The litter layer of the soil is home to a myriad of small creatures. Most are familiar to the gardener, who has dug a little and turned over stones and compost piles. For the most part, these are unremarkable; the group includes worms (annelids), millipedes, centipedes, and roly-polys (a crustacean) that scurry for safety when uncovered. One creature that may not be so familiar is the pseudoscorpion.
Pseudoscorpions (order Pseudoscorpiones) are arachnids, as are spiders, ticks, mites, and true scorpions. Arachnids have eight legs, two body sections, and no antennae. They are close relatives to the insects, which have six legs, three body sections, and one pair of antennae.
Pseudoscorpions are tiny (2-4 mm; there are 24.5 mm/inch), and in a superficial way, they resemble their larger cousin the true scorpion but lack its stinging tail. Pseudoscorpions are not only small but secretive, and for these reasons, and perhaps because of taxonomic complexity made more problematic by their small size, pseudoscorpions have not been extensively studied. Compared with other arachnids relatively little is known about them. Pseudoscorpions are also a small (though well defined) group. Roughly 100,000 arachnid species have been described, and of these, only about 3000 are pseudoscorpions. In the continental USA and Canada, there are about 350 pseudoscorpion species.
Pseudoscorpions are “cosmopolitan”. In addition to the soil litter layer, they are found in a wide variety of habitats that include soil well below the litter layer; compost piles; tree hollows; rotting stumps; under bark and stones; caves; marine intertidal zones; in the nests of insects, birds, and mammals; and sometimes in houses.
They are predaceous, and have one pair of relatively large pincer-like claws (called pedipalps), hence the resemblance to true scorpions. In the interior of the pedipalps is a poison gland that pseudoscorpions use for defense and to capture prey. As with many arachnids, they inject their prey with digestive juices to predigest them before eating them. They feed on small soil invertebrates (mites and collembola–springtails) and various flies, ants, beetle larvae, booklice, and an occasional caterpillar. In houses, they kill and eat the cloth moth larvae that are so destructive to woolens.
Some pseudoscorpion species perform elaborate courtship “dances”, but male and female never touch during mating. For reproduction to occur, the male produces a spermatophore, which is capsule or mass containing sperm. The female searches for this and rubs over it to absorb the sperm that will fertilize her eggs. Pseudoscorpions have a silk gland. The females may store her eggs in a silk sac to protect them. When the eggs hatch, she will tend the young for a short time. She will have only one brood in a year with fewer than 25 young. In some pseudoscorpion species, males are very rare, suggesting that parthenogenesis (asexual reproduction in which the embryo develops without fertilization) may occur.
Pseudoscorpions, like all arachnids, are wingless. This has not inhibited their dispersal, though; they have evolved an effective method of getting around–they are “phoretic”, meaning that they hitch rides on other animals. They use flies, beetles, and wasps as well as small mammals and birds. Pseudoscorpions clinging to insects have been found preserved in 25 million year old fossilized Baltic amber.
Pseudoscorpions are harmless to people, and if you have seen a pseudoscorpion, it may well have been in your bathroom sink. One species–Chelifer cancroides–is common and often found in houses. It is attracted to the moisture of bathrooms. Once on the slippery porcelain of a sink, it may fall in and be unable to get out.
In a square meter of the litter of a temperate forest like those in the eastern USA, you might find as many as 5000 pseudoscorpions. But their distribution is spotty. One sample of leaf litter might yield dozens of pseudoscorpions, while another might have none.
If you’re inclined, an easy way to isolate pseudoscorpions is by means of a Berlese funnel, named for its inventor, Italian entomologist Antonio Berlese (died 1927). A Berlese funnel is easy to construct and use, and there are many internet sites that describe how to make and use one; the following site is one of many but has good illustrations: http://pnwsteep.wsu.edu/edsteep/SoilInvertebrates/Berlese.doc; verified 17 June 2010. Sample your compost pile or the leaf litter in a familiar woodlot. If you are successful and want to try identifying your finds, see Photographic Key to the Pseudoscorpions of Canada and the Adjacent USA (http://www.biology.ualberta.ca/bsc/ejournal/b_10/b_10_main.html; verified 17 June 2010).
The general ecology of pseudoscorpions is another area that has not received much attention. Recent research, however, indicates that pseudoscorpions may be a good indicator of biological diversity. So if you find them in your garden or compost pile, consider that a good sign.