Silverfish belong to a group of very primitive insects that were already around before insects developed wings more than 350 million years ago. They changed little over hundreds of millions of years but are now restricted to specific habitats, and some have found a living in our homes.
They are easily recognised by the presence of three tail filaments and the lack of wings. Some species have eyes, others do not; some are covered in dark scales, others in yellowish scales and others lack scales.
There are about 600 species described worldwide but this only scratches the surface. The group as a whole has been very poorly studied.
For example, only 50 native Australian species have been described but probably double this number lie undescribed in museum collections, and most field trips result in the collection of more undescribed than described species.
Where do they live?
Silverfish live mainly in three unusual habitats:
Dry leaf litter, the bark of trees, sheltered under rocks. Silverfish are generally found in the driest material. If there are cockroaches in the leaf litter, it is probably too humid for silverfish. They thrive in hot conditions and are quite common in desert regions.
Most of the family Lepismatidae, including those found in our homes, fall into this category, including one (Thermobia domestica) that prefers the hot dry conditions surrounding ovens in bakeries.
Subterranean. These eyeless species are found in soil or in caves and recently have been found in deep subterranean habitats (deeper than 20 m) during survey work in holes drilled for iron ore exploration. Most of the family Nicoletiidae are in this category.
Living with ants or termites. One subfamily of silverfish has moved into the nests of ants and termites where they apparently feed off refuse in the tunnels or perhaps even steal food from their hosts. They avoid being killed by their hosts through their speed and agility and also probably by the use of disguising pheromones.
Apart from a lack of wings, silverfish have two other primitive traits that distinguish them from their winged cousins.
First, they do not have a final defined adult stage but will continue to moult throughout their life. They reach sexual maturity after about the ninth moult but will continue to moult as many as 50 times during their life.
They mate after moulting so they can lay eggs. They can live for several years, moulting every one to three months depending on temperature and their diet. One advantage of such regular moults is that silverfish can regenerate lost appendages at the following moult, be it a leg or an antenna.
Second, silverfish do not have internal fertilisation. Instead the males and females engage in a mating dance. The male produces a group of threads with a bundle of sperm onto which the female, after suitable encouragement, then sits to take up the spermatozoa.
Silverfish in the home
Several species of silverfish have found suitable habitat within our houses, but one of them, the grey silverfish (Ctenolepisma longicaudata), predominates in most countries.
This is quite a large silvery-grey species often found in empty bathtubs, and this fact can be explained by two other peculiarities of the family.
First, silverfish do not need to drink. They can absorb moisture from the air – through their rectum! So they are attracted to the bathroom because there is more humidity in the air.
Second, silverfish have quite simple feet of just three claws. They lack the specialised structures that allow other insects to land on smooth surfaces such as glass, so if they fall into the bathtub they just can’t climb out again.
Silverfish used to be very common household pests in Australia in the first half of the 1900s but are declining with the introduction of different household furnishings and cleaning methods. They are basically omnivorous but in houses they prefer starchy materials supplemented with protein from dead insects, insect hairs, fungal spores and pollen, or whatever else they can find.
In our homes they will eat paper, especially old books and the cardboard covering of plasterboard. Wallpaper sizing was often attacked, but wallpaper has gone out of fashion and new glues and sizing compounds are perhaps not so palatable. Silverfish will eat rayon and cotton but not wool, natural silk or fur pelts, unless these have been stored with foodstuff spilled on them.
In general, silverfish are not that important as pests in Australian homes, unless you are a collector of rare books or old photographs. They are slow breeders, and a good spring clean, including seldom disturbed cupboards, will go a long way towards keeping their numbers under control.
If all else fails they are fairly susceptible to most household insecticides and can even be caught in sticky traps baited with rolled oats.