Ants you'd be pleased to see on the coffee table

Evolution of the Insects

December 15, 2006

I often find it quite unsettling when I try to grasp the nature and origins of the universe. An ever-expanding universe containing billions of galaxies and myriad stars is easy to state but almost impossible to comprehend. Similarly, the diversity and interactions between the millions of forms of life sharing the Earth with us is hard to grasp, and when one starts considering the pathways of its evolution the subject can seem almost as mind-numbingly complex. It has been suggested that 99.9 per cent of all species that ever existed are extinct - in other words, at least 2 billion multicellular species have at some time existed on Earth. Yet, many authors believe that the conservative estimate of 2 million living species wildly misses the mark and suggest that insect species alone may number 5 million.

To describe and catalogue all living species is a mammoth task, one that would take a few hundred years if enough resources were devoted to it, but then if the theory of creation were correct, that would be that.

However, most scientists find the theory of evolution a more satisfactory explanation of our observations, especially the fossil remains of plants and animals that have no living counterparts. Consequently, many biologists believe that to understand modern life forms we should attempt to reconstruct the "family tree" of life based on the fossil record to see how they relate to living and other extinct species. Lineage reconstruction, or phylogeny, is now an important part of the modern taxonomist's work, adding to the original vital role of describing and cataloguing species. More than half of all known animal species are insects. Since insects first evolved about 400 million years ago, they have had profound influences on the evolution of many other groups, especially the flowering plants.

Entomologists such as myself are concerned only with a few insect species, especially those that interact with man - either pest species or beautiful, endangered species such as butterflies. Seldom do we give much thought to how and when our study organisms evolved, and when we do, the relevant information is often hard to find. Fortunately, two American authors, David Grimaldi and Michael Engel, have brilliantly synthesised the potentially mind-boggling diversity of information in Evolution of the Insects.

This book is well written, logically presented, well referenced, easy to read and marvellously illustrated, mostly in colour. In a work of this scope, there will be errors; and in a field that is so open to interpretation and reinterpretation, I am sure some specialists will find cause for disagreement. However, it will be immensely useful to non-taxonomists, entomologists who are ecologists, behaviourists and physiologists. One of its strengths is that it is a general reference source that can be dipped into according to need but also has a logical thread and can be read cover to cover.

The key disciplines of taxonomy are introduced in the first chapter, "Diversity and evolution", and taken on its own this is a nice essay on the role, disciplines and history of modern taxonomy, with portraits of the "founding fathers". Anthropologists have noted that tribal peoples who live close to nature have an intuitive grasp of what a species is and have names for many plants and animals. Yet how can their often detailed knowledge be communicated to others if natural historians used different names and different classification systems? Karl Linnaeus resolved this when he proposed a binomial system of nomenclature. Chapter one shows how his successors developed his system, with its need for type specimens, into the present convention where a rigorous priority in nomenclature ensures the avoidance of duplicate names. Johann Fabricius, a student of Linnaeus and the first modern entomological taxonomist, is quoted as saying: "If the names are lost, the knowledge also disappears" - a sentiment still relevant today. In my view, inadequate training in taxonomy is the main reason why many modern ecological studies will be worthless or, worse still, totally confusing for future scientists. If a species is wrongly identified and no "voucher specimens" are preserved, then what use to a future generation is a detailed behavioural or population study of that organism? This is nicely summed up in a possibly apocryphal Chinese proverb cited by the authors:

"The first part of knowledge is getting the names right."

Defining what exactly constitutes a species is not as easy as it seems and is a topic still open to much philosophical debate. DNA studies, which might help clarify this, are briefly reviewed. However, the sheer numbers of insects and the fact that DNA recovery from dried or otherwise preserved museum specimens is technically very difficult will prevent it replacing conventional morphologically based taxonomy in the near future. The chapter concludes on how phylogenies can be constructed using morphological characters.

It is virtually impossible to recover genetic material from fossils except from exceptionally well-preserved specimens in certain types of amber, an observation that takes us on to chapter two, where the plausibility of a Jurassic Park-type scenario is discussed at length. Here, Grimaldi and Engel review and illustrate the surprising range of media that contain insect fossils. They list the richest sources and map the position of the landmasses and their probable climates in the different geological epochs when the best known fossil insect deposits were formed.

Chapter three covers the origin of multicellular life in the Precambrian era some 600 million years ago and the Cambrian "explosion" (roughly 550 million years ago) during which many complex forms of multicellular animals evolved. Fossils belonging to the phylum Arthropoda (the jointed animals) were particularly abundant at that time. The ancestral insects evolved about 400 million years ago, after the arthropods colonised the land. The older reader will notice here that the classification of arthropods has changed considerably in the past few decades, for example Collembola, the spring-tails, are no longer included in class Insecta, rather they are in the closely related class Entognatha.

The next chapter, "The insects", starts by outlining the major features of insect morphology, a knowledge of which is needed to understand some of the book's technical sections. It gives potted biographies of pioneering entomologists who constructed insect phylogenies and finally, in a remarkably clear figure, gives the phylogeny, which is used as the basis for the next nine chapters.

It is impossible to review all these here but take as an example chapter eight, "The Paraneopteran orders" - lice, aphids and bugs. These belong to the Neoptera, a major lineage of insects, defined by their folding wings, which are believed to have diverged from the older lineages of winged insects sometime during the late Devonian or early Carboniferous periods, about 380 million years ago. By the start of the Permian (280 million years ago) the major Paraneopteran groups of lice, thrips and bugs and plant lice had already evolved. A very clear figure illustrates the relationships among the orders and suborders, with diets indicated in various colours, while the morphological features that characterise the divergences are listed in a separate table. The structure of the mouthparts figures prominently in this, and the various types are beautifully illustrated by coloured line drawings. Each of the major orders and suborders is then treated in turn with detailed discussion of recently published phylogenies.

Fossils, where known, are illustrated and placed into context. Ecology and behaviour of the groups are outlined with fossil evidence for these behaviours. The parasitic lifestyle of the louse means there are few fossilised lice; nevertheless, much about their relationships can be deduced from their modern range of host species. Aphids and scale insects are famous for having close relationships with ants that "farm"

them for the honeydew they produce. A lovely colour photograph shows a young queen Acropyga ant trapped in Miocene amber about 25 million years ago carrying a mealybug from her mother colony's "herd", which she would use to start her own herd had she not been trapped in resin. As an ecologist, I find it awesome to think that some of the complex mutualisms that I study today had already evolved so long ago, probably long before the Miocene epoch. The 69 pages of chapter eight contain more than 50 colour photographs, 18 black-and-white scanning microscope pictures and numerous tables, figures and line drawings often enhanced by prudent use of colour.

Each of the other chapters is an equal pleasure to read, and the average of at least one illustration per page is maintained throughout. Apart from being relevant to the text, some of the photographs of both living and fossil insects are so beautiful that the book might even have a casual "coffee table" appeal. I am sure that I will use it much more frequently than many other reference books that I own, and I have no hesitation in recommending it to anyone with an interest in entomology. At £48.00, it is a bargain.

Graham Elmes is a Centre for Ecology and Hydrology research fellow, Natural Environment Research Council's CEH Dorset Laboratories.

Evolution of the Insects

Author - David Grimaldi and Michael S. Engel
Publisher - Cambridge University Press
Pages - 772
Price - £48.00
ISBN - 0 521 82149 5

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