Pseudoscorpion

Pseudoscorpion

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Pseudoscorpion

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
Reproduction and growth
Geographical distribution
Evolution
Historical references
Classification
Footnotes
References
Further reading

Physical characteristics

Pseudoscorpions are small arthropods with a flat, pear-shaped body and pincers that resemble those of scorpions. They usually range from 2 to 8 mm ( to ? inch) in length.^[1] The largest known species is Garypus titanius of Ascension Island at up to 12 mm.^[2] ^[3]

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.^[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.

Reproduction and growth

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. 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.

Geographical distribution

A book scorpion (Chelifer cancroides) could get lost in a 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, 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

Until recently, the oldest pseudoscorpion fossils known were only 35 million years old, but in the late 1980s a fossil of Dracochela deprehendor was found dating back over ten times as far, 380 million years, to the Devonian period.^[6] The fossil has all of the traits of a modern pseudoscorpion, indicating that the order matured very early in the history of land animals.^[7]

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." ^[8]

Classification

This list follows Joel Hallan's Biology Catalog^[9]. 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)

Footnotes

References

(1787): Essays on the Microscope. First edition. (London: Robert Hindmarsh)
(1993): "Mating biology resolves trichotomy for cheliferoid pseudoscorpions (Pseudoscorpionida, Cheliferoidea)", Journal of Arachnology, Vol. 21, No. 2, (1993)
(1991): The first Paleozoic pseudoscorpions (Arachnida, Pseudoscorpionida). American Museum novitates 3009. Abstract - PDF
(1966): Spermatophore Web Formation in a Pseudoscorpion. Science 153(3744): 1647-1649.

(1931): The Arachnid Order Chelonethida. Stanford University Publications in Biological Science. 7(1): 1-284.
(1958): List of the Pseudoscorpions of North America North of Mexico. American Museum Novitates. 1875. PDF
(1967): Pseudoscorpione vom kontinentalen S�dost-Asien. Pacific Insects 9(2): 341-369. PDF
(1970): Validity of Life History Analyses of Pseudoscorpions. Journal of Natural History 4: 1-15.
(1982): Pseudoscorpionida. In "Synopsis and Classification of Living Organisms." Vol. 2. Parker, S.P.
(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.
(1991): Catalogue of the Pseudoscorpionida. (edited by V . Mahnert). Manchester University Press, Manchester.