Pseudoscorpions

Scientific Classification

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Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Class:	Arachnida
Order:	Pseudoscorpiones

Pseudoscorpions, also referred to as false scorpions or book scorpions, are small arachnids with pincer-like appendages that resemble those of scorpions. These appendages, called pedipalps, function similarly to claws and are relatively large in comparison to their overall size. Generally reaching lengths of about 2–8 millimeters at maturity, pseudoscorpions often go undetected or are mistaken for ticks. However, unlike ticks, pseudoscorpions are not regarded as harmful to humans. In fact, they prey upon many small arthropods, aiding with pest control.

Characteristics and Morphology

Drawings of a pseudoscorpion and a scorpion from JH Comstock’s book. Note that the scale is different for each drawing.

Pseudoscorpions superficially resemble true scorpions; however, they lack an elongated metasoma (tail) and stinger at the end of their abdomens. Another crucial difference between the two types of arachnids is size. The common house pseudoscorpion, Chelifer cancroides, will usually only grow between 2.5–4.5 millimeters (0.10–0.18 in). Generally, pseudoscorpions have an average size of 3 mm (0.1 in), much smaller than the average scorpion.

Pseudoscorpions have flat, teardrop shaped bodies with rounded ends. The lower abdomen of a pseudoscorpion, also called the opisthosoma, is composed of 12 segmented sections. These segments are protected by plates made of chitin. Located on the sides of its cephalothorax (head and thorax), a pseudoscorpion may have 1–2 pairs of eyes, or no eyes at all.

As members of the class Arachnida, these small invertebrates have 8 legs. Pseudoscorpions move by shuffling their legs similar to crabs, allowing them to move as quickly backwards as they do forwards.

Most pseudoscorpions have yellowish to brown mahogany abdomens, though the pedipalps often contrast in color. These pedipalps grow over twice the length of their legs, measuring 7–9 mm across when fully extended. Pedipalps consist of an immobile hand and a finger. The finger is mobile and controlled by adductor muscles. A venom gland and venom duct are usually located in the finger. Additionally, pseudoscorpions possess a gland located within their chelicerae (jaws) for the purpose of producing silk. Pseudoscorpions use this silk to make cocoons for mating, molting, or enduring cold weather.

Female Gymnobisium sp. — size reference.

Reproduction and Life Cycle

Male pseudoscorpions produce packets of sperm called spermatophores, which they deposit on surfaces for females to take into their bodies. Males often deposit spermatophores even in the absence of a female. Males of certain pseudoscorpion species also perform mating dances to attract females after depositing a spermatophore, during which they grasp the female with their pedipalps and guide her to the spermatophore to fertilize her eggs. Males of other pseudoscorpion species may directly transfer spermatophores into the female with their pedipalps.

The eggs mature within a female's brood pouch, which is connected to the abdomen. In a single brood, 20–50 young can hatch. Young remain with the mother for a brief period after hatching, eventually dispersing. Most pseudoscorpion species have a lifespan of several years, during which they go through three molting stages before reaching maturity. Pseudoscorpions spend an average of 2 to 3 years as mature adults.

Diet and Feeding Behaviors

Pseudoscorpions are carnivorous, preying on a variety of small insects, mites, and larvae. Often targeting common house pests such as booklice, clothes moths, dust mites, and ants, pseudoscorpions are able to survive indoors for extended periods. Pseudoscorpions use the venom glands in their pedipalps to immobilize prey. They deposit semi-corrosive saliva on their prey to facilitate smooth external digestion before ingesting the liquid remains. This venom is a key mechanism in the feeding process of pseudoscorpions, yet it is harmless to humans and pets.

Distribution

There are over 3,300 identified species of pseudoscorpions. They are found worldwide, ranging from tropical regions, where the densest and most diverse populations are found, to temperate cold regions. Pseudoscorpions have also spread to island territories, with endemic species being recorded in the Canary Islands and the Maltese Islands. Across these regions, these organisms can be found in many different types of habitats and environments. They are typically located under tree bark, within tree hollows, beneath the soil surface, under leaf litter, under stones, in rock fractures, inside caves, on ocean shores, and within buildings and homes. Pseudoscorpions are commonly found indoors in rooms with many books, feeding on booklice and dust mites that often inhabit bookshelves, which led to the coining of the name 'book scorpions'.