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On the Wing: Insects, Pterosaurs, Birds, Bats and the Evolution of Animal Flight

This book is an unusual form of technology. The author reviews the lives of four taxa of animals—the birds, the bats, the pterosaurs, and the insects— which are not related to each other but which typically have wings and can fly. What is it about wings that makes these animals so very different from any others? In his first chapter, Alexander explains that flight allows animals to cover ground faster than walking or running, and that is true also for the bigger species. As to flying slowly, only insects can do that, and it takes a lot of energy. Large or mediumsized birds cannot even think about migrating slowly. A large bird has to go faster than it can run before it can fly. Why is that? In Chapter 2 Alexander goes through the special properties that wings have, such as generating lift and drag from the fluid passing over them, and the ratio of lift to drag. He defines those variables carefully in Chapter 3, along with some less obvious ones, such as force and power. Having noted that these variables are important to engineers, he does nothing further with them. If the reader wants a lift-to-drag ratio for a bat, he will not find it here. Alexander knows that the lift has to be adjusted in level flight to match the weight, and also that the muscles do work against the drag—and these actions are different from those of locomotion in water or on the ground. He does not say why this makes a bird or bat any more or less efficient than any other animal. The book is full of biological detail, but there is no theory here from which an animal’s flight performance could be calculated. Starting with gliding animals, there are many interesting details about the flight of rib-winged dragon lizards, followed by a chapter each on insects, birds, bats, and pterosaurs. In addition to having wings, these power-flying animals also have flight muscles that flap the wings up and down. Insects have the small-scale world to themselves, from about 5 g body mass downward, and that is because they have their own type of flight muscle, whose contraction frequency is not limited by the mechanics of the wings that they flap. Insect flight muscles produce rather little power but work well at frequencies higher than those of vertebrates, into