Zygentoma are an order in the class Insecta, and consist of about 550 known species. The Zygentoma include the so-called silverfish or fishmoths, and the firebrats. A conspicuous feature of the order are the three long caudal filaments. The two lateral filaments are cerci, and the medial one is an epiproct or appendix dorsalis. In this they resemble the Archaeognatha, although the cerci of Zygentoma, unlike in the latter order, are nearly as long as the epiproct.
Until the late twentieth century the Zygentoma were regarded as a suborder of the Thysanura, until it was recognized that the order Thysanura was paraphyletic, thus raising the two suborders to the status of independent monophyletic orders, with Archaeognatha as sister group to the Dicondylia, including the Zygentoma.
Etymology
The name "Zygentoma" is derived from the Greek (), in context meaning "yoke" or "bridge"; and (), "insects", literally "cut into" because of the segmented anatomy of typical insects. The idea behind the name was that the taxon formed a notional link between the Pterygota and the Apterygota. This view is now totally obsolete, but the phylogeny of the Insecta was in its infancy in the late 19th and early 20th century, and the name was firmly established by the time that more sophisticated views were developed.
Description and ecology
Silverfish are so-called because of the silvery glitter of the scales covering the bodies of the most conspicuous species (family Lepismatidae). Their movement has been described as "fish-like" as if they were swimming. Most extant species have a body length less than long, though Carboniferous fossils about 6 cm long are known.
Zygentoma have dorsiventrally flattened bodies, generally elongated or oval in outline. Their antennae are slender and mobile. The compound eyes tend to be small, and the twe two families Nicoletiidae and Protrinemuridae, and some troglobitic species, lack eyes entirely. The Lepismatidae have compound eyes composed of 12 ommatidia on each side of the head. Ocelli are absent in all species except for Tricholepidion gertschi, the only member of the family Lepidotrichidae. The mandibles are short, and the mouthparts unspecialised. Tricholepidion, Nicoletiidae and Protrinemuridae have eight pairs of short appendages called styli on their abdominal segments 2 to 9, but in Lepismatidae styli are only found on segment 7 to 9 or 8 to 9, and sometimes just on the ninth segment. Or styli can be completely absent. A distinctive feature of the group is the presence of three long, tail-like filaments extending from their last segment. These three generally subequal, except in some members of the family Nicoletiidae, in which they are short, and the cerci are hard to detect. The two lateral filaments are the abdominal cerci and the medial one is the epiproct.
Silverfish may be found in moist, humid environments or dry conditions, both as free-living organisms or nest-associates. In domestic settings, they feed on cereals, paste, paper, starch in clothes, rayon fabrics and dried meats. In nature, they will feed on organic detritus. Silverfish can sometimes be found in bathtubs or sinks at night, because they have difficulty moving on smooth surfaces and so become trapped if they fall in.
Wild species often are found in dark, moist habitats such as caves or under rocks, and some are commensals living in association with ant colonies, e.g., Trichatelura manni and Allotrichotriura saevissima, which lives inside nests of fire ants in Brazil.
There are no current species formally considered to be at conservation risk, though several are troglobites limited to one or a few caves or cave systems, and these species run an exceptionally high risk of extinction.
