A quick burst of silver goes darting across your bathroom floor and startles you, but then it's gone. In another moment, it's back, and it stops just long enough for you to get a good look at it. It's a small, carrot-shaped silver insect, commonly known as a silverfish, and its presence indicates high moisture in your home.
Any species in the insect order Thysanura are commonly referred to as silverfish, although both silverfish and firebrats belong to this order. Approximately 18 of the 370 or so species of Thysanura inhabit North America; four species are commonly encountered in homes. All species prefer relatively high humidity, although the indoor species have adapted to survive with a little less.
Four species of bristletails are the primary house-dwelling members of Thysanura: three silverfish and one firebrat. The three silverfish species are the common silverfish (Lepisma saccharina), gray silverfish (Ctenolepisma longicaudata) and four-lined silverfish (Ctenolepisma quadriseriata). The firebrat species (Thermobia domestica) is the primary house-invading firebrat. The silverfish species prefer temperatures slightly above room temperature while the firebrat prefers temperatures around 100 degrees Fahrenheit and is commonly found in boiler rooms and similar areas.
The three silverfish and even the firebrat all look very similar. The common and four-lined silverfish, along with the firebrat, are each 1/2-inch long. The gray silverfish is slightly bigger at 3/4-inch long. The common silverfish is uniformly silver with a definite sheen and no distinctive markings; the gray silverfish is uniformly gray with no markings; the four-lined silverfish is grayish-tan with four lines running down its back. The firebrat, on the other hand, is mottled gray with dark, mottled patches. The members of Thysanura are all carrot-shaped, or rounded and oblong with a distinct tapering from the head to the rear. They get their common name "bristletail" from the three appendages on the last abdominal segment. Antennae are long and curved back around the sides toward the rear.
Many other silverfish occur in North America, although they're more comfortable outdoors. Outdoor species live under leaf litter, bark, tree stumps or other organic material where the cover provides humidity and shade. One Thysanura family, Nicoletiidae, prefers to live underground in caves or mammal burrows.
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