Silverfish are nocturnal insects typically 13–25 mm (0.5–1.0 in) long. Their abdomens taper at the end, giving them a fish-like appearance. The newly hatched are whitish, but develop a greyish hue and metallic shine as they get older. They have two long cerci and one terminal filament at the tip of the abdomen between the cerci. They also have two small compound eyes, despite other members of Zygentoma being completely eyeless, such as the family Nicoletiidae.
Like other species in Apterygota, silverfish are completely wingless. They have long antennae and move in a wiggling motion that resembles the movement of a fish. This, coupled with their appearance and silvery scales, inspires their common name. Silverfish typically live for two to eight years.
Silverfish are agile runners and can outrun most of their predators (including wandering spiders and centipedes). However, such running is possible only on horizontal surfaces, as they lack any additional appendages, and therefore are not fast enough to climb walls at the same speed. They also avoid light.
Before silverfish reproduce, they carry out a ritual involving three phases, which may last over half an hour. In the first phase, the male and female stand face to face, their quivering antennae touching, then repeatedly back off and return to this position. In the second phase, the male runs away and the female chases him. In the third phase, the male and female stand side by side and head to tail, with the male vibrating his tail against the female. Finally, the male lays a spermatophore, a sperm capsule covered in gossamer, which the female takes into her body via her ovipositor to fertilize her eggs.
The female lays groups of fewer than 60 eggs at once, deposited in small crevices. The eggs are oval-shaped, whitish, about 0.8 mm (0.031 in) long, and take between two weeks and two months to hatch. A silverfish usually lays fewer than 100 eggs in her lifetime.
When the nymphs hatch, they are whitish in colour, and look like smaller adults. As they moult, young silverfish develop a greyish appearance and a metallic shine, eventually becoming adults after three months to three years. They may go through 17 to 66 moults in their lifetimes, sometimes 30 in a single year—many more than most insects. Silverfish are among the few types of insect that continue to moult after reaching adulthood.