|MadSci Network: Zoology
The ability to fly upside-down has been noted in some flies and some dragonflies. There may be other types of insect which have been documented as having this ability, but I have only read reliable accounts from the two groups mentioned.
Halteres are not essential for flying upside-down (dragonflies do not have them) but they are amazing little devices. Most people do not realise that when they see a common house-fly or a mosquito, they have in front of them the animal with most complex flight system on earth. It is also that amazing that some flies are able to beat their wings up to a thousand times per second.
One of the major advances in insect evolution was the reduction from four wings to two. After all, two wings are much easier to control. Most insects still have four wings, but many of them have effectively reduced the number to two by coupling the front and hind pairs together. The evolution of flies led to a different approach. The size of the hind wings have been reduced so much, that they do not provide any power for flight. It is often the case in evolution that structures do not disappear, but merely get used for something else. So it is with the hind wings of flies, which still beat like wings but are now used as halteres. Take a look at this diagram of a haltere .
The haltere is a knobbed structure. The heavy knob of the haltere gives this organ a high level of inertia. As a result, when a fly turns, the halteres try to continue beating in exactly the same plane as before. To do this they have to bend. Sensory hairs at the base of the haltere measure the amount and direction of bending. With this information, a fly is able to calculate the direction in which it has turned. The halteres will bend if the fly turns in either the horizontal or the vertical plane. The "calculations" are not done in the fly's brain. A signal is sent directly from the halteres to the flight system resulting in extremely quick response times. It is bit like having your car on automatic steering.
The turn in mid-flight is carried out by the forewings alone. By varying the amount of forward thrust or lift in these wings is the fly able to flip over in mid-flight. If you would like to know more about insect flight, then check out the university library, which should have several titles.
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