Pseudoscorpion

Pseudoscorpions Fossil range: 380–0 Ma PreЄ Є O S D C P T J K Pg N Devonian to Recent
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.

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.

Physical characteristics

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.^[1] The largest known species is Garypus titanius of Ascension Island at up to 12 mm.^{[2] [3]}

Pseudoscorpion hitchhiking on a diptere

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.^[3]

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.

Behavior

Some species have an elaborate mating dance, where the male pulls a female over a spermatophore previously laid upon a surface.^[4] In other species, the male also pushes the sperm into the female genitals using the forelegs.^[5]

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. Many species molt in a small, silken igloo that protects them from enemies during this vulnerable period.^[6] 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, i.e., live on trees. Some others are phagophiles. Some species are phoretic.^[7] They may sometimes be found feeding on mites under the wing covers of certain beetles.

Geographical distribution

A book scorpion (Chelifer cancroides) on top of an open book

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. 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]

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." ^[10]

Classification

A complete online catalogue of pseudoscorpions has been published by Mark Harvey (Perth) as the Pseudoscorpions of the World [1].

This list below follows Joel Hallan's Biology Catalog^[11]. 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)

References

• 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)
• Schawaller, Wolfgang; Shear, William A. & Bonamo, Patricia M. (1991): The first Paleozoic pseudoscorpions (Arachnida, Pseudoscorpionida). American Museum novitates 3009. Abstract - PDF
• Weygoldt, Peter (1966): Spermatophore Web Formation in a Pseudoscorpion. Science 153(3744): 1647–1649. doi:10.1126/science.153.3744.1647

Further reading

• 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. ISBN 978-0674074255 (Full text e-book). http://www.calameo.com/viewer.swf?bkcode=0000094842f94b0bb9237&langid=en. 
• 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.