Pseudoscorpions

Pseudoscorpions belong to the phylum Arthropoda, class Arachnida, order Pseudoscorpionida, and two suborders (Epiocheirata and Iocheirata). There are more than 3,300 species within about 430 genera, with more described as new species regularly. They are cosmopolitan in distribution and occur in a variety of habitats from temperate to cold regions from Northern Ontario, Canada, and above the timberline in the Rocky Mountains of Wyoming in the United States to the karst system of the Jenolan Caves of New South Wales, Australia. However, pseudoscorpions are most dense with diverse populations in the tropics and subtropics, where they can be found on island territories such as the Canary Islands, where about twenty-five endemic species have been reported. There are also two endemic species on the Maltese Archipelago. Favorite microhabitats include soil, leaf and pine litter, and bark and tree hollows, as well as under stones, in caves, at the seashore in the intertidal zone, and within fractured rocks. Three types of pseudoscorpions are found in caves in Arkansas.

The oldest known fossil pseudoscorpion dates back to the Devonian period (380 million years ago). It possesses all of the morphological traits of a modern specimen, demonstrating that the order evolved very early in the geological history of terrestrial animals. As with most other orders of arachnids, pseudoscorpions have changed very little since they first appeared, retaining nearly all the morphologies of their original ancestor.

Aristotle was apparently the first to describe a pseudoscorpion; he probably found them among scrolls in a library where they would have been feeding on booklice. In 1665, microscopist Robert Hooke referred to a “land-crab” in his famous tome Micrographia. More recently, William B. Muchmore (1920‒2017) provided a great deal of information on pseudoscorpions. He spent his entire career at the University of Rochester in New York and was a world authority on pseudoscorpions (particularly cave forms), discovering and naming more than 278 new species, and had six species and a new genus named in his honor by the time of his death.

Pseudoscorpions are not known to have any economic or medical importance but can be generally beneficial to humans since they prey on a variety of detrimental arthropods, including ants, house dust mites, booklice, clothes moth larvae, carpet beetle larvae, and small flies. They possess a venom gland and duct, and the venom is used to capture and immobilize prey. During digestion, pseudoscorpions emit a mildly corrosive fluid over their prey, then they ingest the liquefied (partially digested) remains.

They are very small and inoffensive, and are rarely observed due to their small size, despite being common in many environments. They typically range from two to eight mm (0.08 to 0.31 in.) in length, averaging about three mm (0.1 in.) in length. The largest known species is Garypus titanius at up to 12 mm (0.47 in.) in length. It is endemic to Boatswain Bird Island, off the coast of Ascension Island, in the British overseas territory of Saint Helena, Ascension, and Tristan da Cunha.

These small arachnids possess a flat, pear-shaped body and pincers that resemble those of true scorpions. The opisthosoma (abdomen) is composed of up of twelve segments, each protected by plates made of chitin (tergites above, sternites below). The abdomen is short and rounded at the rear, rather than extending into a segmented tail and stinger like true scorpions. They have eight legs with five to seven segments, and the number of fused segments is used to taxonomically distinguish between families and genera. They also have two very long pedipalps with palpal chelae (pincers) that strongly resemble the pincers found on a true scorpion. They may have two, four, or no eyes. The pedipalps generally consist of an immobile “hand” and “finger,” with a separate movable finger controlled by an adductor skeletal muscle. To make disk-shaped cocoons for mating, molting, or waiting out cold weather, pseudoscorpions spin silk from a gland in their jaws. However, unlike true scorpions, they do not have book lungs but instead they breathe exclusively through spiracles. Their body color can be yellowish tan to dark brown, with the paired claws often with a contrasting color.

During reproduction, some species take part in an elaborate mating dance, where the male pulls a female over a spermatophore previously laid upon a surface. In other species, the male also pushes the sperm into the female genitalia using his forelegs. The female carries the fertilized eggs in a brood pouch attached to her abdomen, and once hatched, the young ride on the mother for a short period. An average of thirty young hatch in a single brood, and there may be more than one brood per year. Before reaching adulthood and over the course of several years, the young go through three molts. During this vulnerable period, most molt in a small, silken igloo that protects them from predators. After reaching adulthood, pseudoscorpions have a life span of about two to three years.

Pseudoscorpions are most active in the warmer months of the year, overwintering in silken cocoons when the weather begins to get cooler. Smaller species live in debris and humus. Some species are arboreal, while others are phagophiles, eating parasites in an example of cleaning symbiosis. Pseudoscorpions often carry out a form of commensalism (phoresy), in which one organism uses another for the purpose of transport. Examples include those that feed on mites under the wing covers of certain beetles.

The most common pseudoscorpion found in residences is Chelifer cancroides, a species often observed in rooms housing dusty books. Here they feed on booklice and house dust mites. They enter homes by phoresy attached to insects or they are brought in with firewood.

In Arkansas, three troglobitic pseudoscorpions are known from caves. Apochthonius diabolus was described from a single male collected in 1958 from Devil’s Den Cave (Washington County). Another, A. titanicus, was described from several specimens collected near the base of “The Titans” in Blanchard Springs Caverns (Stone County). The third is the fairly widespread Hesperochernes occidentalis known from caves in ten counties across the state. In addition, specimens of potentially new troglobitic Apochthonius have been collected from caves in several Arkansas counties but remain undescribed.

For additional information:
Allen, Robert T. “Additions to the Known Endemic Flora and Fauna of Arkansas.” Proceedings of the Arkansas Academy of Science 42 (1988): 18–21. Online at http://scholarworks.uark.edu/jaas/vol42/iss1/8/ (accessed September 1, 2021).

Cokendolpher, James C. “New Troglobitic Tyrannochthonius from Fort Hood, Texas (Pseudoscorpionida: Chthoniidae).” Texas Memorial Museum Speleological Monographs 7 (2009): 67–78.

Hoff, C. C., and J. E. Bolsterli. “Pseudoscorpions of the Mississippi River Drainage Basin Area.” Transactions of the American Microscopical Society 75 (1956): 155‒179.

McAllister, Chris T., Henry W. Robison, and Michael E. Slay. “The Arkansas Endemic Fauna: An Update with Additions, Deletions, a Synthesis of New Distributional Records, and Changes in Nomenclature.” Texas Journal of Science 61 (2009): 203–218.

McDaniel, V. Rick., Kenneth N. Paige, and C. Renn Tumlison. “Cave Fauna of Arkansas: Additional Invertebrate and Vertebrate Records.” Journal of the Arkansas Academy of Science: Vol. 33 (1979): 84‒85. Online at https://scholarworks.uark.edu/jaas/vol33/iss1/35/ (accessed September 1, 2021).

Muchmore, William B. “Additional Pseudoscorpions, Mostly from Caves, in Mexico and Texas (Arachnida: Pseudoscorpionida).” Texas Memorial Museum, Speleological Monographs 1 (1986) 17–30.

———. “Cavernicolous Pseudoscorpions from Texas and New Mexico (Arachnida: Pseudoscorpionida)”. Texas Memorial Museum, Speleological Monographs 3 (1992): 127–153.

———. “Clarification of the Genera Hesperochernes and Dinocheirus (Pseudoscorpionida, Chernetidae).” Journal of Arachnology 2 (1974): 25‒36.

———. “New Cave Pseudoscorpions of the Genus Apochthonius (Arachnida: Chelonethida).” The Ohio Journal of Science 67 (1967): 89–95.

———. “Review of the Genus Tartarocreagris, With Descriptions of New Species (Pseudoscorpionida: Neobisiidae).” Texas Memorial Museum, Speleological Monographs 5 (2001): 57–72.

Proctor, Heather C. “Mating Biology Resolves Trichotomy for Cheliferoid Pseudoscorpions (Pseudoscorpionida, Cheliferoidea).” Journal of Arachnology 21 (1993): 156–158.

Reddell, James R., and James C. Cokendolpher. “Catalogue, Bibliography, and Generic Revision of the Order Schizomida (Arachnida).” Texas Memorial Museum Speleological Monographs 4 (1965): 1–170.

Robison, Henry W., and Robert T. Allen. Only in Arkansas: A Study of the Endemic Plants and Animals of the State. Fayetteville: University of Arkansas Press, 1995.

Robison, Henry W., and Chris T. McAllister. “The Arkansas Endemic Flora and Fauna: An Update with 13 Additional Species.” Journal of the Arkansas Academy of Science 69 (2015): 78–82. Online at http://scholarworks.uark.edu/jaas/vol69/iss1/16/ (accessed September 1, 2021).

Robison, Henry, Chris McAllister, Christopher Carlton, and Robert Tucker. “The Arkansas Endemic Biota: With Additions and Deletions.” Journal of the Arkansas Academy of Science 62 (2008): 84–96. Online at http://scholarworks.uark.edu/jaas/vol62/iss1/14/ (accessed September 1, 2021).

Robison, Henry W., and Kenneth L. Smith. “The Endemic Flora and Fauna of Arkansas.” Proceedings of the Arkansas Academy of Science 36 (1982): 52–57. Online at http://scholarworks.uark.edu/jaas/vol36/iss1/17/ (accessed September 1, 2021).

Schawaller Wolfgang, William A. Shear, and Patricia M. Bonamo. “The First Paleozoic Pseudoscorpions (Arachnida, Pseudoscorpionida).” American Museum Novitates 3009 (1991):

Shear, William A., Wolfgang Schawaller, and Patricia M. Bonamo. “Record of Palaeozoic Pseudoscorpions.” Nature 342 (1989): 527–529.

Weygoldt, Peter. “Spermatophore Web Formation in a Pseudoscorpion.” Science 153 (1966): 1647–1649.

Chris T. McAllister
Eastern Oklahoma State College

Henry W. Robison
Sherwood, Arkansas