Silverfish: identification, why they are in your home, and whether they matter

Card #140 is one of the oldest designs still walking around: silverfish belong to a group that has looked roughly like this for hundreds of millions of years, longer than there have been flying insects. It never grew wings, it never learned to bite, and it ended up in your bathroom because your bathroom is a cave with a library in it.

How to recognise one

• Shape: a teardrop or carrot, wide at the head and tapering to the tail, flattened top to bottom so it slips into cracks.
• Colour: silvery grey with a metallic sheen, from the tiny scales that cover it. Young ones are paler.
• Size: about 1 to 1.5 cm long as an adult, not counting the bristles.
• Ends: two long antennae at the front, three long bristles at the back. That three-tailed rear is the giveaway.
• Movement: fast, with a side-to-side wiggle that looks like a fish swimming. Hence the name. It cannot climb smooth vertical surfaces well, which is why you find it trapped in the bath in the morning.

The firebrat is the same shape, mottled grey-brown instead of silver, and prefers hot places such as around boilers and ovens. Same habits, same answer.

What it eats

Starch and sugar in almost any form: the glue in book bindings and wallpaper paste, paper, cardboard, the sizing in fabric, flour, oats, dead insects, dandruff. It can go months without eating. The damage looks like irregular holes and scraped surfaces on paper, yellow stains, and tiny black pepper-like droppings.

Why it is in your house

Two things, and it needs both:

1. Humidity. Silverfish dry out easily and look for rooms above roughly 75% relative humidity: bathrooms, kitchens, cellars, laundry rooms, a bedroom with a damp outside wall.
2. Food. Books, cardboard boxes in the loft or garage, stacked paper, wallpaper, a flour bag with a hole in it.

One silverfish in the bath is a wanderer. Several a week, or damage to books or wallpaper, means there is a humid spot with a food supply and probably a population in the wall void or under the floor behind it.

Is it dangerous?

No. It does not bite or sting, and it does not spread disease. People sometimes react to the scales and droppings the way they react to dust, but that is rare. The only thing a silverfish threatens is your paperwork.

What actually reduces them

• Dry the room. This is most of the answer. Ventilate after showers, fix the leak, run a dehumidifier. Below about 50% humidity they stop breeding and move on.
• Take away the food. Paper and dry goods in sealed plastic boxes rather than cardboard. Old cardboard in the garage is a silverfish hotel.
• Close the doors. Seal gaps around pipes, behind skirting boards and where the bath meets the wall. They live in the voids and come out at night.
• Vacuum the corners. Eggs are laid in cracks and crevices; a vacuum nozzle along the edges of the room removes them.
• Traps to measure, not to solve. A sticky trap or a jar with a strip of masking tape up the side and a spoon of flour in the bottom tells you whether numbers are falling. Sprays kill the ones that walk through them and do nothing about the damp that brought them.

Front desk note: if you have silverfish and a house centipede, you have a predator and its prey in the same building. The house centipede eats them. Most people would still rather have the silverfish.

Look-alikes

• Firebrat: same family, mottled brown, in hot spots.
• Earwigs: darker, with pincers at the back rather than bristles, and they come from the garden. Card #139.
• Woodlice: grey and segmented but much slower, and they curl up. Card #171.
• Springtails: much smaller and they jump. Card #141.

If you are not sure which one is in the bath, a photo in the identifier will tell you, and card #140 has the seasonal pattern from iNaturalist records and the safety note.