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Apterygota (Ametabola; subphylum Uniramia, class Insecta) The smaller subclass of insects, containing two orders (Archaeognatha and Thysanura), whose members are primitively wingless (in contrast to secondarily wingless insects, e.g. fleas, lice, etc.), terrestrial insects with ectognathous, biting mouth-parts. The subclass formerly included the Protura, Collembola, and Diplura, which are now regarded as distinct subclasses. Most Apterygota are free-living; elongate or oval; 3–15 mm long; and with direct larval development, moulting occurring throughout the life of the animal. They feed on fungi, lichens, pollen, etc., domestic species subsisting on a diet of carbohydrates, dextrins, and a small amount of protein from glue and sizes. Approximately 580 species occur world-wide, and are found almost everywhere, even at high altitudes and in polar regions.