Pseudoscorpions: Cryptic Predators of the Soil

by Jan A. Neethling

Pre-Devonian in origin, the Pseudoscorpiones are one of the oldest extant lineages and over the past 392 Ma have diverged into more than 3400 known species in 26 families.

Female Ellingsenius sculpturatus (Lewis, 1903), a species that occurs mutually within beehives.
(Photo: J.A. Neethling)

Most are less than five millimetres in length, though they range from less than one millimetre in some Chthoniidae to just over ten millimetres in females of Garypus titanius Beier, 1961. They superficially resemble true scorpions, but lack the elongated metasoma (tail) and telson (sting). They do, however, share the six-segmented pedipalps, with the tibia and tarsus modified into a chela with a movable finger. Their unusual appearance and secretive nature has been recorded as far back as Aristotle, though even Linnaeus failed to clearly define the group, instead grouping the first two described species (Acarus crancroides Linnaeus, 1758 and A. scorpioides Linnaeus, 1758) together with mites and harvestmen.

Pseudoscorpions occur in almost every part of the world, though most species are found within the tropics and subtropics. Their unique morphology makes these secretive generalists a very important predatory component of many terrestrial habitats, where they can readily be found among humid soil, leaf litter, compost piles, under stones, bark and logs, as well as harsh environments such as intertidal zones. Although most species are ground dwellers, many arboreal representatives are also present.  On the other hand, Chelifer cancroides (Linnaeus 1758), a cosmopolitan species, can often be found within manmade structures. Some species furthermore occupy very specific niches, such as caves.

Female Horus obscurus (Tullgren, 1907) within her nest constructed of soil particles and silk.
(Photo: J.A. Neethling)

Due to their solitary lifestyle, pseudoscorpions generally avoid interacting socially, as there is often inter- and intraspecific aggressive behaviour. There are, however, a few social species that go beyond merely aggregating in large numbers. Paratemnoides elongatus (Banks, 1895), for example, exhibits one of the highest levels of social organisation currently known among pseudoscorpions, with communal spinning by immatures as well as communal moulting. Adults and tritonymphs also engage in cooperative predation, which enables the colony to capture prey many times their own size. Another observed behaviour is of particular interest, namely the occurrence of matriphagy in P. nidificator (Balzan, 1888). Here the females, which are responsible for all brood care, would allow nymphs to attack and feed on them during periods of food deprivation.  They hypothesized that this behaviour could lead to a decrease in cannibalism among nymphs, as well as contribute towards the evolution of social behaviour in the species.

One of the better documented habits of pseudoscorpions is their phoretic association with many flying insects, whereby

Chela of Ellingsenius sculpturatus (Lewis, 1903), demonstrating the species’ morphological adaptations to prevent mechanical damage during phoresis with bees.
(Photo: J.A. Neethling)

the pseudoscorpion attaches itself to a host insect by grasping it with its chela, and travels along with its host to a new location where it would then detach. Since this is only an occasional behaviour most pseudoscorpion species that engage in phoresis do not have chelae specifically modified to prevent damage to the host when grasping, often leaving scars on the host’s cuticle where the pseudoscorpion was attached.  But what if you depend on your host for survival? Would it not then be beneficial to reduce any potential damage? Examples of this can be seen with the bee-associated pseudoscorpions of the Ellengsenius genus, where they survive by feeding on parasites within beehives. Thus if a bee colony moves the pseudoscorpions have to tag along with them without causing damage to their flying partner, since an injured worker bee may not be able to keep up with the rest of the hive. Thus, to prevent injury to their bee hosts the associated pseudoscorpions have chelae modified with both a ventral cured movable finger and a groove on the anterior of the fixed chelal finger. Together these modifications allow the pseudoscorpion to effectively grasp around the bee’s leg and not directly onto the leg, possibly causing damage by crushing the exoskeleton of the leg.

Female Gymnobisium species on my finger for size reference.
(Photo: J.A. Neethling)

With 152 described species in 17 families and with 70% of these species being endemic to the country, South Africa has the eight largest diversity on the planet. Not bad considering that more than 80% of these species were described by one man, Dr. Max Beier between 1932 and 1966. Also, that is roughly the sum of all our knowledge. We sorely lack detailed biological and ecological data on most of these species, as most articles concerning the South African fauna are either brief species descriptions or species lists. Combining that with the fact that nearly all of the species were described by foreign scientist, resulting in only a few of the types being deposited with South African institutions, and we are left with a very little data to work on. To remedy this situation, a long-term project has been initiated with the aim of revising our entire South African pseudoscorpion fauna.  The study hopes to produce a series of papers that aim to be the most comprehensive summary of the more than 100 years of research done on South African pseudoscorpions, and in so doing bring the taxonomy of the group into the modern era.

  

REFERENCES

Aguiar, N.O. & Bührnheim, P.F. 1998. Phoretic pseudoscorpions associated with flying insects in Brazilian Amazônia. Journal of Arachnology 26: 452-459.

Beier, M. 1958. Pseudoscorpionidea (false scorpions) of Natal and Zululand. Annals of the Natal Museum 14: 155-187.

Harvey, M.S. 1992. The phylogeny and classification of the Pseudoscorpionida (Chelicerata: Arachnida). Invertebrate Taxonomy 6: 1373-1435.

Legg, G. 2008. Taxonomy and the dangers of sex with special reference to pseudoscorpions. Monographs 12: 247-257.

Murienne, J., Harvey, M.S. & Giribet, G. 2008. First molecular phylogeny of the major clades of Pseudoscorpiones (Arthropoda: Chelicerata). Molecular Phylogenetics and Evolution 49: 170-184.

Tizo-pedroso, E. & Del-Claro, K. 2005. Matriphagy in the neotropical pseudoscorpion Paratemnoides nidificator (Balzan 1888) (Atemnidae). Journal of Arachnology 33: 873-877.

Zeh, J.A. & Zeh, D.W. 1990. Cooperative foraging for large prey by Paratemnus elongatus (Pseudoscorpionida: Atemnidae). Journal of Arachnology 18: 307-311.

 

CONTACT

Contact: J.A. Neethling at ja.neethling@nasmus.co.za