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Submit an article to Indago - a peer reviewed journal
Submit an article to Indago - a peer reviewed journal
Submit an article to Indago - a peer reviewed journal

Travelling through Brazil you find yourself in the Cerrado, a vast tropical savannah just southeast of the Amazon rainforest. Stopping under one of the trees, you sit down for a quick rest and to take in the natural beauty around you. Your eye then caches the movement of a fly landing on the rough bark of the tree and, ever ready as you are, you reach for your camera and inch closer to try and take a good macro photo. While getting the insect into focus you notice the fly begins to struggle, though at first you fail to see why. It is then that you notice a myriad of small, scorpion-like pincers emerging from the crevices in the bark around the fly, grasping at it and keeping it from flying away. Then you see the owners of the pincers emerge, one or two at first then more and more as they grow bolder. Before you are tiny creatures that look like a scorpion without its tail, furiously working together trying to subdue the fly before dragging it under the bark where they emerged from. What you have just witnessed is an extremely rare event… cooperative hunting in pseudoscorpions.

Paratemnoides nidificator adults and nymphs interacting in and around a cooperatively constructed silken brood chamber. © José Roberto Peruca, 2015 (CC-BY-2.0)

Pseudoscorpions (Class: Arachnida, Order: Pseudoscorpiones) are a group of arachnids originating in the pre-Devonian, making them one of the oldest extant lineages. Since then, thus over the last 392 Ma, the group has diverged into 4197 currently known species. They range in size from less than one millimetre in some Chthoniidae to just over ten millimetres in females of Garypus titanius Beier, 1961, though most are 2–3 mm long. False scorpions superficially resemble true scorpions, but lack the elongated tail and sting. They do, however, share the six-segmented pedipalps modified into grasping pincers. Members of the suborder Iocheirata also possess venom teeth on either one or both of the grasping claws’ fingers, making the Pseudoscorpiones one of only three arachnid orders that possesses venom, the other two being the true spiders (Araneae) and scorpions (Scorpiones). Pseudoscorpions are distributed almost worldwide, though most species occur in the tropics and subtropics. Secretive generalists, they comprise a very important predatory component of many terrestrial habitats, where they can readily be found in humid soil, leaf litter, compost piles, under stones, bark and logs, as well as in harsh environments such as intertidal zones.

The vast majority of pseudoscorpion species are solitary in nature, with intraspecific conflict arising whenever two individuals meet. That said, all pseudoscorpion species present some degree of parental care. Their solitary nature is present even in their mating habits, where indirect sperm transfer via a spermatophore is used. The spermatophore of a pseudoscorpion is very simple, consisting of a stalk bearing a globular sperm package on its top. The spermatophore is deposited by the male onto a suitable surface and then sperm transfer occurs via one of two means. In more solitary species, the male simply deposits the spermatophore and the female is then tasked to locate it via chemical signals. Males of some species construct silken trails that lead to the spermatophore, aiding the female in locating it.

Members of more social species, on the other hand, perform elaborate mating dances that can last up to three hours, with the male finally positioning the female over his spermatophore, thus ensuring the transfer of his genes.

For many species these mating interactions are the only contact they have with each other. The species with greater social interaction may construct silken moulting or brood chambers together under the same stone. The first are used by individuals when they shed their exoskeleton during moulting, while the latter are used by females to care for the eggs she is carrying. In both instances, the individuals eventually disperse again.

Social behaviour is scarce among arachnids in general and, until recently, permanent-sociality has been only known in a few spider species. That was until scientists began observing Cerrado pseudoscorpions in the genus Paratemnoides where, for the first time, they saw the presence of non-territorial permanent sociality in pseudoscorpions. While a number of temporary social interactions were known in pseudoscorpion species, such as the facultative communal construction brood chambers by females of Neobisiuim muscorum, or the facultative aggregation of multiple individuals of Apocheiridium ferum in silk chambers, Paratemnoides nidificator presented a myriad of unique social behaviours previously thought to be absent from pseudoscorpions.

Paratemnoides nidificator pseudoscorpions (Family: Atemnidae) are found living under the bark of a variety of trees in the Cerrado savannah. Unlike other less social species though, members of Paratemnoides nidificator live communally in permanent colonies, ranging from only a few to over 150 individuals. The colonies are mostly female dominated, with a ratio of 3:2 females to males, and contain both adults and nymphs. Members cooperatively build nests with some females even utilizing the moulting chambers built by nymphs as brooding chambers. Maternal care is also more complex in this species, with females caring for their brood sacs and then even feeding and grooming the emerging nymphs. These nymphs will naturally leave the brood chamber as they mature, but it can happen that the female produces another brood sac while the previous nymphs still occupy the silk chamber. The female will then nudge the nymphs out of the chamber with her pincers before sealing herself in. The evicted nymphs will then often cooperate and construct a new moulting chamber for the group.

Hunting is done by both adult males and females. While small prey are often captured and consumed by a single individual, larger prey are hunted cooperatively and fed upon by both adults and nymphs. This cooperative hunting strategy allows these pseudoscorpions to take on prey up to four times the size of any one individual. After the adults subdue the prey, it is dragged under the bark to the colony where nymphs then join in feeding. After feeding the remains of the prey item are discarded out of the colony.

Paratemnoides nidificator females not only serve as guards at the entrances to protect the colony from dangers, but in times of famine some females will enact a strategy call matriphagy, sacrificing themselves to ensure the survival of the nymphs and prevent any cannibalism that may occur. During matriphagy the female will exit her silk chamber, raise her pincers and wait motionless for nymphs to start feeding on her. The nymphs will attach themselves to the female’s joints, and the female will remain motionless while the nymphs devour her. Eventually her remains will be discarded from the nest like any other prey item.

Being tiny, dispersion is particularly difficult for most pseudoscorpion species. While some species utilize phoresis to disperse to new habitats, Paratemnoides nidificator colonies also use fission. Here, larger colonies will split, with a number of adults moving to another unoccupied piece of bark to start a new colony. Phoresy is still used, with larger colonies engaging in more phoretic behaviour. Up to seven individuals may attach themselves to a single vector, usually on the legs or antennae of larger insects, and then ride along to a new place. When the attached pseudoscorpions arrive at a location they deem suitable, the vector is often killed and consumed as the first prey item of the new colony.

Due to the cryptic lifestyles of pseudoscorpions and a small community of scientists specializing in their study, very little is known about the total number of pseudoscorpion species, their ecology and their social behaviour. It is no surprise then that it took until the early 2000’s to discover these unique social behaviours in pseudoscorpions. It also highlights the importance of the continued study of these fascinating arachnids and other arthropods.

References

Brach, V. 1978. Social behaviour in the pseudoscorpion Paratemnus elongatus (Banks) (Pseudoscorpionida: Atemnidae). Insectes Sociaux 25(1), 3–11. https://doi.org/10.1007/BF02224481

Del-Claro, K. & Tizo-Pedroso, E. 2009. Ecological and evolutionary pathways of social behaviour in Pseudoscorpions (Arachnida: Pseudoscorpiones). Acta Ethologica 12, 13–22. https://doi.org/10.1007/s10211-009-0052-y

Klausen, F.E. 2005. The male genitalia of the family Atemnidae (Pseudoscorpiones). Journal of Arachnology 33(3), 641–662. https://www.jstor.org/stable/4129867

Tizo-Pedroso, E. & Del-Claro, K. 2005. Matriphagy in the neotropical pseudoscorpion Paratemnoides nidificator (Balzan 1888) (Atemnidae). The Journal of Arachnology 33(3), 873–877. https://www.jstor.org/stable/4129893

Tizo-Pedroso, E. & Del-Claro, K. 2007. Cooperation in the neotropical pseudoscorpion, Paratemnoides nidificator (Balzan, 1888): feeding and dispersal behaviour. Insectes Sociaux 54, 124–131. https://doi.org/10.1007/s00040-007-0931-z

Weygoldt, P. 1969. The Biology of Pseudoscorpions. Cambridge, MA, USA: Harvard University Press. xiv+145 pp.

World Pseudoscorpiones Catalog. 2022. World Pseudoscorpiones Catalog. Natural History Museum Bern. https://wac.nmbe.ch/order/pseudoscorpiones/3 (accessed 19 January 2024)

Zeh, J.A. & Zeh, D.W. 1990. Cooperative foraging for large prey by Paratemnus elongates (Pseudoscorpionida, Atemnidae). Journal of Arachnology 18, 307–311. https://www.biodiversitylibrary.org/partpdf/226352

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