BRISTLETAIL

(order Thysanura), any of approximately 350 species of primitive, wingless insects that measure from 5 to 20 mm (0.2 to 0.8 inch) in length when they are fully grown and have three slender, bristlelike appendages at the tip of the abdomen. The compound eyes are small, the mouth parts external. Some species have scales covering the body. Young bristletails resemble adults except in size. Sexual maturity is attained in two to three years. A bristletail molts as many as 35 times during its life (3 to 5 times per year); the life span of some species may be as long as seven years. Bristletails eat starchy material, often causing excessive damage to books and papers. Bristletail is also the common name for members of the order Microcoryphia, which is now separated from the Thysanura. The silverfish (Lepisma saccharina), a slender, flat, wingless insect with three tail bristles, is so called because it is covered with silvery scales and moves quickly. It normally lives indoors and is worldwide in distribution. Because it eats materials containing high percentages of starch (e.g., paste, bookbindings, and wallpaper) it can cause damage to books and fabrics. The antennae and the tail bristles of the silverfish are shorter than the body. Superficially the male resembles the female. Unlike other groups of true insects, which copulate, the silverfish performs courtship movements that end with the male depositing a sperm packet, which the female places in her vagina. The oval, whitish eggs are believed to be inserted into cracks and soil litter. The young, which hatch in several days, are scaleless and have short appendages; they molt every few days, gradually acquiring adult features. The silverfish continues to molt throughout the two or more years of its life, even after reaching sexual maturity. Control is possible with poisonous baits or other insecticides. The firebrat (Thermobia domestica) is typically white with light and dark spots. Its life cycle and habits are similar to those of the silverfish. Its name derives from the fact that it often inhabits warm places such as bakehouses and fireplaces. Chlordane sprays and dusts are used to control firebrat populations. The oldest known insect fossil for which there is significant remaining structure (head and thorax fragments) is a bristletail (Archaeognatha), estimated to be 390 to 392 million years old. It was discovered on the north shore of Gasp Bay, Quebec, Can., at a site that was only 10 above the equator during the Devonian time of this insect.