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.

Contents

Physical characteristics

Geographical distribution

Evolution

Growth

Classification

Historical References

References

Further reading

Physical characteristics

Pseudoscorpions are small arthropods with a flat, pear-shaped body and pincers that resemble those of scorpions. They range from 2 to 8 mm ( to ⅓ inch) in length."Entomological Notes: Pseudoscorpion Fact Sheet", Pennsylvania State University, Department of Entomology
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.^[1]
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.

Geographical distribution

There are more than 3,300 species of pseudoscorpions recorded, with more being discovered on a regular basis. They range worldwide, even in temperate to cold regions like Michigan 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. There have even been reports of pseudoscorpions in domestic refrigerators.

Evolution

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.

A book scorpion could get lost in a book.

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.

Growth

During the elaborate mating dance, the male of some pseudoscorpion species pulls a female over a spermatophore previously laid upon a surface.^[2] In other species, the male also pushes the sperm into the female genitals using the forelegs.^[3]
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. 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.

Classification

This classification is based on that of Muchmore (1982).^[1]

★ Superfamily Chthonioidea

★ Superfamily Neobisioidea

★ Superfamily Cheiridioidea

★ Superfamily Garypoidea

★ Superfamily Feaelloidea

★ Superfamily Cheliferoidea

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. The next reference to them is from the 1660s, when Robert Hooke wrote of: "A lobster-insect, found in a public house by some working men, and borne away by an ingenious gentleman, who brought it to my lodging."

References

1. Read, Wolf. Pseudoscorpions.
2. Weygoldt, Peter. "Spermatophore Web Formation in a Pseudoscorpion" Duke University Marine Laboratory, Beaufort, North Carolina.
3. Proctor, Heather C. "Mating biology resolves trichotomy for cheliferoid pseudoscorpions (Pseudoscorpionida, Cheliferoidea)", Journal of Arachnology, Vol. 21, No. 2, (1993)
4. Read, Wolf. Pseudoscorpions.

Further reading

★ Chamberlin, J. C. ''The Arachnid Order Chelonethida''. Stanford University Publications in Biological Science. 7(1): 1-284. 1931.

★ Coddington, J. A., Larcher, S. F., Cokendolpher, J. C. ''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. No 3. Ksoztatrab, M. and Schaefer, C. W. Editors. Virginia Agricultural Experiment Station, Virginia Plytechnic Instidute and State University. 1990.

★ Gabbutt, P. D., "Validity of Life History Analyses of Pseudoscorpions". ''Journal of Natural History.'' 4: 1-15. 1970.

★ Harvey, M. S., ''Catalogue of the Pseudoscorpionida.'' (edited by V . Mahnert). Manchester University Press, Manchester. 1991.

★ Hoff, C. C. "List of the Pseudoscorpions of North America North of Mexico". ''American Museum Novitates''. No 1875. 1958.

★ Muchmore, W. B. ''Pseudoscorpionida''. In "Synopsis and Classification of Living Organisms." Vol. 2. Parker, S. P. 1982.

★ Weygoldt, P. ''The Biology of Pseudoscorpions''. Harvard University Press. Cambridge. 1969.
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