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ON THE WING: INSECTS, PTEROSAURS, BIRDS, BATS AND THE EVOLUTION OF ANIMAL FLIGHT. By David E. Alexander. Oxford University Press, Oxford, UK. 2015: 224 pages, 43 illustrations. ISBN: 978-0-1999-9677-3. $29.95 (hardcover), $ 21.99 (digital edition).- This essay by a notable animal aerodynamicist provides an accessible commentary on general principles of flight in animals, ranging from insects to the varied vertebrate groups that took to the air with powered flight. Although innumerable animals have taken to the air, the book concentrates on four groups (insects, pterosaurs, birds, and bats) that developed powered flight, and searches for biomechanical and aerodynamic principles common to all. Advantages of this locomotor method include speed, economical travel, large foraging ranges, escape from predators, predation itself, and of course, migration. Following an introductory section, the book includes chapters on: How to fly?, Gliding animals, Insects, Birds, Bats, Pterosaurs, and Pedestrians which have lost the power of flight. The essay concludes with a chapter on Unifying Themes. Following a chapter comparing the varied wings, from insects to bats and birds, chapter 3 provides a simplification of the principles of flight in all groups, including the physics of wings, liftand drag, the Reynolds Number, and the production of thrust, the function of flapping wings. The author perceptively points out that although a number of paleontologists have argued that gliding could not have led directly to flapping, this is indeed a false assumption as experiments and modeling show that even low-amplitude, weak flapping can produce enough thrust to be useful. He uses "partially powered flight" (p. 56) to refer to intermediate behavior between gliding and fully powered flight.
Chapter 4 examines the myriad gliding animals, including lizards, snakes, frogs, and even the ancient bizarre Sharovipteryx, which presumably had a hind-limb membrane that produced a deltashaped glider. The six groups of mammal species that glide include rodents
(including 40 or so species of flying squirrels), marsupials, and colugos or "flying lemurs." It is noted that among the primary advantages of gliding is energy conservation, flying from one tree to another and therefore covering much more territory faster and more economically than a similar nonglider. An important point is "the...