Defining Features Of The Insects

What then defines an insect? Certainly six legs is what most individuals think of; however, entognaths as well as stem-group panhexapods have or had six legs. The insects are without a doubt a monophyletic group, universally supported by morphological and molecular features. The defining features of the Insecta include the following:

• Loss of musculature in the antenna beyond the scape

• The presence of a chordotonal organ in the antennal pedicel (the Johnston's Organ)

• The development of the posterior tentorium into a transverse bar

• The loss of articulations between the coxae and the sterna

• The subsegmentation of the tarsus into units called tar-someres

• The articulation of the pretarsal claws with the apicalmost tarsomere rather than the pretarsal base

• The presence in females of an ovipositor formed by gonapophyses on the eighth and ninth abdominal segments (although this trait may be more primitive since progenitors of these structures were apparently present in marine panhexapods)

• The presence, at least primitively, of a long terminal filament on the dorsum of the eleventh abdominal segment (Kristensen, 1991).

As we will see through this discourse, most of these traits may be secondarily modified or reduced in one or more insect lineages. For example, the ovipositor is vestigial or independently lost in several groups, like the orders Zoraptera, Phthiraptera, and Coleoptera and the orders of Panorpida.

With this concept of an insect in mind, we can begin to unravel their evolutionary history. As we will see, their history as we know it begins around 410 mya, in the alien world of the middle Paleozoic. Before considering the various lineages, we provide here a brief outline of the major groups of insects and the history of studies that contributed to our present understanding of their relationships.

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