Evolution and paleontology ( insect )
Origin of insects
The most primitive insects known are found as fossils in rocks of the Middle Devonian Period and lived about 350,000,000 years ago. The bodies of those insects were divided then, as now, into a head bearing one pair of antennae, a thorax with three pairs of legs, and a segmented abdomen. Those insects originated with the terrestrial branch of the phylum Arthropoda. The Arthropoda, whose origin is thus far unknown, probably arose in Precambrian times, perhaps as much as 1,000,000,000 years ago. Some arthropods colonized the open sea and have become the present-day class Crustacea (crabs, shrimps) and the now-extinct Trilobita. Other arthropods colonized the land. This terrestrial line persists chiefly as the classes Onychophora, Arachnida (spiders, scorpions, ticks), the myriapods (consisting of Diplopoda [millipedes], Pauropoda, Symphyla, and Chilopoda, or centipedes), and finally the class Insecta. The most primitive insects today are found among the wingless (apterous) hexapods; sometimes known collectively as apterygotes, they include proturans, thysanurans, diplurans, and collembolans. It is agreed generally that insects are related most closely to the myriapod group, among which the Symphyla exhibit most of the essential features required for the ancestral insect form (i.e., a Y-shaped epicranial suture, two pairs of maxillae, a single pair of antennae, styli and sacs on the abdominal segments, cerci, and malpighian tubules). There is, therefore, general agreement that the insects probably arose from an early symphylan-like form.
Insect fossil record
The insect fossil record has many gaps. Among the primitive apterygotes, only the collembolans (springtails) have been found as fossils in the Devonian Period. Ten insect orders are known as fossils, mostly of Late Carboniferous and Permian times. No fossils have yet been found from the Late Devonian or Early Carboniferous periods, when the key characters of present-day insects are believed to have evolved; thus, early evolution must be inferred from the morphology of extant insects. It has become evident that insect evolution, like that of other animals, was far more active at some periods than at others. There have been geological epochs of “explosive†evolution during which many new forms have appeared. Those epochs may have followed some modification or innovation in body function, or new developments favoured by climatic changes or evolutionary advances of other animals and plants. During those periods of evolutionary change, new methods of feeding and living led to diversity of insect mouthparts and limbs, the origin of metamorphosis, and other changes.
[tags]insects[/tags]
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