The oldest insect fossils so far discovered are tiny imprints of wingless insects found in sandstone rocks of the mid-Devonian period (c. 380 million years old). These earliest fossils closely resemble modern springtails (Collembola) and even by this time they show most of the specialised evolutionary features that characterise the present-day members of this insect order (e.g., the reflexed 'tail' used for jumping). This would suggest that the insect-stock from which they arose must have been already very old in Devonian times, and that the first primitive wingless insects probably appeared much earlier, at least in the Silurian, around 410-440 million years ago.

The earliest fossils of winged insects come from the coal-measures of the mid-Carboniferous (c. 320 million years old). These include the remains of cockroaches (Dictyoptera), some of which are little different from present-day species. It would seem that this group have survived more or less unchanged perhaps from Devonian times (i.e., for about 360-400 million years). The same coal deposits also contain the fossils of diverse archaic insects belonging to ancient lines that, unlike the cockroaches, gradually disappeared before the end of the Paleozoic era, leaving no modern descendants. Many of these archaic forms are huge dragonfly-like insects, with a wing-span of 20 cm or more, like the Palaeodictyoptera and Protodonata illustrated below - true 'dinosaurs' of the insect world.

Apart from cockroaches with their early origins in the Carboniferous or even earlier, fossils of other winged insects that clearly belong to modern orders first appear in the rock deposits of the Permian (c. 250-280 million years old), and include a diverse assemblage of dragonflies (Odonata), mayflies (Ephemeroptera), stoneflies (Plecoptera), lacewings (Neuroptera), crickets (Orthoptera) and bugs (Hemiptera). These are followed by caddis flies (Trichoptera), beetles (Coleoptera), flies (Diptera), earwigs (Dermaptera) and sawflies (Hymenoptera) in Triassic and Jurassic deposits (c. 150-240 million years old). The fossil remains of a few groups appear much later. For example, there are no authenticated fossils of moths and butterflies (Lepidoptera) before the Eocene period and most of those found so far come from Oligocene and Miocene deposits (c. 25-40 million years old). The relatively recent arrival of this latter group onto the insect scene (with mouthparts specially adapted for sucking nectar from flowers) is almost certainly linked to the appearance and spread of flowing plants in the late Mesosoic and early Tertiary eras. The approximate first appearance of these various groups of insects as fossils in the geological record, and the appearance of other animals and plants, is summarised in the table at the top of this page.

The fossils of insects found in rock deposits are often only fragments of body parts, like those illustrated above. Although such fragments might be easily identified as belonging to a general group or particular insect order, those that an expert could recognise again with confidence as being the same as a fossil species already described, or perhaps identical or similar to a species that still exists today, are very few. This is not the case for most insects trapped and 'fossilized' in amber (the solidified resin of pine trees), where specimens are often beautifully preserved in their entirety, so allowing accurate identification of species.

Baltic amber ²  from the lower Oligocene (c. 35-40 million years old) is perhaps the richest source of amber-preserved insects. All the insects so far discovered in amber deposits are familiar types that clearly belong to modern orders and families, and in many cases they belong to species that still exist today. These include, among the wingless insects (Apterygota), a great variety of springtails (Collembola) and bristletails (Thysanura); among the winged insects (Pterygota), many cockroaches and mantids (Dictyoptera), stoneflies (Plecoptera), stick-insects (Phasmida), earwigs (Dermaptera), termites (Isoptera), bugs (Hemiptera), thrips (Thysanoptera), beetles (Coleoptera), flies (Diptera) and so on - in fact, representatives of most of the insect groups recognised in our present-day classification. Three examples are illustrated above.

Thus, as in nearly all groups of animals and plants, the process of evolution over the ages has resulted in many diverse insect faunas, each comprised of different insect groups and species. The fossil record shows that many ancient forms flourished for a time and then became extinct, to be replaced by more successful types better suited to the ever changing climatic and other environmental conditions of the Earth's land masses. In a general view, however, the insects as a whole must be looked on as very ancient inhabitants of the Earth, probably at least twice as old as the reptiles and three times as old as the mammals - perhaps nearly a thousand times as old as our own mammalian group, the hominids. In many cases, even the common and familiar insect species that we encounter nearly everyday in our homes and gardens have existed in their present form for at least 35-40 million years before mankind first appeared on Earth - something to ponder the next time you swat a fly!