Species: Silverfish (Lepisma saccharina); Firebrat (Thermobia domestica)

What to look for

  1. Outline. Maryland describes a distinctly fish-like shape: flat, tapered at both ends and covered by overlapping scales.
  2. Length. Maryland gives about 1/2 inch (12.5 mm) or less and Penn State gives 12 mm head to tip of abdomen. UC IPM says the common silverfish is about 1/2 inch long although it can grow as long as 3/4 inch (19 mm).
  3. The three tails. Penn State says the common name bristletail comes from the three tail-like appendages protruding from the last abdominal segment, and that both silverfish and firebrats carry them.
  4. Antennae. A silverfish has antennae as long as its body, while a firebrat's antennae are longer than its body.
  5. Color separates the two species. Silverfish carry silvery metallic scales while firebrats carry tufts of brown scales giving a mottled look. UC IPM puts it as silver or pearly gray against mottled gray or brown.
  6. Which room it was in. Maryland says the two are best told apart by habitat: silverfish in damp, cool places such as basements, laundry rooms and under sinks, firebrats in very warm, moist places near ovens, heating units, fireplaces and hot water pipes.

Where and when

Maryland extension puts silverfish in damp, cool parts of the house and notes both species travel at night through the house to the spots that suit them. Penn State says silverfish prefer cooler temperatures and are usually found in basements, while firebrats prefer temperatures over 90 degrees Fahrenheit and turn up near furnaces, hot water pipes, attics and roofing shingles. Maryland gives a maximum lifespan of 2 to 3 years.

Does it matter?

Penn State says silverfish and firebrats are mostly a nuisance but can damage cereals, books, papers and wallpaper with their feeding and excrement, leaving irregular holes, and that they do not feed on wool or other animal products. UC IPM notes their jaws are very weak, so they scrape paper rather than bite it. Penn State's approach is dehumidifying, sealing harborages and airtight food storage.

Related guides

  • Carpet beetle: Carpet beetles are small oval beetles, 1/16 to 1/8 inch long, that Kentucky extension describes as black or mottled with white, brown, yellow and orange.
  • Camel cricket: A camel cricket is the humpbacked, wingless, long-legged cricket that jumps out of a basement corner.
  • House centipede: The fast, many-legged thing in a bathroom is almost always a house centipede.

Sources