Silverfish are ½ inch to 1 inch long while Firebrats only reach ½ inch.
Silverfish (and firebrats, a type of silverfish) range from silvery gray to dark gray and have dark lines running front to back on top, which is why they are often referred to as four-lined silverfish. They are both elongate insects with very long, thin antennae and three long appendages at their rear end – a pair of long, thin cerci that project out sideways and a central, longer filament.
Silverfish prefer to live in damp, cool places such as basements and laundry rooms. They are often found in bathtubs, sinks or washbasins because they climb in and are unable to climb out. Firebrats, on the other hand, prefer to live in hot, humid places such as near furnaces, fireplaces and heat pipes in winter and in attics in summer. Attics often have an abundance of firebrat feces lying on insulation and rafters as they leave this black, pepper-like material everywhere they go. Both silverfish and firebrats are also commonly found outdoors in woodpiles or fences. Evidence of their damage will be holes made by their rasping mouthparts.
Both silverfish and firebrats feed on a wide variety of materials, including human foods, paper products, fabrics, and the glue in books and wallpaper.
Females deposit groups of very small eggs into cracks and crevices and they lay fewer than 100 eggs during their lifespan. Eggs hatch within one month. Full development to the adult stage can be anywhere from a few months to as long as three years, depending on the living conditions and they generally live for several years.
Silverfish and firebrats are covered with scales, flattened top to bottom, and have the ability to squeeze into tiny cracks to hide or to gain entry. They have very quick movements and usually dart for cover when lights are turned on.