Insect flight
Of these insects, some (flies and some beetles) 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.
Some very small insects make use not of steady-state aerodynamics, but of the Weis-Fogh clap and fling mechanism, generating large lift forces at the expense of wear and tear on the wings.
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.
To balance this evolutionary trade-off, insects that evolved indirect flight have also developed a separate neuromuscular system for fine-grained control of the wingstroke.
[6] Recent work has begun to address the complex non-linear muscular dynamics at the wing hinge and its effects on the wingtip path.
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.
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 wings.
Because the flow has separated, yet it still provides large amounts of lift, this phenomenon is called stall delay, first noticed on aircraft propellers by H. Himmelskamp in 1945.
[13] 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.
[17] As insect sizes become less than 1 mm, viscous forces become dominant and the efficacy of lift generation from an airfoil decreases drastically.
The implementation of a heaving motion during fling,[23] flexible wings,[21] and a delayed stall mechanism were found to reinforce vortex stability and attachment.
[21] Bristles on the wing edges, as seen in Encarsia formosa, cause a porosity in the flow which augments and reduces the drag forces, at the cost of lower lift generation.
[26][27] Some insects, such as the vegetable leaf miner Liriomyza sativae (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.
A special class of objects such as airfoils may reach a steady state when it slices through the fluid at a small angle of attack.
The corresponding lift is given by Bernoulli's principle (Blasius theorem):[8] The flows around birds and insects can be considered incompressible: The Mach number, or velocity relative to the speed of sound in air, is typically 1/300 and the wing frequency is about 10–103 Hz.
For example, selecting only flight sequences that produced enough lift to support a weight, will show that the wing tip follows an elliptical shape.
Regardless of their exact shapes, the plugging-down motion indicates that insects may use aerodynamic drag in addition to lift to support its weight.
[14] One can now compute the power required to maintain hovering by, considering again an insect with mass m 0.1 g, average force, Fav, applied by the two wings during the downward stroke is two times the weight.
[14] Some four-winged insect orders, such as the Lepidoptera, have developed morphological wing coupling mechanisms in the imago which render these taxa functionally two-winged.
[31] The mechanisms are of three different types – jugal, frenulo-retinacular and amplexiform:[32] The biochemistry of insect flight has been a focus of considerable study.
[44] 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.
[45] This leaves two major historic theories: that wings developed from paranotal lobes, extensions of the thoracic terga; or that they arose from modifications of leg segments, which already contained muscles.
According to this theory these tracheal gills, which started their way as exits of the respiratory system and over time were modified into locomotive purposes, eventually developed into wings.
[48] The paranotal lobe or tergal (dorsal body wall) hypothesis, proposed by Fritz Müller in 1875[49] and reworked by G. Crampton in 1916,[47] Jarmila Kukalova-Peck in 1978[50] and Alexander P. Rasnitsyn in 1981 among others,[51] suggests that the insect's wings developed from paranotal lobes, a preadaptation found in insect fossils that would have assisted stabilization while hopping or falling.
This model implies a progressive increase in the effectiveness of the wings, starting with parachuting, then gliding and finally active flight.
[48] 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.
[54] Biologists including Averof,[55] Niwa,[56] Elias-Neto[57] and their colleagues have begun to explore the origin of the insect wing using evo-devo in addition to palaeontological evidence.
