The most important event in insect evolution during the Mesozoic was the radiation of the angiosperms or flowering plants in the Cretaceous. Before the end of the Cretaceous, the angiosperms had become the dominant plant element in most terrestrial ecosystems. The radiation of the angiosperms sparked a number of subsequent radiations among various insects that are today the dominant herbivorous insect groups. Most of these belong to the Holometabola; prominent examples of herbivorous holometabolans primarily feeding on angiosperms are the leaf beetles, weevils, bees and butterflies and moths. In many cases, the angiosperms are not merely exploited by the herbivorous holometabolan insects, but a mutually beneficial interaction has evolved. The best known example of this is probably pollination of flowers, for example, by bees. Many flowering plants are entirely dependent on insect pollinators to be able to reproduce and offer a reward like nectar or pollen to the insect. The close relationship between the plant and its pollinator and the co-evolution between them might have been a driving force for the diversification of both angiosperms and the holometabolous insects. A remarkable example of plant-pollinator interaction is known from Madagascar. An orchid known as Darwin's orchid can be found here. The flower of this orchid has a narrow and very elongate hollow spur, at the bottom of which the nectar container is situated. The spur might be up to 35 cm in length. Are there any insects that can reach the nectar? It turns that the only known species that can perform this feat is a hawk moth which has a proboscis matching the length of the spur of the orchid. As the moth inserts its proboscis to extract the nectar, the pollen packages of the orchid are attached to the base of the proboscis. The moth can then go on to pollinate the next specimen of the orchid it encounters when searching for nectar. Why would the orchid and moth evolve such extreme specializations? It seems to make pollination and finding nectar more difficult. The potential advantage for both the orchid and moth is that it reduces competition. By having the elongate spur, the orchid ensures that it can only be pollinated by one insect species. As long as the insect pollinator does not visit different species of flowers, there is less chance that the insect gets eaten or loses the pollen before it can pollinate another orchid of the same species. The orchid thus reduces competition for pollinators. Similarly, by specializing in a single orchid species where the nectar cannot be reached by other insects, the hawk moth does not compete for nectar except with members of its own species. A possible case where a change in vegetation composition affected the insect fauna has recently been documented in a study of fossil scorpion flies from the late Jurassic - early Cretaceous. Scorpion flies are a small order of holometabolous insects; its living members are mostly scavengers, do not have a proboscis and rarely visits flowers. In the late Jurassic, three lineages of scorpion flies independently developed proboscises; one of these scorpion flies is shown here. It has been suggested that they were important pollinators of gymnosperms, as shown in this reconstruction. Flowering plants had still not diversified at this time. Other insects with proboscises which might have pollinated gymnosperms occurring at the same time are members of the true flies. When angiosperms diversified at the expense of the gymnosperms later in the Cretaceous, there were some dramatic changes in the composition of these potential pollinators. The proboscis-bearing scorpion flies all went extinct before the end of the Cretaceous, declining together with the gymnosperms and apparently failing to switch to feeding on flowering plants. The proboscis-bearing true flies are still with us today and have radiated primarily on angiosperms, possibly having switched from the gymnosperms sometime in the Cretaceous. Finally, a new group of pollinators arose at the same time as the angiosperms radiated: the glossatan or proboscis-bearing moths which are today predominantly associated with flowering plants. Insects as parasites on warm-blooded vertebrates are first documented in the Early Cretaceous. Possible stem group fossils of both lice and fleas are known from this time. This fossil, Tarwinia australis, is a possible early flea. Like modern fleas, it has the flattened body and specialized combs on the body that is useful for clinging to the fur or feathers of a host, but it has longer antennae than modern fleas and the legs are not expanded at the base to accommodate the musculature used in jumping. The fleas probably evolved their exceptional jumping abilities only later. Today, lice and fleas attack birds and mammals. It is highly likely that in the Cretaceous some of them would also target non-avian dinosaurs, at least some of which were probably warm-blooded. A prominent feature of terrestrial ecosystems today that had its origin in the Mesozoic is the evolution of insect societies. Social insects are primarily found within two insect groups, the termites and the ants, bees and social wasps. The most impressive insect societies are formed by termites and ants, which may reach a colony size of millions of individuals. Such a colony effectively functions as a single super-organism with the biomass and feeding capacity of a large herbivorous vertebrate, for example a cow. The termites are non-holometabolous insects and have been convincingly shown to be subordinate within the cockroaches. Termites have symbiotic protists in their gut that enable them to digest cellulose and thus process huge amounts of plant material that is unavailable to most other insects. However, not all termites feed on cellulose. Termites are also famous for constructing huge mounds, like this from Queensland, Australia. It is about 3 meters tall. The initial radiation of the termites probably took place in the early Cretaceous, which is when they first appear in the fossil record. The remaining fully social insects belong to the Hymenoptera; eusociality has evolved more than once within this order. This occurred at various times in the Cretaceous, probably first in the ants. This is Sphecomyrma, one of the earliest ant fossils from the upper Cretaceous. The social bees are herbivores, the social wasps are predators; both groups have close relatives that are solitary, so sociality is not characteristic of bees and vespids as a whole. In contrast, all ants are social or social parasites. The ants have by far the most diverse lifestyle among the social insects, including herbivorous as well as carnivorous members. Recently, ants have invented large scale agriculture, as seen in the South American leafcutter ants. These herbivorous ants cut up fresh vegetation, bring it home to their nests, and feed from the fungi that they cultivate on the harvested plant material. The most advanced version of this ant/fungus association, where the fungi cannot grow independently of their ant hosts, apparently evolved fairly recently, having arisen some 10 million years ago. The leaf cutter ants are the dominant herbivores in South and Mesoamerican rainforests today. By the end of the Cretaceous, insect life very much resembles what we see today. All the orders living today had evolved, as well as most of the families. Insects seem to have been at most moderately affected during the end Cretaceous extinction events, certainly less than by the end Permian extinctions, at least at the ordinal and family level. The transition to what is essentially the modern insect fauna happened gradually during the Cretaceous, in particular in conjunction with the radiation of the flowering plants. For most of the past 400 million years, insects have been a dominant part of the terrestrial ecosystems, and they will continue to be so in the future.
