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.
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