We have now explored how insects arose, evolved wings and developed interactions with the earliest land plants to form the first complex terrestrial ecosystems. We will now continue to explore the origins of the present day terrestrial insect fauna and its correlation with the angiosperms or flowering plants, the dominant group of land plants today. Of the more than one million living insects described, more than 80% belong to the endopterygotan or holometabolan insects. Ten orders of holometabolan insects are recognized, of which four are considered megadiverse, that is, having more than 120,000 described species each. These orders are: the Coleoptera - the beetles; the Lepidoptera - the butterflies and moths; the Hymenoptera - wasps, ants and bees; and finally the Diptera - or true flies. Three out of four described insects belongs to one of these four orders, which together constitute half of the known biodiversity on the planet. Given that many poorly explored insect groups, for example parasitic wasps and many beetle groups, are found within the four megadiverse orders, their proportion of the true insect diversity is likely to be even higher. The holometabolan insects differ from other insects in how they develop from egg to adult. A non-holometabolan insect, for example a bug, hatches from the egg as a nymph. The nymph is essentially a smaller version of the adult insect, except that the wings and external genitalia are not fully developed. The insect develops through a series of nymphal stages that gradually comes to resemble the adult stage more and more; the nymphs often eats the same food as the adults. In contrast, a holometabolous insect, for example a butterfly, hatches from the egg as a larva that does not bear any resemblance to the final adult stage. The larva has no external wing buds and no complex eyes, and in many cases has further reduced anatomy, having lost legs, eyes and/or some of the mouthparts. After a series of larval instars, there is a pupal stage, which is a non-feeding, immobile stage. Inside the pupae, the entire anatomy of the insect is reorganized and eventually the adult emerges from the pupa. Often, the pupa serves as a resting stage for insects that have to overwinter or otherwise avoid unfavorable conditions for an extended period of time. The adult often feeds from a different food source than the larva, if it feeds at all. The first holometabolan insects occurred in the fossil record well before the end of the Permian. There are some reports of holometabolan insects from the late Carboniferous more than 300 million years ago. The earliest fossils that can be assigned to at least a stem group of one of the living holometabolan orders are from the Permian more than 250 million years ago. However, even though representatives were present in the late Paleozoic, it seems that the holometabolan orders that are dominant today did not start to radiate in earnest and begin their rise to dominance until after the end Permian extinctions. Beetles, true flies, and wasps all appeared in the Triassic, moths and butterflies in the Jurassic. Close relatives or stem group fossils of at least Coleoptera and true flies are known from the Permian. Beetles are characterized by having very hardened fore wings that fit tightly over the abdomen; the wing venation is usually entirely absent or at least substantially altered. We can see this in this living archostemmatan beetle, which belongs to one of the basal most beetle lineages that have a fossil record going back to the Triassic. Notice the pattern on the hardened forewings with long rows of punctures extending front to back; this has been interpreted as the remains of the wing venation. The earliest proto-beetles are known from the lower Permian. Here we see an example of one of these. When comparing with the living archostemmatan beetle, we see that in the proto-beetle the forewings extend beyond the tip of the abdomen and apparently do not fit tightly. Also, the forewings might not have been as hardened as in modern beetles, and there are even remains of a more typical wing venation pattern. Is there anything in the biology of holometabolans that facilitated their radiation in the early Mesozoic? The end Permian extinction event was a result of major upheavals in the ecosystems across the planet. Break-down of normal tropic interactions resulted in wide-ranging turnover in biotas as many ecosystem functions were suspended for months or even years. One might speculate that the holometabolan life cycle which includes a pupal stage that is frequently dormant for an extended time might have allowed holometabolans to better survive the extreme conditions during the extinction event. Afterwards, their comparatively higher survival rate might have given the holometabolans a head start when the terrestrial ecosystems were repopulated after the end Permian event. The subsequent radiation of the holometabolous insects might also have been helped by their particular life cycle. As mentioned previously, the larva and adult of holometabolans often specialize in different food sources. In this way, the larva and adults avoid competing for the same food source, in contrast to the non-holometabolous insects which often eat the same food throughout their life. By the early Mesozoic, most holometabolan orders had evolved and were starting to diversify. The evolution of their current diversity is closely linked with the radiation of another group of organisms that dominate terrestrial ecosystems today: the flowering plants. In the final video on insect evolution, we will look at how they have interacted with flowering plants for the past 150 million years.
