An introductory chapter looks at biodiversity, definitions of species, the history of evolutionary ideas, phylogenetics, taxonomy, and paleontology, all from the perspective of the insects. This is followed by an overview of insect fossils: lakes are the most common sites, wings are the most commonly fossilised body part (and wing venation is taxonomically important), and impression, compression, concretion, encapsulation, inclusion in amber and other processes have produced many different fossil forms. Grimaldi and Engel also present a chronological and regional survey of the most important fossil insect deposits.
A chapter covers the position of the arthropods among their nearest relatives — onychophora (velvet worms) and tardigrada (water bears) — of Atelocerata (Tracheata) among the arthropods — with sister subphyla Arachnomorpha (spiders, ticks, trilobites) and Mandibulata (including crustaceans and myriapods) — and of the Insecta among the Hexapoda (alongside springtails and other Entognatha). Next comes an introduction to the insects, covering insect anatomy, the taxonomy of the major insect orders, and (briefly) the history of entomology.
The bulk of Evolution of the Insects is made up of chapters on the major groups, in phylogenetic branching order: "Earliest Insects" (bristletails, silverfish), "Insects Take to the Skies" (Pterygota and the origins of flight, mayflies, extinct beaked insects, dragonflies and relatives), "Polyneoptera" (stick insects, roaches, termites, mantises), "The Paraneopteran Orders" (lice, "true" bugs), "The Holometabola" (and the origins of complete metamorphosis), "Coleoptera and Strepsiptera", "Hymenoptera: Ants, Bees, and Other Wasps", "Panorpida: Antliophora and Amphiesmenoptera", and "Amphiesmenoptera: The Caddisflies and Lepidoptera". Broader topics are treated where they fit most naturally: cospeciation following the section on lice, for example, and eusociality in the chapter on the hymenoptera.
Some of the most engaging material is left for last. The penultimate chapter is an overview of Cretaceous and Tertiary history, covering the origins and radiation of angiosperms and their links to insects as pollinators, the radiations of phytophagous insects, austral/gondwanan relicts and disjuncts and other aspects of biogeography, mass extinctions and the K/T boundary, and the mammalian radiations. And a brief final chapter looks at why there are so many insect species, and at anthropogenic insect extinctions.
With no footnotes and in-line (Harvard-style) references, the text of Evolution of the Insects is rather dense in places. Sometimes it goes into anatomical and taxonomic details that are hard for a non-specialist to get much grip on, though Grimaldi and Engel keep links to broader evolution and ecology to the fore and the "footnoteable" bits and references can easily be skipped over.
Some insect taxa will also be of more interest than others. Many readers, for example, will glance over a page on the glosselytodeans, which "as presently constituted may be an unnatural group, consisting on the one hand of the Early Permian Permoberothidae and on the other hand as [sic] an assemblage of Late Permian and Triassic-Jurassic families". Immediately following this, however, come two pages on "The Origins of Complete Metamorphosis" which are of much broader interest.
A few extracts:
"The cells of the wing are the regions between veins and crossveins. There are numerous naming systems for the cells, none of which are consistent across orders. One of the more common naming systems uses the name of the vein that marks the anterior margin of the cell, though numerous other names are in use, like marginal, submarginal, discal, and discoidal. Only two terms are universal in entomology when referring to wing cells: Closed cells are those that are bordered on all sides by veins, while open cells are those... at the edge of the wing."
"Mantises are tone deaf and auditory cyclops: They cannot discriminate frequency or direction. Their hearing, though, is acute between 25-30 kHz, the region of ultrasound most frequently used by echolocating bats. ... Field tests indicate mantises are highly effective at evading bats. ... [W]hat may be a unique origin of the ear in the Mantoidea is perhaps ... contemporaneous with the origin of insectivorous bats in the Late Paleocene to Early Eocene".
"Two traits are required for the evolution of an advanced society: living in a nest or other persistent domicile, and close genetic relatedness. The nest provides a discrete enclosure where individuals meet and store food, and, most importantly, it allows for the cooperative protection and extended care of the brood. If it is a particularly sturdy structure, this will allow multiple generations to occupy it. The construction of a nest or cooption of a preexisting structure is a feature common to all eusocial organisms, which in fact appears to have phylogenetically preceded social behavior. For example, social aphids in the Pemphiginae and Hormaphidinae, and Cladothrips and Oncothrips Thysanoptera, belong to lineages that construct persistent galls, but most species in these lineages are not social."
The colour photographs in Evolution of the Insects are mostly of fossils, but include photographs of living insects and some scanning electron micrographs. These are supplemented by excellent anatomical drawings and plenty of phylogenetic trees. It is a splendid volume.