of the power used in hovering, the examples used neglected the kinetic energy of the moving wings. The wings of insects, light as they are, have a finite mass; therefore, as they move they possess kinetic energy. Because the wings are in rotary motion, the maximum kinetic energy during each wing stroke is:
:
Here ''I'' is the moment of
inertia
Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...
of the wing and ω
max is the maximum
angular velocity
In physics, angular velocity or rotational velocity ( or ), also known as angular frequency vector,(UP1) is a pseudovector representation of how fast the angular position or orientation of an object changes with time (i.e. how quickly an objec ...
during the wing stroke. To obtain the
moment of inertia
The moment of inertia, otherwise known as the mass moment of inertia, angular mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is a quantity that determines the torque needed for a desired angular acceler ...
for the wing, we will assume that the wing can be approximated by a thin rod pivoted at one end. The moment of inertia for the wing is then:
:
Where l is the length of the wing (1 cm) and m is the mass of two wings, which may be typically 10
−3 g. The maximum angular velocity, ω
max, can be calculated from the maximum linear
velocity
Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity is a ...
, ν
max, at the center of the wing:
:
During each stroke the center of the wings moves with an average linear velocity ''ν''
av given by the distance ''d'' traversed by the center of the wing divided by the duration Δt of the wing stroke. From our previous example, d = 0.57 cm and Δt = 4.5×10
−3 s. Therefore:
:
The velocity of the wings is zero both at the beginning and at the end of the wing stroke, meaning the maximum linear velocity is higher than the average velocity. If we assume that the velocity oscillates (
sinusoidal
A sine wave, sinusoidal wave, or just sinusoid is a mathematical curve defined in terms of the '' sine'' trigonometric function, of which it is the graph. It is a type of continuous wave and also a smooth periodic function. It occurs often in m ...
ly) along the wing path, the maximum velocity is twice as high as the average velocity. Therefore, the maximum angular velocity is:
:
And the kinetic energy therefore is:
:
Since there are two wing strokes (the upstroke and downstroke) in each cycle of the wing movement, the kinetic energy is 2×43 = 86
erg
The erg is a unit of energy equal to 10−7joules (100 nJ). It originated in the Centimetre–gram–second system of units (CGS). It has the symbol ''erg''. The erg is not an SI unit. Its name is derived from (), a Greek word meaning 'work' o ...
. This is about as much energy as is consumed in hovering itself.
Elasticity
![Resilin in insect wing crossection](https://upload.wikimedia.org/wikipedia/commons/2/20/Resilin_in_insect_wing_crossection.svg)
Insects gain kinetic energy, provided by the muscles, when the wings
accelerate
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...
. When the wings begin to decelerate toward the end of the stroke, this energy must dissipate. During the downstroke, the kinetic energy is dissipated by the muscles themselves and is converted into heat (this heat is sometimes used to maintain core body temperature). Some insects are able to utilize the kinetic energy in the upward movement of the wings to aid in their flight. The wing joints of these insects contain a pad of elastic, rubber-like protein called
resilin
Resilin is an elastomeric protein found in many insects and other arthropods. It provides soft rubber-elasticity to mechanically active organs and tissue; for example, it enables insects of many species to jump or pivot their wings efficiently ...
. During the upstroke of the wing, the resilin is stretched. The kinetic energy of the wing is converted into
potential energy
In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors.
Common types of potential energy include the gravitational potentia ...
in the stretched resilin, which stores the energy much like a spring. When the wing moves down, this energy is released and aids in the downstroke.
Using a few simplifying assumptions, we can calculate the amount of energy stored in the stretched resilin. Although the resilin is bent into a complex shape, the example given shows the calculation as a straight rod of area A and length. Furthermore, we will assume that throughout the stretch the resilin obeys
Hooke's law
In physics, Hooke's law is an empirical law which states that the force () needed to extend or compress a spring (device), spring by some distance () Proportionality (mathematics)#Direct_proportionality, scales linearly with respect to that ...
. This is not strictly true as the resilin is stretched by a considerable amount and therefore both the area and
Young's modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied leng ...
change in the process of stretching. The potential energy U stored in the stretched resilin is:
:
Here ''E'' is the Young’s modulus for resilin, which has been measured to be 1.8×10
7 dyn/cm
2. Typically in an insect the size of a bee, the volume of the resilin may be equivalent to a cylinder 2×10
−2 cm long and 4×10
−4 cm
2 in area. In the example given, the length of the resilin rod is increased by 50% when stretched. That is, Δℓ is 10
−2 cm. Therefore, in this case the potential energy stored in the resilin of each wing is:
:
The stored energy in the two wings for a bee-sized insect is 36 erg, which is comparable to the kinetic energy in the upstroke of the wings. Experiments show that as much as 80% of the kinetic energy of the wing may be stored in the resilin.
Wing coupling
![Frenulo-retinacular wing coupling in male and female moths](https://upload.wikimedia.org/wikipedia/commons/a/a1/Frenulo-retinacular_wing_coupling_in_male_and_female_moths.png)
Some four-winged insect orders, such as the Lepidoptera, have developed morphological
wing coupling
Some four-winged insect orders, such as the Lepidoptera, have developed a wide variety of morphological wing coupling mechanisms in the imago which render these taxa as "functionally dipterous" (effectively two-winged) for efficient insect flight. ...
mechanisms in the
imago
In biology, the imago (Latin for "image") is the last stage an insect attains during its metamorphosis, its process of growth and development; it is also called the imaginal stage, the stage in which the insect attains maturity. It follows the f ...
which render these taxa functionally two-winged.
All but the most basal forms exhibit this wing-coupling.
[Stocks, Ian. (2008). Sec.''Wing Coupling'' in Capinera (Ed) (2008) "Encyc. Entom.", Vol 4. p. 4266]
The mechanisms are of three different types – jugal, frenulo-retinacular and amplexiform:
* The more primitive groups have an enlarged lobe-like area near the basal posterior margin, i.e. at the base of the forewing, a ''jugum'', that folds under the hindwing in flight.
[Powell, Jerry A. Ch. ''Lepidoptera'' (pp. 631-664) in Resh, & Cardé (Eds). "Encyclopedia of Insects". 2003.]
* Other groups have a frenulum on the hindwing that hooks under a retinaculum on the forewing.
* In almost all
butterflies
Butterflies are insects in the macrolepidopteran clade Rhopalocera from the Order (biology), order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The ...
and in the
Bombycoidea
Bombycoidea is a superfamily of moths. It contains the silk moths, emperor moths, sphinx moths, and relatives. The Lasiocampoidea
The Lasiocampidae are a family of moths also known as eggars, tent caterpillars, snout moths (although this also ...
(except the
Sphingidae
The Sphingidae are a family of moths (Lepidoptera) called sphinx moths, also colloquially known as hawk moths, with many of their caterpillars known as “hornworms”; it includes about 1,450 species. It is best represented in the tropics, bu ...
), there is no arrangement of frenulum and retinaculum to couple the wings. Instead, an enlarged humeral area of the hindwing is broadly overlapped by the forewing. Despite the absence of a specific mechanical connection, the wings overlap and operate in phase. The power stroke of the forewing pushes down the hindwing in unison. This type of coupling is a variation of frenate type but where the frenulum and retinaculum are completely lost.
Biochemistry
The biochemistry of insect flight has been a focus of considerable study. While many insects use
carbohydrates
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or may ...
and
lipids
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...
as the energy source for flight, many beetles and flies use the
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
proline
Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group but is rather a secondary amine. The secondary amine nitrogen is in the prot ...
as their energy source. Some species also use a combination of sources and moths such as ''
Manduca sexta
''Manduca sexta'' is a moth of the family Sphingidae present through much of the Americas. The species was Species description, first described by Carl Linnaeus in his 1763 ''Centuria Insectorum''.
Commonly known as the Carolina sphinx moth and ...
'' use carbohydrates for pre-flight warm-up.
Sensory Feedback
Insects use
sensory feedback
Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled c ...
to maintain and control flight. Research has demonstrated the role of sensory structures such as
antennae,
halteres
''Halteres'' (; singular ''halter'' or ''haltere'') (from grc, ἁλτῆρες, weights held in the hands to give an impetus in leaping) are a pair of small club-shaped organs on the body of two orders of flying insects that provide infor ...
and
wings
A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expresse ...
in controlling flight posture, wingbeat amplitude, and wingbeat frequency.
Evolution and adaptation
![Mazothairos1](https://upload.wikimedia.org/wikipedia/commons/d/d7/Mazothairos1.jpg)
Sometime in the
Carboniferous Period
The Carboniferous ( ) is a Period (geology), geologic period and System (stratigraphy), system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago (Myr, Mya), to the beginning of the Permian Period, ...
, some 350 to 400 million years ago, when there were only two major land masses, insects began flying.
Among the oldest winged insect fossils is ''
Delitzschala'', a
Palaeodictyoptera
The Palaeodictyoptera are an extinct order of medium-sized to very large, primitive Palaeozoic paleopterous insects. They are informative about the evolution of wings in insects.
Overview
They were characterized by beak-like mouthparts, used ...
n from the Lower Carboniferous;
''
Rhyniognatha
''Rhyniognatha'' is an extinct genus of arthropod of disputed placement. It has been considered in some analyses as the oldest insect known, as well as possibly being a flying insect. ''Rhyniognatha'' is known from a partial head with preserved m ...
'' is older, from the
Early Devonian
The Early Devonian is the first of three Epoch (geology), epochs comprising the Devonian period, corresponding to the Lower Devonian Series (stratigraphy), series. It lasted from and began with the Lochkovian Stage , which was followed by the P ...
, but it is uncertain if it had wings, or indeed was an insect.
How and why insect wings developed is not well understood, largely due to the scarcity of appropriate fossils from the period of their development in the Lower Carboniferous. There have historically been three main theories on the origins of insect flight. The first was that they are modifications of movable
abdominal
The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso. ...
gill
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
s, as found on aquatic
naiads
In Greek mythology, the naiads (; grc-gre, ναϊάδες, naïádes) are a type of female spirit, or nymph, presiding over fountains, wells, springs, streams, brooks and other bodies of fresh water.
They are distinct from river gods, who ...
of
mayflies
Mayflies (also known as shadflies or fishflies in Canada and the upper Midwestern United States, as Canadian soldiers in the American Great Lakes region, and as up-winged flies in the United Kingdom) are aquatic insects belonging to the orde ...
. Phylogenomic analysis suggests that the
Polyneoptera
The cohort Polyneoptera is a proposed taxonomic ranking for the Orthoptera (grasshoppers, crickets, etc.) and all other Neopteran insects believed to be more closely related to Orthoptera than to any other insect orders. These winged insects, no ...
, the group of winged insects that includes grasshoppers, evolved from a terrestrial ancestor, making the evolution of wings from gills unlikely.
Additional study of the jumping behavior of mayfly larvae has determined that tracheal gills play no role in guiding insect descent, providing further evidence against this evolutionary hypothesis. This leaves two major historic theories: that wings developed from paranotal lobes, extensions of the
thoracic
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...
terga
A ''tergum'' (Latin for "the back"; plural ''terga'', associated adjective tergal) is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the 'base' and posterior edge is called the 'apex' or 'ma ...
; or that they arose from modifications of leg segments, which already contained muscles.
Epicoxal (abdominal gill) hypothesis
![Mayfly nymph dorsal view wing buds paired gills (rotated)](https://upload.wikimedia.org/wikipedia/commons/8/86/Mayfly_nymph_dorsal_view_wing_buds_paired_gills_%28rotated%29.JPG)
Numerous
entomologist
Entomology () is the scientific study of insects, a branch of zoology. In the past the term "insect" was less specific, and historically the definition of entomology would also include the study of animals in other arthropod groups, such as arach ...
s including
Landois in 1871,
Lubbock
Lubbock ( )
is the 10th-most populous city in the U.S. state of Texas and the seat of government of Lubbock County. With a population of 260,993 in 2021, the city is also the 85th-most populous in the United States. The city is in the northwe ...
in 1873,
Graber
Graber (also known as Carrosserie Hermann Graber) was a coachbuilder based in Wichtrach in central Switzerland. Between 1927 and 1970 the firm supplied coach-built bodies for fitting on the chassis of various European and US auto-makers. G ...
in 1877, and
Osborn in 1905 have suggested that a possible origin for insect wings might have been movable abdominal gills found in many aquatic insects, such as on naiads of mayflies. According to this theory these
tracheal
The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends from the la ...
gills, which started their way as exits of the respiratory system and over time were modified into locomotive purposes, eventually developed into wings. The tracheal gills are equipped with little winglets that perpetually vibrate and have their own tiny straight muscles.
Paranotal (tergal) hypothesis
The paranotal lobe or tergal (dorsal body wall) hypothesis, proposed by
Fritz Müller
Johann Friedrich Theodor Müller (31 March 1822 – 21 May 1897), better known as Fritz Müller, and also as Müller-Desterro, was a German biologist who emigrated to southern Brazil, where he lived in and near the German community of Blumenau, ...
in 1875 and reworked by G. Crampton in 1916,
Jarmila Kulakova-Peck in 1978
and Alexander P. Rasnitsyn in 1981 among others,
suggests that the insect's wings developed from paranotal lobes, a
preadaptation
Exaptation and the related term co-option describe a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common ...
found in insect
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s that would have assisted stabilization while hopping or falling. In favor of this hypothesis is the tendency of most insects, when startled while climbing on branches, to escape by dropping to the ground. Such lobes would have served as
parachute
A parachute is a device used to slow the motion of an object through an atmosphere by creating drag or, in a ram-air parachute, aerodynamic lift. A major application is to support people, for recreation or as a safety device for aviators, who ...
s and enable the insect to land more softly. The theory suggests that these lobes gradually grew larger and in a later stage developed a joint with the thorax. Even later would appear the muscles to move these crude wings. This model implies a progressive increase in the effectiveness of the wings, starting with
parachuting
Parachuting, including also skydiving, is a method of transiting from a high point in the atmosphere to the surface of Earth with the aid of gravity, involving the control of speed during the descent using a parachute or parachutes.
For ...
, then
gliding
Gliding is a recreational activity and competitive air sport in which pilots fly unpowered aircraft known as gliders or sailplanes using naturally occurring currents of rising air in the atmosphere to remain airborne. The word ''soaring'' is al ...
and finally active
flight
Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...
. Still, lack of substantial fossil evidence of the development of the wing joints and muscles poses a major difficulty to the theory, as does the seemingly spontaneous development of articulation and venation, and it has been largely rejected by experts in the field.
Endite-exite (pleural) hypothesis
![Agnostus leg](https://upload.wikimedia.org/wikipedia/commons/a/ae/Agnostus_leg.jpg)
In 1990, J. W. H. Trueman proposed that the wing was adapted from
endites and exites, appendages on the respective inner and outer aspects of the primitive
arthropod limb, also called the pleural hypothesis. This was based on a study by Goldschmidt in 1945 on ''Drosophila melanogaster'', in which a variation called "pod" (for podomeres, limb segments) displayed a mutation that transformed normal wings. The result was interpreted as a triple-jointed leg arrangement with some additional appendages but lacking the tarsus, where the wing's costal surface would normally be. This mutation was reinterpreted as strong evidence for a dorsal exite and endite fusion, rather than a leg, with the appendages fitting in much better with this hypothesis. The innervation, articulation and musculature required for the evolution of wings are already present in the limb segments.
[Trueman, J. W. H. (1990), Comment: evolution of insect wings: a limb exite plus endite model. '']Canadian Journal of Zoology
The ''Canadian Journal of Zoology'' is a peer-reviewed scientific journal that covers zoology
Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the ...
''.
Other hypotheses
Other hypotheses include
Vincent Wigglesworth
Sir Vincent Brian Wigglesworth CBE FRS (17 April 1899 – 11 February 1994) was a British entomologist who made significant contributions to the field of insect physiology. He established the field in a textbook which was updated in a number ...
's 1973 suggestion that wings developed from
thoracic
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...
protrusions used as
radiator
Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics.
A radiator is always a ...
s.
Adrian Thomas and Åke Norberg suggested in 2003 that wings may have evolved initially for sailing on the surface of water as seen in some
stoneflies
Plecoptera is an order of insects, commonly known as stoneflies. Some 3,500 species are described worldwide, with new species still being discovered. Stoneflies are found worldwide, except Antarctica. Stoneflies are believed to be one of the mo ...
.
Stephen P. Yanoviak and colleagues proposed in 2009 that the wing derives from directed aerial gliding descent—a preflight phenomenon found in some
apterygota
The name Apterygota is sometimes applied to a subclass of small, agile insects, distinguished from other insects by their lack of wings in the present and in their evolutionary history; notable examples are the silverfish, the firebrat, and the ...
, a wingless sister taxon to the winged insects.
Dual origin
Biologists including Averof,
Niwa,
Elias-Neto
and their colleagues have begun to explore the origin of the insect wing using
evo-devo
Evolutionary developmental biology (informally, evo-devo) is a field of biological research that compares the developmental processes of different organisms to infer how developmental processes evolved.
The field grew from 19th-century beginni ...
in addition to palaeontological evidence. This suggests that wings are
serially homologous Serial homology is a special type of homology (biology), homology, defined by Owen as "representative or repetitive relation in the segments of the same organism."R:Webster 1913 in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913 Ern ...
with both tergal and pleural structures, potentially resolving the centuries-old debate.
Jakub Prokop and colleagues have in 2017 found palaeontological evidence from Paleozoic nymphal wing pads that wings indeed had such a dual origin.
See also
*
Bird flight
Bird flight is the primary mode of locomotion used by most bird species in which birds take off and fly. Flight assists birds with feeding, breeding, avoiding predators, and migrating.
Bird flight is one of the most complex forms of locomotion ...
*
Flying and gliding animals
A number of animals are capable of aerial locomotion, either by powered flight or by gliding. This trait has appeared by evolution many times, without any single common ancestor. Flight has evolved at least four times in separate animals: inse ...
*
Gliding ant
Gliding ants are arboreal ants of several different genera that are able to control the direction of their descent when falling from a tree. Living in the rainforest canopy like many other gliders, gliding ants use their gliding to return to the ...
*
Insect migration
Insect migration is the seasonal movement of insects, particularly those by species of dragonflies, beetles, butterflies and moths. The distance can vary with species and in most cases, these movements involve large numbers of individuals. In som ...
*
Unmanned aerial vehicles
An unmanned aerial vehicle (UAV), commonly known as a drone, is an aircraft without any human pilot, crew, or passengers on board. UAVs are a component of an unmanned aircraft system (UAS), which includes adding a ground-based controller ...
References
Further reading
*
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External links
{{Commons category, Insect flight
An Insect’s Role In The Development Of Micro Air VehiclesInsect-like Flapping-Wing Micro Air VehiclesThe Novel Aerodynamics Of Insect Flight: Applications To Micro-Air VehiclesThe aerodynamics of insect flightFlight muscles
Flight
Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...
Animal flight