insects

insects Insects are invertebrates belonging to the phylum Arthropoda, which is composed of animals with a segmented body, many jointed appendages, and a chitinous exoskeleton (an external skeleton made of chitin, a hard brown substance). They occupy first place among animals in terms of biological success: almost 75 per cent of all known animals are arthropods (750 000 species), and of these almost 500 000 are insects. Insects form an easily distinguishable natural group of organisms in which the body is divided into a head with jointed appendages adapted as mouthparts, a thorax with legs and two pairs of wings, and an abdomen. They are separated into major groups based on gross morphology; Palaeoptera have wings that cannot be folded when the animal is at rest (e.g. dragonflies), while Neoptera have wings that can be folded. In addition, the palaeopterans have immature forms that are voracious aquatic predators, quite unlike the adults, while neuropterans have immature forms that are either smaller versions of the adults or are initially worm-like and then pass through a pupa stage in which they are transformed into adults.

The fossil record of land arthropods is scanty, like that of all land animals, and hence does not necessarily give a true picture of numbers of species or time of earliest appearance. Although almost 500 000 species of insect have been described, only a few thousand fossil insect species are known; most of these are from a few special regions where fossilization took place under unusual conditions. Baltic amber deposits from the Tertiary are well known; deposits of Cretaceous age are also known from Alaska and New Jersey. The Late Carboniferous (Pennsylvanian) Mazon Creek Fauna from Illinois is an important Palaeozoic locality and contains over a hundred insect species, almost half of the animals recorded from there. Insects are also known from trace fossils; brood chambers of mining bees are known from the Eocene of Wyoming and the Upper Cretaceous of Arizona; insect trails have been reported from the Permian of Germany; and insect-cut leaves are known from the Eocene of Kentucky.

Insects probably developed in the Devonian from an unknown arthropod ancestor. The earliest described insects are flightless; they occur in Scottish Devonian rocks, and are similar to modern springtails. The first important evolutionary event was the development of wings, which must have taken place in the Early Carboniferous (Mississippian) although currently no Mississippian insects are known. By the Late Carboniferous (Pennsylvanian) large flying insects had developed, including dragon-flies with wingspans up to 750 mm, the largest insects known. The ability to fold the wings close to the body developed during the Late Palaeozoic, and allowed insects to escape predators by running along the ground and hiding in crevices. The development of a pupal stage probably took place at about the same time; the first examples are found in the Permian. This development avoided competition between larval and adult forms for food sources and also provided for overwintering in the pupal stage. The last major development was the co-evolution of insects and flowering plants, which took place in the Late Mesozoic. Although the record is poor it is clear that insect pollinators such as moths and butterflies, which appear in the Cretaceous, arose in concert with the flowering plants, while the appearance at the same time of fleas marks their development together with their hosts, the mammals.

David K. Elliott