Pseudoscorpian

They’re called pseudoscorpions, belonging to the Pseudoscorpiones group of arachnids related to true scorpions and spiders. There are over 3,500 species of them worldwide [1], a whopping number when you consider that there are less than 300 species of mammals belonging to the Carnivora group [2]. North America in particular has approximately 420 species [3].

These creatures are incredibly diverse and exist worldwide [3,4,5]. Despite their high levels of diversity, they are rarely seen because they live under bark or stones, in leaf litter or mosses, and between the boards of buildings [5]. One particular species even lives in houses, hunting the small insects that would otherwise be house pests [4]. Should we be scared? After all, they do have pinching claws that produce venom [3].

Pseudoscorpions are not harmful to humans, and are in fact helpful! They do not damage property, eat tiny pests like mites, and their venom is not dangerous to us in such small amounts [4].

A pseudoscorpion, (false scorpion or book scorpion), is an arachnid belonging to the order Pseudoscorpionida, ( Pseudoscorpiones or Chelonethida)

Pseudoscorpions are generally beneficial to humans since they prey on clothes moth larvae, carpet beetle larvae, booklice, ants, mites, and small flies. They are small 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 (0.079 to 0.31 in) in length.The largest known species is Garypus titanius of Ascension Island at up to 12 mm.

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 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 palpal chelae (pedipalps or 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 poison 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. Another trait they share with their closest relatives, the spiders, is breathing through spiracles. However, they do not have book lungs as most spiders do.

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, but have their most dense and diverse populations in the tropics and subtropics. 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.[1] They may sometimes be found feeding on mites under the wing covers of certain beetles.

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 to 4,5 mm) can find their food like booklice and house dust mites. They enter homes by "riding along" with larger insects (known as phoresy), or are brought in with firewood.

Evolution
The oldest known fossil pseudoscorpion dates back 380 million years to the Devonian period.[8] It has all of the traits of a modern pseudoscorpion, indicating that the order evolved very early in the history of land animals.[9]

Historical references
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 labouring men who were drinking their porter, and borne away by an ingenious gentleman, who brought it to my lodging.