Pseudoscorpions are arachnids belonging to the order pseudoscorpiones, also known as pseudoscorpionida or chelonethida. They are sometimes referred to as false scorpions or book scorpions.
They are small and therefore difficult to see in all of their particulars. To humans that can see them, they are creepy looking. Nevertheless, pseudoscorpions are generally beneficial to humans, since they prey on clothes moth larvae, carpet beetle larvae, booklice, ants, mites, and small flies.
Though they can migrate on their own, pseudoscorpions often travel by means of phoresy, a form of commensalism in which one organism uses another for the purpose of transport.
These tiny arachnids have a flat, pear-shaped body and pincers that resemble those of scorpions. They usually range from 2 to 8 millimeters or 0.08 to 0.31 in in length.
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, which is a derivative of glucose. The abdomen is short and rounded at the rear, rather than extending into a segmented tail and stinger like true scorpions. Their visual similarity to scorpions, aside from not having a stinger tail, accounts for them being called pseudoscorpions. 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.
Pseudoscorpions have 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. i.e., 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 and 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. Pseudoscorpions have sometimes been 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. 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 can find their food like booklice and house dust mites. They enter homes by “riding along” attached to insects.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. As with most other arachnid orders, the pseudoscorpions have changed very little since they first appeared, retaining almost all the features of their original form.
Aristotle described pseudoscorpions, apparently after coming across 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 to them was made in the 1780s, when George Adams wrote of “a lobster-insect, spied by some labouring men who were drinking their porter, and borne away by an ingenious gentleman, who brought it to my lodging.”
In San Bernardino County pseudoscorpions are known inhabitants of caves and caverns located in the Mojave Desert.
From Wikipedia.