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Bristletails are about 300-400 species of small, elongate, terrestrial insects in the order Thysanura. Bristletails have an ancient evolutionary lineage, and they are believed to be relatively primitive, that is, similar in form and function to the most early evolved insects.

Bristletails have a simple metamorphosis, with three life-history stages: egg, nymph, and adult. Both the nymphal and adult stages are wingless, and they are rather similar in their physical appearance, the main difference being in size and sexual maturity. The adults are large and mature.

Bristletails are easily distinguished by the three thread-like appendages that emerge from the end of their abdomen, by their long, backward-pointing antennae, and by their body covering of glistening scales. Bristletails have chewing mouth parts, and they feed on a wide range of types of soft, usually decaying organic matter. Bristletails hide during the day, and if they are disturbed they run quickly away to seek a new hiding place. Otherwise, bristletails are active at night. Bristletails are unusual in that they continue to grow and molt even after they have become sexually mature adults. Almost all other insects stop growing after they become breeding adults.

The most familiar bristletails are those in the family Lepismatidae, including the light-colored, common silverfish (Lepisma saccharina) and the darker-colored firebrat (Thermobia domestica), which often occurs in warm, moist places, for example, near hearths, stoves, and furnaces. Both of these species can be common in moist places in buildings, where they feed on a wide variety of starchy foods, and can sometimes cause significant damages to books, wallpaper, fabrics, and stored foods.

The jumping bristletails (family Machilidae) are widespread in North America, living under organic debris in various types of natural, terrestrial habitats. The primitive bristletails (Lepidotrichidae) and nicoletiid bristletails (Nicoletiidae) are all rather rare in North America.

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