Hexapoda

The Insects

The Insects - Four hundred million years of glorious evolution

Evolution of the insects, from Prothero, 1990, url. Although insect evolution is not quite as linear as this, insect evolution, like that of other major groups, does involve a number of progressive evolutionary grades or stages.

The most diverse and abundant (or perhaps second most abundant, after nematodes) type of animal life on Earth today, insects, or hexapods (six legged animals), are small terrestrial arthropod distinguished by their six legs and division of the body into three parts: head, thorax, and abdomen. they are also the only invertebnrates with the power and flight, and this may well be a factor in their success, although some wingless types, such as the tiny Collembola, are astonishingly successful and abundant.

The diagram shown at the right presents a simple "tree of life" representation of the evolutionary relationships of the insects. Insects through their long evolutionary history show a number of distinct evolutionary stages towards greater complexity, represented artistically and metaphorically as ascent up the tree (for reasons expressed elsewhere the present writer (MAK) does not agree with the current trend to reject all reference to evolutionary advancement to greater complexity). However if the time dimension were added, then the tree would resemble a sort of multiple bush, with sudden bursts of evolutionary novelty, then nothing happening for a hundred million years or so and the various lineages continuing alongside each other, then another burst of novelty, and so on. Hence during the Early Devonian there is a sudden evolutionary radiation of primitive wingless insects. Nothing much happens for a long time, then suddenly during the mid Carboniferous one of these lineages gives rise to paleopterous (dragonfly-like) insects, and there is another extraordinary adaptive radiation (while the flightless insects continue humbly in the background). One of these dragonfly-like types very quickly (geologically speaking) evolves into the ancestral folding wing insects, and there so there are two huge evolutionary radiations alongside each other, the dragonfly types (called Paleoptera) and the early cockroach and grasshopper types (called Neoptera). Nothing happens for a while, with both groups existing alongside each other. Then at some time during the middle Permian one of the folding wing types evolves into the ancestral holometabolic insects which, as their name indicates, undergo transformation through their life cycle. These last named are the most successful of the whole group, including flies (diptera), beetles (coleoptera) and, only much later, butterflies and moths (lepidoptera), and ants bees and wasps (hymenoptera)

Although "insect" has been used in the above paragraph in a colloquial sense, technically speaking we should distinguish here between hexapods and insects, as these two terms are sometimes confused or synonymised. Although colloquially, all hexapods are also insects, and although some (e.g. Cavalier-Smith 1998) have synonymised the two (in which case they are referred to by their earlier name, Class Insecta), in current cladistic classifications, insects are a subgroup of hexapods, albeit the largest and most diverse one. Hence we have divided these units according to evolutionary grades. The five grades are:

Hexapoda - six legs, three tagma, spiracles (the latter shared with myriapods)
Insecta - similar to above, but distinguished by further synapomorphies
Pterygota - wings
Neoptera - folding wings
Holometabola (or just Metabola - ref) - change of form during life cycle

Updating the insect tree of life diagram, this can be arranged in the form of a dendrogram, as follows

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