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Pseudoscorpions
Lasiochernes cretonatus
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Class: Arachnida Subclass: Dromopoda Order: Pseudoscorpionida Haeckel, 1866
Superfamilies
Chthonioidea Neobisioidea Garypoidea Cheiridioidea Feaelloidea Cheliferoidea

A pseudoscorpion, (also known as a false scorpion or book scorpion), is an arachnid belonging to the order Pseudoscorpionida, also known as Pseudoscorpiones or Chelonethida.

Contents 1 Physical characteristics 2 Geographical distribution 3 Evolution 4 Growth 5 Classification 6 Historical References 7 Footnotes 8 References 9 Further reading

Pseudoscorpions are small arthropods with a flat, pear-shaped body and pincers that resemble those of scorpions. They range from 2 to 8 mm (¹⁄₁₂ to ⅓ inch) in length.^[1]

The abdomen, known as the opisthosoma, is made up of twelve segments, each guarded by plate-like tergites above and sternites below. 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.^[2]

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. Most spiders have one pair of spiracles, and one of book lungs, but pseudoscorpions do not have book lungs.

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. Chelifer cancroides is the species most commonly found in homes. Other 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] There have even been reports of pseudoscorpions in domestic refrigerators.^{[citation needed]}

Until recently, the oldest pseudoscorpion fossils known were only 35 million years old, but some have now been found dating back over ten times as far, 380 million years, to the Devonian period, near the time when the first land-animal fossils appear. This is not surprising, however, since they are more closely related to spiders than real scorpions, and yet of course could not have branched off from spiders before the latter lost any pinchers their ancestors may have had. Since spiders gained their modern traits in the early Devonian, it's entirely possible that pseudoscorpions were actually around long before even the 380 million year old fossil.

This is also supported by the advanced nature of that oldest fossil; it has all of the traits of a modern pseudoscorpion, rather than being some kind of primitive transitional stage on its way to developing the features of this group. This is also not-unusual, as many of the very first land-arthropod fossils show advanced terrestrial features, giving the impression that they had been on land a long time, but simply not preserved as fossils up to that time. Part of this would be a result of geological activity; the suitable land areas have mostly become seabeds, or been subsumed entirely into the mantle by continental drift. Indeed, fossils from the very earliest land animals are extremely rare already, for those very reasons; going back slightly farther, mere chance could result in few or no fossils surviving.

During the elaborate mating dance, the male of some pseudoscorpion species pulls a female over a spermatophore previously laid upon a surface.^[3] In other species, the male also pushes the sperm into the female genitals using the forelegs.^[4]

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.^[1] Up to two dozen 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. Adult pseudoscorpions live 2 to 3 years. They are active in the warm months of the year, overwintering in silken cocoons when the weather grows cold.

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. They usually enter the home by "riding along" with larger insects (known as phoresy), or are brought in with firewood. They are often observed in bathrooms or laundry rooms, since they seek humidity. They may sometimes be found feeding on mites under the wing covers of certain beetles.

This list follows Joel Hallan's Biology Catalog^[5]. Recent genus and species numbers are given in parentheses.

• suborder Epiocheirata

• Chthonioidea

• Chthoniidae (31,605)
• †Dracochelidae — one fossil species (Devonian)
• Lechytiidae (1,22)
• Tridenchthoniidae (17,68)

• Feaelloidea

• Feaellidae (1,12)
• Pseudogarypidae (2,7)

• suborder Iocheirata

• infraorder Hemictenata

• Neobisioidea

• Bochicidae (10,38)
• Gymnobisiidae (4,11)
• Hyidae (3,9)
• Ideoroncidae (9,53)
• Neobisiidae (34,498)
• Parahyidae (1,1)
• Syarinidae (16,93)

• infraorder Panctenata

• group Elassommatina

• Cheliferoidea (224,1261)

• Atemnidae (19,172)
• Cheliferidae (59,292)
• Chernetidae (112,643)
• Withiidae (34,154)

• Sternophoroidea

• Sternophoridae (3,20)

• group Mestommatina

• Garypoidea

• Cheiridiidae (6,69)
• Garypidae (10,74)
• Geogarypidae (3,61)
• Larcidae (2,13)
• Pseudochiridiidae (2,12)

• Olpioidea

• Menthidae (4,8)
• Olpiidae (53,329)

• incertae sedis

• Megathis Stecker, 1875 (nomen dubium, 2 species)

Pseudoscorpions were first described by Aristotle, who probably found them among scrolls in a library where they would have been feeding on booklice. The next reference to them seems to be from 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." ^[6]

• Adams, George (1787): Essays on the Microscope. First edition. (London: Robert Hindmarsh)
• Proctor, Heather C. (1993): "Mating biology resolves trichotomy for cheliferoid pseudoscorpions (Pseudoscorpionida, Cheliferoidea)", Journal of Arachnology, Vol. 21, No. 2, (1993)
• Weygoldt, Peter (1966): Spermatophore Web Formation in a Pseudoscorpion. Science 153(3744): 1647-1649. doi:10.1126/science.153.3744.1647

• Chamberlin, Joseph C. (1931): The Arachnid Order Chelonethida. Stanford University Publications in Biological Science. 7(1): 1-284.
• Hoff, Clarence Clayton (1958): List of the Pseudoscorpions of North America North of Mexico. American Museum Novitates. 1875. PDF
• Beier, Max (1967): Pseudoscorpione vom kontinentalen Südost-Asien. Pacific Insects 9(2): 341-369. PDF
• Weygoldt, P. (1969): The Biology of Pseudoscorpions. Harvard University Press. Cambridge.
• Gabbutt, P.D. (1970): Validity of Life History Analyses of Pseudoscorpions. Journal of Natural History 4: 1-15.
• Muchmore, W.B. (1982): Pseudoscorpionida. In "Synopsis and Classification of Living Organisms." Vol. 2. Parker, S.P.
• Coddington, J.A., Larcher, S.F. & Cokendolpher, J.C. (1990): The Systematic Status of Arachnida, Exclusive of Acari, in North America North of Mexico. In "Systematics of the North American Insects and Arachnids: Status and Needs." National Biological Survey 3. Virginia Agricultural Experiment Station, Virginia Polytechnic Institute and State University.
• Harvey, Mark S. (1991): Catalogue of the Pseudoscorpionida. (edited by V . Mahnert). Manchester University Press, Manchester.