TLS: Evolution of the insects

Two and six
Quentin D. Wheeler

EVOLUTION OF THE INSECTS. By David Grimaldi and Michael S. Engel. 755pp.
Cambridge University Press. Pounds 45 (US $80). 0 521 82149 5

If you needed a one-word description for the biological diversity of life on Earth you could scarcely do better than "insects". An overview of insect evolution is breathtaking in its scope and complexity. In kinds, numbers and habits insect diversity is biblical in proportion, swarms of locusts and all.
However you take their measure, insects tell an evolutionary success story without equal. For 400 million years insects have crept, crawled, run, burrowed, swum, gnawed and flown their way into the fabric of just about every terrestrial and freshwater ecosystem. Insects are so diverse that, after more than 250 years of diligent exploration, entomologists estimate that three- quarters or more of the earth's living insect species remain unknown to science. Much of the history of insect evolution is thus yet to be written; but if it is anything like the parts that have been revealed, it will make good reading.
The seemingly endless variety of flowers that decorate our hillsides and woodlands were not put there for our enjoyment. They are, instead, the result of an elaborate and well-rehearsed evolutionary dance of attraction, dependency and deceit taken up with insects millions of years ago. Many insects engage in the dispersal of plant pollen, but the awesomely successful ones are the bees, moths and butterflies that have diversified into thousands of species of wildly divergent habits. The close relationship between these insects and their equally impressively diverse plant hosts is like a "big bang" among evolutionary radiations; the paradigmatic model for co-evolution, insect and plant species adapting in successive stages one to the other. While flowering plants only first appeared in the lower Cretaceous about 150 million years ago, by the middle of the Cretaceous, 50 million years later, virtually every major branch in the angiosperm family tree was in place, as were the major lineages of insect pollinators. The importance of insects to the function of our world, of course, extends far beyond, and began long before insects' relationship with flowering plants.
Sir David Attenborough, in his television series Life in the Undergrowth, drew our attention to two parallel worlds, one of men, the other of insects. As he soberly explained, were humans to disappear tomorrow, insects would continue much as they have for hundreds of millions of years, our absence completely unnoticed. On the other hand, were the insects to disappear suddenly, terrestrial and freshwater ecosystems would collapse and the future of our own lives would be thrown very much into question.
Our anthropocentric view is a potentially dangerous illusion; considering the diversity, abundance and importance of insects to the life systems of the earth, we would be wise to explore and learn about and from the insects.
Books addressing the evolution of insects in the past have all suffered from limitations. Most are highly technical, laden with the jargon of professional entomology. Some focus on the insects of one region such as Europe or Australia, giving only a partial picture of living species. Those seeking to be more comprehensive have summarized living or fossil insect evidence, never both. To gain an appreciation for the larger picture, the whole of insect diversity and evolution, was an arduous process at best. Evolution of the Insects by David Grimaldi and Michael S. Engel is the first book that has attempted to pull together and synthesize both fossil and recent evidence for insect evolution, and to present the information in an accessible, engaging way. They have succeeded to an unprecedented degree, and anyone with an active or passing interest in insects owes it to themselves to have a look.
A few comparisons serve to put the audacity of Grimaldi and Engel's subject in perspective. Were their topic instead all known mammals, it would cover about
5,000 species, were it all known birds, about 9,000 species, and we would marvel at the size of their accomplishment. Of the approximately 1.7 million species of living things ever described on earth, 925,000 of them are insects.
Any single volume providing a credible introduction to such a massive proportion of life is a very special and welcome addition.
Insect specialists can and will find fault in details of chapters addressing their favourite insect group, but how could it be otherwise? Our knowledge of insect evolution is so incomplete and progressing so rapidly that controversies surround theories about all but the most mundane aspects of insect history. In the past decade alone there have been discoveries of significant fossils, phylogenetic insights based on newly acquired DNA sequences, and even the report of a new ordinal level discovery. Like most stories worthy of your attention, this one is fast-paced, contentious and compelling. Grimaldi and Engel have given to us a beginning, not an end; a milestone in a scientific quest to explore insect diversity launched in the eighteenth century and far, far from ended. This ever-present sense that the next major discovery lies just around the corner only adds to the poignancy of a grand evolutionary tale and entices bright young minds to join in the search.
Evolution of the Insects cleverly intertwines history and the theoretical foundations of reconstructing evolution, illustrated by insect examples.
Grimaldi has introduced an excitement and accessibility to the study of fossil insects that had been absent in a previously turgid and excessively technical literature. His gift for showmanship surfaced a number of years ago when he produced a book on amber and launched it complete with amber-coloured spotlights illuminating the Empire State Building against the Manhattan night sky. Here, the most important deposits that have given up fossils are presented in both their geographic and plate-tectonic contexts, helping us to grasp just how much, or how little, we know and the complex interplay of space and time in the origin of insects.
There is much about the structure of insects that makes them utterly foreign to us: their skeletons are worn on the outsides of their bodies, they fly on two pairs of wings, and they scurry about on six legs. Multiple pairs of mouthparts, long antennae and compound eyes confront us with faces that only a mother bug could love. Appreciating the beauty and complexity of insects involves teasing apart the complex structures that evolution has sutured together into the most successful feeding and breeding machines in history.
This is no easy task. Many body parts represent the fusion of multiple segments of ancestral bodies; this is most easily pictured by thinking of a multi-segmented ancestor resembling a millipede or centipede. The evolutionary process of consolidating many segments into efficient functional units is called tagmatization. Insects, as most children can explain, have three tagmata: the head, thorax and abdomen. The head alone is made up of half a dozen ancient body segments and is an awesomely efficient sensory and feeding zone. The thorax delivers locomotion, containing as it does both legs and wings. And the abdomen is mostly concerned with food and sex or, more clinically, with digestion and reproduction.
Major discoveries in the insect world happen quietly outside the gaze of the media, yet rival the most impressive ones ever made. While the spotted owl was being effectively used as an emblem for the cause of saving old growth Douglas fir forests on America's Pacific Coast, there was a smaller, admittedly less charismatic, candidate which was of infinitely more scientific interest. A family of primitively flightless insects related to silverfish, the Lepidotrichidae, were believed to have become extinct 40 million years ago.
Then, like the more celebrated coelacanth, a living species of lepidotrichid was discovered alive and well in northern California in 1961 by Pedro Wygodzinski of the American Museum.
Silverfish are the sister group of the hundreds of thousands of species of winged insects, so this "living fossil" presented extremely important insights into early insect structure and habits.
When we think of flying animals, we think first of birds and bats, but neither was first. Nor were the pterodactyls. The first, preceding birds by about 200 million years, were the insects. Not only did insects command the skies first, they did so with a flair only elsewhere attempted in myth and religion: they evolved wings in addition to their existing limbs. Insects accomplished what humans could only imagine in the form of Pegasus and angels. This combination of both wings and ambulatory limbs proved a potent one, although the de novo origin of wings is much more difficult to explain than the modifications of forelimbs in flighted vertebrates. There are two primary competing ideas. One, based largely on certain fossils, suggests that lateral lobes of the thorax made gliding possible and were the precursors of wings. The other favours an origin from gills; limb bases modified into breathing structures in some aquatic insects are the candidate structures. Regardless of the mechanism, insects took to the air, which opened vast adaptive horizons. Early fliers could not flex their wings like modern insects, so that they rested roof-like over the abdomen when not in use.
Dragonflies hold their wings outstretched at rest, while damselflies and mayflies fold the wings upright over the back. Complex changes in muscles associated with flexion again opened new options for evolution. While anyone who has admired dragonflies on the wing can attest to their aerobatic prowess on four wings, there have been repeated transformations to one pair of flight wings among higher insects. Beetles use their hind wings for flight, with the front wings modified into hardened protective sheathes. Flies, on the other hand, have the hind wings reduced to club-like organs and are motored instead by the forewings. In wasps and butterflies and moths various mechanisms have evolved to couple the fore and hind wings; retaining four wings, the two wings on each side function in unison as though they were but one, again changing the landscape of insect evolution.
Beyond its authoritative and encyclopedic approach to the subject, what sets Evolution of the Insects apart is the quality and quantity of illustrations: beautiful colour photographs; spectacular digital images; scanning electron micrographs; clever uses of colour to unravel bits of complex anatomy; crisp diagrams; evolutionary trees; and the kind of skilful scientific illustrations that are widely identified with David Grimaldi's contributions to insect morphology. More than 900 in total, almost every one original, these images serve to inform, explain, elaborate and inspire. Yogi Berra once said, "You can observe a lot by just looking"; it is certainly true of this visually engaging volume.
Evolution of the Insects will fill a number of important roles; a text for the serious student of insects, a reference for a wealth of information on insect evolution, and a source of joy for the casual reader who picks it up and opens it to any of the hundreds of intriguing stories and examples it contains. It is a call to action as well. David Grimaldi and Michael S. Engel end with a discussion of the future, of the threats of extinction faced by many insects, and the enormous opportunities that lie ahead to expand and refine our understanding of the world of insects. We may be late arrivals to their world, but we have now the best travel guide that has ever existed to make the most of our visit.