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the order of primitive wingless insects most closely related to winged insects in regard to their development. The spindle-shaped body is usually covered with thin, shiny scales and in some cases is naked. It measures 10–20 mm in length. The insects have long, filamentous antennae, a mouth apparatus of the chewing type, and two compound eyes with two to three simple occelli between them. The thorax is continuous with the 11-segmented abdomen, which ends in a long, segmented axial filament and paired cerci. Fertilization is external and internal.
The order embraces approximately 360 species, which are encountered on all continents, predominantly in warm areas. The Thysanura live under rocks, in rock crevices, and in plant remains. The USSR has about ten species, including the silverfish. The insects have not been studied thoroughly.
Certain more primitive insects bearing a superficial resemblance to the Thysanura have been assigned to an independent order, Archeognatha, which was earlier considered the suborder Machiloidea of Thysanura. There are about 220 species of Archeognatha, fewer than ten of which inhabit the USSR.