An order of ancestrally wingless insects, commonly known as bristletails, in the subclass Apterygota; about 350 species are known in two families, Machilidae and Meinertellidae. All have slender, spindle-shaped bodies of about 0.4–0.8 in. (1–2 cm) in length, tapered tail-ward to a long, multisegmented median caudal filament and paired cerci. The relatively thin integument is covered with pigmented scales after the first instar. The antennae are long and filiform, without intrinsic musculature in the flagellum, as in all true insects; and large, contiguous compound eyes are accompanied by a trio of ocelli. The mandibles are exposed and attached to the cranium by a single joint (condyle), and the maxillary palpi are extended, with seven segments, while the labial palpi have only three segments. Each leg has a large coxa, two-segmented trochanter, femur, tibia, and a two- or three-segmented tarsus terminating in a two-clawed pretarsus. The posterior pair or two pairs of coxae and most of the ventral abdominal coxites bear styles, which may represent modified legs. Outwardly projecting vesicles that are apparently water absorbing also occur on most coxites. Internally, the gut is relatively simple, with small crop, digestive ceca, and a dozen or more Malpighian tubules. The nervous system is primitive, with three thoracic and eight abdominal ganglia and twin connectives. Tracheae are well developed.
In mating, the male deposits sperm on a thread or in a spermatophore that is picked up by the female, but in at least one species sperm is placed directly on the female ovipositor. Immature forms are similar to adults in body form, though the first instar may lack cerci. Sexual maturity is attained after eight or more molts.
Bristletails live in a variety of habitats worldwide, often on stones or tree trunks where they find the algae and lichens that appear to be their principal sources of food. Most species are primarily nocturnal feeders. See also Apterygota.
