Most insects fly; more than 99.9 per cent of the 900,000 or so species belong to the Pterygota , or winged insects. We take this for granted. It is reflected in the common names we give to some insect orders - flies, stoneflies, butterflies, sawflies, etc. These aerial animals are relatively rare in the seas, being essentially creatures of terrestrial and freshwater environments. Even so, no matter whether global biodiversity is measured in numbers of individuals or species, then insects are pre-eminent. By species they outnumber vertebrates 18:1 and if estimates of numbers of species "yet to be described" are correct, the ratio is nearer to 180:1. An obvious question is: "Is this diversity due to their ability to fly?" Answering this underpins Robert Dudley's new book.
Somewhat unusually for such a broad-scope book, Dudley fully addresses all three aspects of the subtitle. He has packed it with a plethora of interesting facts, observations and questions that should interest a wide audience.
Insect ecologists tend to accept flight as an integral behaviour of the animals they study, and give more thought to its consequences, such as mobility and dispersion, than to the evolutionary constraints imposed by the biomechanical and physiological adaptations necessary for flight in the first place. I vividly remember a marvellous seminar at York University given by T. Weis-Fogh shortly before his untimely death. Nearly a quarter of a century later I can still see him standing on a bench in his academic gown to demonstrate the basic principles of flapping flight. I came away thinking: "So that's how they do it", but sadly gave it little further thought until I read Dudley's book.
Despite being in a review style, it is easy to read and is not overloaded with figures and tables. Those provided are clear and easily understood, although the six colour plates are poor and unnecessary.
Glossaries of technical terms and parameters used in equations are very helpful and although in parts the book is quite technical, these sections do not feel too heavy for a general reader. The eight chapters more or less stand alone as mini-reviews with their own summaries. Nevertheless, it is well cross-referenced and indexed and is quite easy to use. Non-biologists will probably find much that interests them if they read just the first and last chapters, which act as an introduction and summary. Budding biomechanical engineers, perhaps even aeronautical engineers, will home in on chapters two, three and five which deal with morphology, functional design and aerodynamics. Evolutionary ecologists will find chapters four, six and seven more interesting, which deal with physiology, energetics and evolution and their consequences for diversity.
We are reminded throughout of how the success of Pterygota species depends upon complex interactions between biochemical, physiological and mechanical processes that are constrained by the physical characteristics of the environment and mitigated by behaviour. For example, during the Carboniferous period, atmospheric oxygen levels rose, peaking 300 million years ago at about 25 per cent more than present levels. The change in density, viscosity and diffusivity of the air is believed to have led directly to the evolution of giant insects - dragonflies and mayflies with up to 70cm and 45cm wingspans. On the other hand, later selection for exceedingly small flying insects, such as some parasitoids, was only possible after the evolution of asynchronous flight muscle, which can contract at rates of up to 1,000Hz in response to single nervous stimuli, compared to about 100Hz for synchronous muscle. More generally, wide dispersal by flying insects made them good pollinators and resulted in fantastic co-evolution between flowering plants and pollinating species. Dudley suggests that the changed physical properties of a warmed atmosphere might affect the biomechanics of insect flight, which might, in some environments, have unpredictable consequences for pollinator behaviours besides affecting their geographical ranges.
This book should be on the library shelves of all universities and entomological research institutions. A cheaper paperback version would make a good gift for any serious biologist.
Graham W. Elmes is a senior merit scientist at NERC's Centre for Ecology and Hydrology, Dorset laboratories.
The Biomechanics of Insect Flight: Form, Function, Evolution
Author - Robert Dudley
ISBN - 0 691 04430 9
Publisher - Princeton University Press
Price - £30.00
Pages - 476