Evolution of Insect Wings
In an attempt to piece together the puzzle of insect evolution many related areas have been extensively studied. First we have to understand the fundamental basis of evolutionary thought and how it came to be. One of the most revolutionary ideas in Western thought is Darwin’s theory of biological evolution1. This was a very controversial and bold development in its time due to its apparent anti-creation position. Darwin’s theory of decent with modification, that all living things have come from one or a few common ancestors, did not fit with the idea that all living things had been created individually in their perfection by one supernatural being. The truth is that science cannot prove or disprove the existence of supernatural phenomena.
So where does the study of insect evolution start? The oldest true insects (Entognatha) are two apparent wingless insects from the later Devonian period of North America2. So where do we see evidence of winged species diverge from these non-winged lineages? Fossil evidence shows that it may be a lot earlier than the originally theorized Carboniferous period, which is where the oldest winged species containing fossil is dated back to. This fossil, however, shows fully winged individuals and several orders are represented, suggesting that wings may have originated considerably earlier3.
The study of insects and their color is vast and in general, a thoroughly explored topic. Wing color can be beneficial to the insect, such as a camoflauge in the case of the peppered moth, or a warning indicating poisonous to predators. It can also be detrimental, also in the case of the peppered moth: how a change in colour caused an increase in predation and subsequently a change in allele frequency1. Wing number, traced through the fossil record, shows a general decline in the number of wings necessary to achieve optimal fitness. This is evident in recent studies where less necessary wings have become increasingly smaller. A well supported theory is that wings not needed for flight have evolved into a protective "helmet". Not only physically beneficial because they are strong, but also psychologically adventageous because they can represent rocks, leaves, etc..., so as to confuse predators9.
There are many topics one can discuss when considering wing shape. Shape and textures and the fitness benefits of both are important to understanding various insect behaviours. Wing shape has also been revolutionary in the world of engineering, by hleping us understand the mechanics of how different features can benefit (or not) a flight pattern. Balance and control in flight is imperative to an insects survival in a constantly changing environment. This asset can be directly coneected to the presence of haltere wings in some insects, the development of which is caused by the presence of the hox gene: Ultrabithorax (Ubx)14.
It is important to understand that bird wings and insect wings are the result of convergent evolution. They each have a very different genetic make-up of one's wing, and mechanics behind flight, to the other's. However, recent studies show a mid-evolution example where and insect wing flight is tending towards a birds dynamics.
From Wingless to Winged
Insect wings are hypothesized to have originated and evolved only once, sometime during the late Devonian, early Carboniferous periods. By the late Carboniferous a diversity of wing morphology and Pterygote lineages was already well established4. There does not, however, exist any conclusive fossil intermediates from wingless to winged insects in the fossil record that hints explicitly to their evolution4. Due to this lack of conclusive fossil evidence a number of hypotheses for insect wing development have arisen, most notably the paranotal lobes hypothesis and the exite (or gill) theory. The former (considered to be the classical view) sees thoracic extensions (paranotal lobes) as the origin of wings where as the latter looks to moveable abdominal gills as the origin of wings. Though both hypotheses are based to a degree on assumptions, the gill theory is beginning to gain momentum as more evidence appears to support this hypothesis4.
The Gill Theory - From Epipods to Winglets
The gill theory hypothesizes that Pterygota insect wings are homologous with moveable abdominal gills as seen in some extant species (such as the aquatic naiads of mayflies)4. That is to say that it is from these gills that wings evolved sometime during the late Devonian4. These abdominal gills originated as modified extensions (exites) on ancestral arthropod basal leg appendages known as epipods4. These epipods (with attached gills) are also seen in extant Crustaceans (figure 1)5 4. Crustaceans have what is considered to be a primitive arthropod tripartite branching leg structures (hypothesized to be similar to that of arthropods of the late Devonian) containing an exopod, endopod and most importantly an epipod branch6. Gill theory suggests that modifications of these epipods into moveable abdominal gills catalyzed the development of eventual Pterygota wings4. These abdominal gills are hypothesized to have originally flattened and elongated to increase surface area for gas exchange in aquatic environments4. With the terrestrialization of Insects came new challenges and selective pressures that may have led to new functions for these elongated abdominal gills, including the potential ability to glide and escape predators4. These abdominal gills would have been modified to become ‘winglets’ for this purpose5. Further modifications and selection eventually gave rise to the first true wings (thought to be similar to that of the Odonata order of the Insect class)4.
Potential Evidence for Gill Theory
Since there are no conclusive fossils that show the intermediate steps of wing development, gill theory supporting evolutionary biologists have looked to Crustaceans as a potential clue to one of evolutions biggest mysteries due to the primitive characteristics of their leg appendages5. Comparisons of morphology and gene expression between extant Crustaceans and Insects has been encouraging, most notably a comparison between Drosophila wings and legs to that of brine shrimp (genus Artemia) tripartite limbs (endopod, exopod, epipod)6. It was seen that similar genes are expressed in similar areas congruent to what gill theory suggests. In particular the gene (pdm) was seen to be expressed in only Drosophila wings and Artemia epipods suggesting a potential evolutionary pathway to wings (and ultimately flight)6. Another potential piece of evidence looks at the similarity in the venation of Artemia abdominal gills to that of Drosophila flies6. Though promising, the research however is far from conclusive.
Wing Number and Colour
Despite millions of years of evolution, insect wings have not converged into the same number of wings. The general plan of insects usually includes a body divided into three parts (head, thorax and abdomen), a pair of antennas, three pairs of legs, and two pairs of wings present on the thorax. One can see this plan in action in Diptera, such as houseflies or mosquitoes, as their hind wings are reduced to small, round appendages called balancers. In Coleoptera, such as ladybugs, beetles, and maybugs, the first pair of wings has been transformed into elytra, colourful wings that protect the hind wings. In some insects, such as fleas and lice, wings have completely disappeared. A team from the Institut de Biologie du Développement de Marseille-Luminy has shattered this [general insect plan] belief by providing proof that the exuberant helmet of Membracidae, a group of insects related to cicadas, is in fact a third pair of profoundly modified wings9. Membracidae could thus be insects with three pairs of wings, one of which is heavily modified and unrecognizable9. This helmet allows a dorsal appendage to be flexible and move, thus it is not simply an outgrowth of the cuticle. This discovery is the first example of a change in the body plan of insects by the addition of an evolutionary innovation.
The colour of objects, including living creatures, is determined by the wavelength-dependent interaction of incident light with the object. The resulting coloration can be fully structural, i.e. it can arise from interference, diffraction and/or scattering of incident light as a result of structural variations of high and low refractive index materials7. The phenotypic traits of dorsal wing colour and wing spots are of importance to the behaviour of some insects. Not only are the colours used for camouflage, but are also used in some cases for long-range mate searching, and reducing predation as well as their survival and immunity. Epidermal cells through a developmental process involving colour patterning are synthesized to produce pigment. The cost of losing pigment in an insects wing could be evolutionary costly to the insect’s survival.
An example of evolution, specifically of convergent evolution, of colour patterns in insects is evident with different species of poisonous vine butterflies. These butterflies mimic each other’s colour patters, and in doing so, join forces to warm common predators of their venomous state. A single gene was identified that controlled the red wing colour patterns in a wide variety of the butterflies. Population genetics studied in hybrid zones, where different color types of the same species naturally interbreed, confirmed it8. Thus, this convergence in colour has caused the butterflies to become adaptive in their behavioural strategies.
In todays world it is easy to see the wide variety of wing shapes and sizes among modern day insects. They can very from Lepidoptera, with two pairs of wings found on the mesothorax (middle) and metathorax (third) segments, to Diptera which have one pair of functional wings and one pair of appendages called halteres on the posterior of the body used primarily for orientation, movement, and balance. Along with these two types is a long list of intermediate types all having unique features for different specializations. Seeing this large amount of variation brings up many interesting questions regarding how evolution brought about these many different designs as well as how they are advantageous for the organism.
Fitness Benefits in Dragonflies
A good start in thinking about the development of wings is to think about how wings are beneficial to an organism. Obviously wings allow flight which increases the fitness of an insect by improving mating, feeding, and avoiding predation. However flight also opens up many other opportunities for an insect such as migration. Johansson and colleagues studied how the distance a dragonfly migrated would affect the shape of its wings. They discovered that the dragonflies with a long migration distance tended to have a lobe one quarter of the way up from the base of the wing and that the outer half of the wing tended to be narrower. It was also found that the migrating dragonflies had a lobe at the base of there hind wings and that the hind wings generally had a more inward pointing frontal tip. These adapted lobes or expansions are related to gliding which could potential increase the distance that these migrating dragonflies are capable of travelling10. In the same study Johansson and colleagues examined how mate guarding influenced the shape of a dragonfly’s wing. In dragonflies there are two main strategies when it comes to mate guarding. The first, called tandem, is were the male physically holds on to the female preventing access by other males. The second, called noncontact, is were the male flies around the female in an attempt to keep other males away. It was found that the noncontact dragonflies tended to have a pointed tip at the outer back of the wing compared to the tandem dragonflies that tended to have the pointed tip at the outer front of the wing10. Also, in general, the outer half of the wing in the tandem dragonflies tended to be broader then in their noncontact counterpart10. These qualities would give the noncontact dragonflies an advantage in maneuverability and hovering ability and the tandem dragonflies an advantage simply in strength and size. These two examples show how simple life history traits can cause morphological differences between similar species.
Textures and Features
It is amazing the number of different adaptations that insect wings have developed each serving a different purpose and benefitting the insect in a different way. They range from Elytra, which is a hard front wing that serves as protection, to Hemelytra which has more of a leathery feel to it and is found on many aquatic insects11. We also see some insects with fringed wings, hairy wings and scaly wings. In order for all of these adaptations to have occurred there most have been some sort of evolutionary pressure that acted upon them. From our back ground knowledge of evolutionary biology we can hypothesize that each adaptation probably started from an advantageous mutation that spread through gene flow throughout a population allowing natural selection to potentially bring the advantageous allele to a fixed state. An example of this type of evolution can be observed in the peppered moth. In England prior to the industrial revolution there were two different forms of the peppered moth. The first form, which at the time made up about 99% of the population, had a pale white appearance and the other form making up the remaining 1% had a dark black appearance. After the industrial revolution got underway air pollution turned the lichens covering the trees from a white color to a black color. With the new improved fitness due to being able to camouflage with the trees, natural selection, in the form of predation, acted upon the moths bringing the black moths numbers up to about 90% of the population in some areas1.
Wings in Engineering
A subject that is quite interesting in the field of engineering is how the shape and structure of insect wings affects the physics and aerodynamics behind the flight of insects. Unlike modern day airplanes most insect wings are extremely unrigged and have multiple twists and folds. Using high speed cameras and computer simulators a team of researchers based out of the University of Oxford recorded insect flight. They then modeled it as if the wings did not have there natural camber and curves. They found that the natural insects were 10% more effective then the camberless insects and a whole 50% more effective then the straight ridged winged insects12. In the modals it appears that the non-ridged wings minimize there drag due to a lack of a vortices caused by the separation of airflow12. This shows the complexity that evolution is a capable of over millions of years and how even human technology is envious of its sophistication.
These are all examples showing how evolution has shaped something so small and seemingly insignificant into multiple complex design’s capable of completing extraordinary tasks. The fact that these delicate wings allow for insects to travel hundreds of kilometers and endure extremely high winds is simply astonishing. It opens up many new opportunities for insects and is potentially the reason why they are so wide spread.
Halteres and Ubx Evolution
Halteres are small flight appendages with sense organs found on the thorax of some flying insects, including those of the orders diptera and strepsiptera. Although their halteres have similar morphology and have the same purpose, they have uniquely different positions; Dipteran halteres are behind the forewing, where the hind wing would be present in other flying insects or mutants. Strepsipterans are the opposite; their halteres are in the forewing position. Halteres been have evolved to maintain stabilized flight by preventing yawing13 and thus act like a gyroscope for the insect in flight.
De Novo evolution of morphology is rare, the same is true of developmental genes. Using a Drosophila melanogaster model, Ultrabithorax (Ubx) is shown to be key haltere development gene. Ubx is a hox gene that has many roles along the anterior-posterior axis14, and regulates downstream haltere development. Ubx suppresses wing growth by binding to the Cis-regulatory element (CRE) of the wing specific gene ana15 as well as others. Ubx increases mitotic cell growth in haltere by regulating expression of the thickviens (tkv), master of thickviens (mtv), and decapentaplegic (dpp). Mutant drosophilas have been found to have both forewings and hind wings, and no halteres; this can happen randomly or can be induced by Ubx mutations16. Ubx has evolved in some insects by being affected by other genes, and it effects other genes that have evolved new CREs that are affected themselves by Ubx. In lepidopterans, Ubx dissimilarly regulates the developmental genes wg, SRF, and AC-S who play important roles in wing growth; this difference is what allowed for the evolution of halteres in dipterans17. Similarly, Ubx and other hox gene evolution has decreased the number of insect segments with wings, having evolved from many winged segments, to two or just one pair of wings (or none).
Bird and Insect Flight
Birds and insects have some apparent commonalities but because of their phylogenetic distance they have more differences. What they do share are the three factors required for flight; a light and powerful engine, wings for generating the force required for flying and a way to control their flight system. Both groups employ different flying strategies and use different wing trajectories.
The first type of flight that evolved in insects, birds and bats is called synchronous flight. The wing beats in this type of flight are controlled directly by the nervous system. Each wing beat is generated by a single nerve impulse. This is ideal for larger insects and birds but when organisms are very small the wing beat frequency required cannot be triggered fast enough by the nervous system. This is because of the limit to the speed at which calcium is moved by the sarcoplasmic reticulum and the time it takes for a muscle contraction relaxation cycle18 by attempting to increase the abilities of the sarcoplasmic reticulum an unrealistic amount of energy is expended. Investigators have resolved that the maximum wing beat frequency for synchronous flyers to be about 100MHz. Synchronous flight works very well for some organisms including butterflies who flap in various directions as well as for gliders19.
A Transition to Asynchronous Flight
The evolution of asynchronous flight has resulted in a different type of communication between the nervous system and the wings; the “flight engine” communicates with the nervous system and then coordinates the wing muscle action. This system uses primary and accessory flight muscles, the accessory flight muscles allow for steering, hovering, reversed flight and other alternative flight patterns20. These accessory flight muscles have a highly organized lattice pattern that was recently discovered using micro-X-ray analysis. This unique sarcomere alignment has most likely evolved independently in bees, flies, beetles and tree bugs. Macroglossum stellatarum, the hummingbird hawk moth, is an interesting example of an organism that is in transition between synchronous and asynchronous flight patterns. Its wing movements are controlled directly by the nervous system but its myofibrils are organized in a lattice pattern. Its wings beats operate near the max frequency of synchronous flight at 85 MHz21.
Aerodynamics of Bird Flight
Bird flight can be broken down into three parts, gliding, soaring and flapping. Gliding requires little energy and occurs when the bird keeps its wings out with the leading edge deflecting air downward, the air travelling down and its opposing force up results in lift. The lift is acting roughly perpendicular to the wing and keeps the bird up in the air. The angle of the wing is called the angle of attack22. If this angle is too great there will be too much drag created by the wing and if this angle is too small there will not be enough lift produced. The resistance of the birds’ body while gliding will eventually cause the bird to slow down unless it employs soaring or flapping. Other taxa that have evolved gliding include lizards, fish, snakes, squirrels, lemurs and opossums23. Soaring is a type of gliding where the bird does not lose vertical distance by taking advantage of rising air currents. A bird moves downward through a thermal that is rising up therefore maintaining its position in the air column. When a bird flaps the theories of gliding still apply but the addition of flapping results in thrust which propels the bird forward.
Aerodynamics of Insect Flight
There are two aerodynamic models of insect flight, most insects flap which creates a spiraling leading edge vortex where the downward stroke moves down and forward and the upward stroke moves up and backward, at the top and bottom of each stroke the wing is flipped or pronated24 which allows for a new stroke. The other method of insect flight is the flight, fling, clap where the wings clap together at the top of the flap which forces the wings apart. As they move apart air between the wings creates a vortex which produces circulation and lift. This is a less common flight method and is not seen in birds because of size constraints and the possibility of tissue damage25.
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