The insect order Thysanura are commonly known as silverfish and firebrats. The name actually means bristletail (thysan is bristle, ura is tail). They are small, pale insects with the requisite six legs and three body parts. Because they lack wings at any time during their development, they are grouped with the most primitive insects, the Apterygota. They are among the best known of the apterygotes because of their economic importance. Unlike the rest, which live inconspicuously in the soil, firebrats and silverfish have invaded the domain of men. Firebrats got their name because of a liking for hearths while silverfish are disliked for their propensity to live in and eat books and clothes.
There are about 370 species of Thysanurans described and grouped into five families. As well as those that invade our homes, thysanurans are also found in moist soils and leaf litter, under rotting logs and in caves. A few live in deserts and have a unique adaptation for dry land living: they can absorb water through their rectums. They are active at night and hide during the day. They have chewing mouthparts and are omnivorous, consuming both plant and animal matter. Bristletails also have long simple antennae, a small pair of compound eyes (although a few species are blind and eyeless), and the tail has three long bristles or cerci on it. One of these points straight back while the other two veer out to either side in a pattern that is characteristic of this order and useful in identification.
If the Thysanura evolved before insects develped wings than they are a very ancient Order indeed. Long before the dinosaurs, when amphibians were first crawling out onto the land, giant dragonflies and other flying insects were already in existence so the flightless insects must have been around long before that. Fossil thysanurans exist from the upper Carboniferous period, some 300 million years ago, but given the poor fossil record for insects, they probably existed long before this. Some scientists now consider the wingless insect orders to be different enough to group in a separate class, the Hexapoda, rather than as part of the Insecta, because they are so primitive. It is probably however that the modern thysanura are the descendants of the animals that became the modern insects so there is a close relational tie between them.
Silverfish reproduce sexually. The males produce spermatophones which are attached to a thread. The male then dances to attract females who pick up the spermatophores and use them to fertilise their eggs (indirect fertilisation). The young nymphs resemble the adults and go through a series of moults as they grow and mature. They are sexually mature after three months and can live for four years or more, a long life for such a small insect. They continue to moult and grow throughout their lives.
Silverfish, firebrats and other bristletails are not endangered and in fact may outlive us. Like cockroaches and rats, they have adapted to the world of humans and learned to use us for their own success. They may be primitive but they are definitely survivors.
for more information: http://www.ento.csiro.au/education/insects/thysanura.html
http://www.earthlife.net/insects/thysan.html
