The Obscure Origin Of Insects And Their Wings
The earliest fossil insect is a wingless springtail found in Scotland's Devonian cherts, which conventional dating schemes tell us are about 350 million years old. Some biologists doubt that springtails should be classified as true insects. In any event, these ancient springtails are considered too specialized to be the ancestors of modern winged insects. The next insects in the fossil record appear suddenly in the Upper Carboniferous (300 million years ago) with fully developed wings. There exists an embarrassing 50-million-year gap between the fossil springtails and the more specialized insects.
Evolution requires that this gap be filled with many random experiments at insect construction, including the first attempts at fashioning wings. Whalley admits the gap and the total mystification of paleontologists about how insects and biological flight first developed. Perhaps, he surmises, wings may have been the natural extrapolation of flap-like outgrowths required for body cooling. Random mutations would have added the muscles needed to orient the flaps and move them to improve circulation!
(Whalley, Paul; "Derbyshire's Darning Needle," New Scientist, 78:740, 1978.)