Pseudoscorpions are generally beneficial to humans since they prey on clothes moth larvae, carpet beetle larvae, booklice, ants, mites, and small flies. They are tiny and inoffensive, and are rarely seen due to their size.
Pseudoscorpions are small arachnids with a flat, pear-shaped body and pincers that resemble those of scorpions. They usually range from 2 to 8 millimetres (0.08 to 0.31 in) in length. The largest known species is Garypus titanius of Ascension Island at up to 12 mm (0.5 in).
The abdomen, known as the opisthosoma, is made up of twelve segments, each protected by plates (called tergites above and sternites below) made of chitin. The abdomen is short and rounded at the rear, rather than extending into a segmented tail and stinger like true scorpions (the fact that they look exactly like scorpions, aside from not having a stinger tail, is the source of the name "Pseudoscorpion"). The color of the body can be yellowish-tan to dark-brown, with the paired claws often a contrasting color. They may have two, four or no eyes.
A pseudoscorpion has eight legs with five to seven segments; the number of fused segments is used to distinguish families and genera. They have two very long pedipalps with palpal chelae (pincers) which strongly resemble the pincers found on a scorpion.
The pedipalps generally consist of an immobile "hand" and "finger", with a separate movable finger controlled by an adductor muscle. A venom gland and duct are usually located in the mobile finger; the venom is used to capture and immobilize the pseudoscorpion's prey. During digestion, pseudoscorpions pour a mildly corrosive fluid over the prey, then ingest the liquefied remains.
Pseudoscorpions spin silk from a gland in their jaws to make disk-shaped cocoons for mating, molting, or waiting out cold weather. However, they do not have book lungs as most of their closest relatives, the spiders, do. They breathe through spiracles, a trait they share with the insects.
Some species have an elaborate mating dance, where the male pulls a female over a spermatophore previously laid upon a surface. In other species, the male also pushes the sperm into the female genitals using the forelegs. The female carries the fertilized eggs in a brood pouch attached to her abdomen, and the young ride on the mother for a short time after they hatch. Between 20 - 40 young are hatched in a single brood; there may be more than one brood per year. The young go through three molts over the course of several years before reaching adulthood. Many species molt in a small, silken igloo that protects them from enemies during this vulnerable period. After reaching adulthood, pseudoscorpions live two to three years. They are active in the warm months of the year, overwintering in silken cocoons when the weather grows cold. Smaller species live in debris and humus. Some species are arboreal, while others are phagophiles, eating parasites in an example of cleaning symbiosis. Some species are phoretic, others may sometimes be found feeding on mites under the wing covers of certain beetles.
There are more than 3,300 species of pseudoscorpions recorded in more than 430 genera, with more being discovered on a regular basis. They range worldwide, even in temperate to cold regions like Northern Ontario and above timberline in Wyoming's Rocky Mountains in the United States and the Jenolan Caves of Australia, but have their most dense and diverse populations in the tropics and subtropics, where they spread even to island territories like the Canary Islands, where around 25 endemic species have been found. Species have been found under tree bark, in leaf and pine litter, in soil, in tree hollows, under stones, in caves, at the seashore in the intertidal zone, and within fractured rocks.
Chelifer cancroides is the species most commonly found in homes, where they are often observed in rooms with dusty books. There the tiny animals (2.5–4.5 mm or 0.10–0.18 in) can find their food like booklice and house dust mites. They enter homes by "riding along" attached to insects (known as phoresy), the insects employed are necessarily larger than the pseudoscorpion, or they are brought in with firewood.
The oldest known fossil pseudoscorpion dates back 380 million years to the Devonian period. It has all of the traits of a modern pseudoscorpion, indicating that the order evolved very early in the history of land animals.
Pseudoscorpions were first described by Aristotle, who probably found them among scrolls in a library where they would have been feeding on booklice. Robert Hooke referred to a "Land-Crab" in his 1665 work Micrographia. Another reference in the 1780s, when George Adams wrote of: "A lobster-insect, spied by some laboring men who were drinking their porter, and borne away by an ingenious gentleman, who brought it to my lodging."
The numbers of recent genera and species are given in parentheses.
- suborder Epiocheirata
- Chthoniidae (31, 605)
- †Dracochelidae – one fossil species (Devonian)
- Lechytiidae (1, 22)
- Tridenchthoniidae (17, 68)
- Feaellidae (1, 12)
- Pseudogarypidae (2, 7)
- suborder Iocheirata
- infraorder Hemictenata
- Bochicidae 7 genera unknown number of species
- Gymnobisiidae (4, 11)
- Hyidae (3, 9)
- Ideoroncidae (9, 53)
- Neobisiidae (34, 498)
- Parahyidae (1, 1)
- Syarinidae (16, 93)
- Vachoniidae 2 genera unknown number of species
- infraorder Panctenata
- group Elassommatina
- Atemnidae (19, 172)
- Cheliferidae (59, 292)
- Chernetidae (112, 643)
- Withiidae (34, 154)
- Cheiridiidae (6, 69)
- Sternophoridae (3, 20)
- group Mestommatina
- Garypidae (10, 74)
- Geogarypidae (3, 61)
- Larcidae (2, 13)
- Pseudochiriididae (2, 12)
- Menthidae (4, 8)
- Olpiidae (53, 329)
- group Elassommatina
- infraorder Hemictenata
- incertae sedis
- Megathis Stecker, 1875 (nomen dubium, 2 species)