from file:///D:/!%20GT-WORK/Insect%20flight/WIKI%20Insect_flight_files/Insect_flight.htm
<[>edit] Evolution and adaptation
Hoverfly in flight (Xanthogramma pedissequum)
Sometime in the Carboniferous Period, some 350 million years ago, when there were only two major land masses, insects began flying. How and why insect wings developed, however, is not well understood, largely due to the scarcity of appropriate fossils from
the period of their development in the Lower Carboniferous. Three main theories on the origins of insect flight are that wings developed from paranotal lobes, extensions of the thoracic terga; that they are modifications of movable abdominal gills as found
on aquatic naiads of mayflies; or that they developed from thoracic protrusions used as radiators.<[>1]
<[>edit] Paranotal hypothesis
The paranotal hypothesis suggests that the insect's wings developed from paranotal lobes, a preadaptation found in insect fossils that is believed to 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 parachutes 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, st
arting with parachuting, then gliding and finally active flight.
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 t
he field.<[>citation needed]
<[>edit] Endite-exite hypothesis
The hypothesis with perhaps the strongest evidence is that which stems from the adaptation of endites and exites, appendages on the respective inner and outer aspects of the primitive arthropod limb. This was advanced by Trueman<[>2] based on a study by Gol
dschmidt in 1945 on Drosophila melanogaster, in which a pod variation displayed a mutation transforming normal wings to what was interpreted as a triple-jointed leg arrangement with some additional appendages but lacking the tarsus, where the wing's costal
surface normally would 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 fo
r the evolution of wings are already present in podomeres.
<[>edit] Epicoxal hypothesis (GT- this is Kukalova)
Some entomologists have suggested that a possible origin for insect wings might have been the movable
abdominal gills found in many aquatic insects, such as on naiads of mayflies. 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, eve
ntually developed into wings. The tracheal gills are equipped with little winglets that perpetually vibrate and have their own tiny straight muscles.
<[>edit] Other hypotheses
Suggestions have been made that wings may have evolved initially for sailing on the surface of water as seen in some stoneflies.<[>3] An alternative idea is that it drives from directed aerial gliding descent</W>a preflight phenomena found in some apterygot
e, a wingless sister taxa to the winged insects.<[>4]
The earliest fliers were similar to dragonflies with two sets of wings, direct flight muscles, and no ability to fold their wings over their abdomens. Most insects today, which evolved from those first fliers, have simplified to either one pair of wings or
two pairs functioning as a single pair and using a system of indirect flight muscles.
Natural selection has played an enormous role in refining the wings, control and sensory systems, and anything else that affects aerodynamics or kinematics. One noteworthy trait is wing twist. Most insect wings are twisted, as are helicopter blades, with a
higher angle of attack at the base. The twist generally is between 10 and 20 degrees. In addition to this twist, the wing surfaces are not necessarily flat or featureless; most larger insects have wing membranes distorted and angled between the veins in suc
h a way that the cross-section of the wings approximates an airfoil. Thus, the wing's basic shape already is capable of generating a small amount of lift at zero angle of attack (see Insect wing). Most insects control their wings by adjusting tilt, stiffnes
s, and flapping frequency of the wings with tiny muscles in the thorax (below). Some insects evolved other wing features that are not advantageous for flight, but play a role in something else, such as mating or protection.
Some insects, occupying the biological niches that they do, need to be incredibly maneuverable. They must find their food in tight spaces and be capable of escaping larger predators - or they may themselves be predators, and need to capture prey. Their mane
uverability, from an aerodynamic viewpoint, is provided by high lift and thrust forces. Typical insect fliers can attain lift forces up to three times their weight and horizontal thrust forces up to five times their weight. There are two substantially diffe
rent insect flight mechanisms, and each has its own advantages and disadvantages - just because odonates have a more primitive flight mechanism does not mean they are less able fliers; they are, in certain ways, more agile than anything that has evolved aft