Invertebrates
Pseudoscorpion
Pseudoscorpiones · also called false scorpion, book scorpion
Sort through a box of old books or a bag of compost and you may spot what looks like a grain of rice with two crab claws, backing away from the light. It is a pseudoscorpion, a real arachnid, a genuine relative of scorpions, and completely harmless.
Overview
The pseudoscorpions are an arachnid order, Pseudoscorpiones, with over 3,000 species worldwide. Most are just 2 to 8 millimetres long, with a flattened body and two pincer-tipped arms like a scorpion but no tail and no sting. They hunt springtails, booklice, mites, and clothes-moth larvae in leaf litter, nests, caves, and houses.
Venom, silk, and milk
Venom is delivered through glands in the pincer fingers, not a tail. Silk glands open on the jaws, and the animal spins a small chamber for moulting, brooding, and overwintering. The female carries her eggs in a sac under her body and the young feed on a milk-like secretion.
Hitchhiking
Too small to travel far, a pseudoscorpion grips the leg of a harvestman, beetle, or fly and rides to a new place — a dispersal trick called phoresy. One common house species, the book scorpion, lives among old paper eating the booklice that damage it.
Status
Not Evaluated as a group and abundant worldwide, though many cave and island species have tiny ranges.
Pseudoscorpion: common questions
Is a pseudoscorpion dangerous to humans?
Not at all. A pseudoscorpion is far too small to break human skin with its pincers, its venom is produced in tiny amounts and tuned to subdue springtails and mites, and it has no sting because it has no tail. If one is on your hand it will usually freeze, then scuttle backwards, sometimes surprisingly fast, and drop off. There are no medically significant pseudoscorpions anywhere in the world. The animals are entirely beneficial around a home, since the prey they hunt includes clothes-moth larvae, carpet-beetle larvae, booklice, and dust mites.
Is it actually a scorpion, or related to one?
It is a true arachnid and a relative of scorpions, but it is not a scorpion and it sits in its own order, Pseudoscorpiones. It shares the pincer-tipped pedipalps and the general body layout with scorpions, and both belong to the wider arachnid group that also includes spiders, harvestmen, mites, and ticks. The obvious differences are the missing tail and sting, the much smaller size, the silk glands in the jaws, and the flattened, teardrop-shaped body. The resemblance is close enough that people who find one in a bathroom or a bookshelf often assume a baby scorpion has appeared.
Why would a pseudoscorpion be riding on a fly or a beetle?
It is hitching a lift. Pseudoscorpions are tiny and slow and cannot travel far under their own power, so to reach a new patch of leaf litter, a fresh nest, or a different tree they grab hold of a larger, more mobile animal and let it carry them. They most often cling to the legs of harvestmen and craneflies or to the bodies of beetles and larger flies, sometimes several pseudoscorpions on one carrier, and let go when the ride reaches somewhere suitable. This behaviour, called phoresy, is how the group has spread so widely, and it is occasionally seen when a pseudoscorpion turns up clamped to a housefly indoors.
Where am I most likely to see one?
In leaf litter and compost, under loose bark, in moss, and in the nests of birds and small mammals, which is where most species live and hunt. Indoors, the classic place is among old books, papers, and boxes in a damp or undisturbed room, and in stored grain, which is why the best-known house species is called the book scorpion. They are also found in bathrooms and around window frames. Because they are only a few millimetres long and move in short bursts, most people see them only when sorting through old paperwork or turning compost.