Silverfish and Firebrat Identification Guide

Key Visual Features

Silverfish (Lepisma saccharinum) and firebrats (Thermobia domestica) are closely related, wingless insects with a distinctive teardrop shape.

• Size: Both measure roughly 12-19 mm long as adults, not counting the long tail filaments.
• Body shape: Elongated, flattened, and tapering smoothly from a wider head/thorax region to a narrow tail, giving a classic teardrop or carrot-like silhouette.
• Silverfish coloring: Covered in fine, silvery-gray to pearlescent scales that give the body a metallic shine and smooth appearance.
• Firebrat coloring: Covered in mottled gray and brown scales with a speckled, less uniform pattern compared to the silvery sheen of true silverfish.
• Tail filaments: Three long, thread-like appendages extend from the rear end — a central filament flanked by two lateral ones (cerci), all roughly as long as the body itself.
• Antennae: Long, thin, and thread-like, often as long as or longer than the body.
• Movement: Extremely fast, fluid runners with a side-to-side, fish-like wiggling motion, and no wings at any life stage.

Where and When You'd See It

Both are nocturnal and shun light, so they are typically spotted at night or when disturbed from hiding spots such as behind baseboards, in bathrooms, basements, closets, or storage boxes. Silverfish prefer cooler, more humid conditions, often found in bathrooms, basements, and damp storage areas. Firebrats favor warmer locations, historically near heat sources such as furnaces, ovens, and boiler rooms, reflecting their name. Both can be active year-round indoors given suitable temperature and humidity.

Similar-Looking Bugs

• Each other: The clearest distinction is color and habitat preference — silverfish have a uniform silvery sheen and favor cooler, damp spots, while firebrats show a mottled gray-brown pattern and favor warmer areas.
• Earwigs: Larger, with a hardened front wing cover and a pair of pincers at the rear, features silverfish and firebrats entirely lack.
• Centipedes: Have many pairs of legs running the length of the body, versus the three pairs and tail filaments of silverfish/firebrats.

Quick ID Checklist

• Wingless, teardrop-shaped body tapering to a point
• Three long tail filaments of roughly equal length at the rear
• Silvery, metallic sheen (silverfish) versus mottled gray-brown speckling (firebrat)
• Long, thread-like antennae
• Fast, fish-like scurrying motion; active at night in dark, undisturbed spots