Ten facts about Pseudoscorpions

1 April 2014 by Christopher Buddle, posted in Natural History, Ten facts

Yesterday I finally finished a draft of a manuscript about the natural history of a northern pseudoscorpion, and that task inspired me to write a ‘ten facts’ post about these truly astounding, tiny, Arachnids. Here goes!

Wyochernes asiaticus (photo by C. Ernst, reproduced here with permission)

Wyochernes asiaticus female (around 2 mm long) with brood sac (photo by C. Ernst, reproduced here with permission)

1. Pseudoscorpions are commonly known as ‘false scorpions’ or ‘book scorpions’. The are their own Order within the Arachnida, and do resemble their cousins, especially the scorpions (hence their name!), except they don’t have a stinger on their back end. It’s suggested they were first described by Aristotle, perhaps as they were wandering among scrolls, hence the ‘book scorpion’ common name…  In Micrographia, Hooke called these little critters ‘land-crabs’.
R. Hooke's 'land-crab': a pseudoscorpion! (from Micrographia, 1665)

Hooke's 'land-crab' (pseudoscorpion) from Micrographia (1665)

2. In 2002 there were just over 3,000 known species of Pseudoscorpions. Mark Harvey considers the pseudoscorpions among a ‘neglected cousins’ within the Arachnida: they are seldom studied, and have received relatively little attention from biologists. Despite this ‘low diversity’ they are still more diverse than some other well-known animals: for example, the Carnivora order of mammals has fewer than 300 species!

3. This Order of Arachnids has an impressively long history, known from fossils going back to the Devonian, 380 million years ago! These arachnids have also been found in cretaceous amber, and it's clear that their body plan hasn't changed in quite some time!

4. Perhaps one reason these arachnids are poorly known is because they are easily missed: the largest pseudoscorpions don’t get over 10 mm in length, and most are much smaller than that. However, one cosmopolitan species often shows up in people’s homes, often in older houses and in damp areas of houses. That species is Chelifer cancroides, first described by Linnaeus in 1758. There’s a good overview about this species on the Encyclopedia of Life website.  It’s an awfully nice-looking arachnid:
Chelifer cancroides

Chelifer cancroides

5. Although they are wingless, pseudoscorpions do get around. I’ve found them under rocks in high elevation areas well above the Arctic circle, and they are known pretty much around the globe. Their success at moving large distances is mostly attributed to their ability to ‘hitch-hike’ on other animals, a process known as phoresy. They can easily grab onto their host and hitch a ride. You can click here to learn more about phoresy in these arachnids, and Nicky Bay has a nice photo showing phoresy.

6. Although they are predators, pseudoscorpions don’t bite humans - they are tiny, and just don’t have the strength to pierce your skin. They tend to feed on tiny little invertebrates, including caterpillars, book lice, and other soft-bodied critters. Their venom glands, interestingly, are located on their giant, oversized pedipalps. They catch their prey, immobile their prey, and then macerate and suck out the insides of their prey, using their chelicerae.

7.  I found found it’s often not particularly easy to find pseudoscorpions in nature, as their distributions tend to be quite clumped. Some good habitats include rotten logs, leaf-litter, and some species are known from organic debris on beaches. Sometimes get lucky and find a lot of specimens in a short period of time. This was the case when I was looking for Arctic pseudoscorpions in the Yukon Territory a few years ago. Here’s a video to illustrate the abundance, and to show how they move around.

8. Pseudoscorpions don’t have sex: sperm transfer is ‘indirect’. Males deposit a spermatophore (a ‘package of sperm’) on the substrate somewhere and the females must come in contact with that package in order for fertilization to occur. In some species, males will do ‘dances’ with females and help her find the spermatophore. Weygoldt’s book describes the full suite of these behaviours. However, in some species, males have never been found! Nelson, for example, collected hundreds of specimens of Microbisium confusum without finding a single male. It’s therefore assumed some species are parthenogenetic. Once eggs are laid (kept within brood sacs of the female), there are three larval stages: protonymphs, deutonymphs, and tritonymphs.

9. The natural history of most pseudoscorpions are largely unknown, but there are some remarkable exceptions including the work by Zeh & Zeh on Cordylochernes scorpioides, which catches a ride on harlequin beetles, and in which the males will defend the beetle’s abdomens as good sites for intercepting (and inseminating) dispersing females.  There’s also a paper that described matriphagy in one species. The authors write: “...the mother went out of the nest and passively awaited the protonymphs’ attack, not reacting to the capture nor to the nymphs feeding on her body”. Yuck.

Pseudoscorpions can easily fit in the palm of your hand!

Pseudoscorpions can easily fit in the palm of your hand!

10. Pseudoscorpions can be tricky to identify, but to become familiar with the group, you should check out Mark Harvey’s websites. If you live in northern North America, you can refer to this photographic key. Otherwise, Muchmore’s key in Dindal’s soil biology guide is probably the best resource.
Screen Shot 2014-03-29 at 10.21.48 AM
(all photos by C. Buddle unless indicated otherwise)

4 Responses to “Ten facts about Pseudoscorpions”

  1. barry Reply | Permalink

    nice. thankyou. i've found these little buggers 2 or three times in my life and i think i had 2 for pets for some time. it's been awhile. you are right, you see them sporadically. maybe i'll see if i can hunt some down this spring!

  2. Will Holz Reply | Permalink

    I didn't know until recently that they actually had social species.

    Well, I suppose the party bus on the harlequin beetle is also kind of social (enthusiastically so!) but I didn't realize they hunted together and had little societies.

    Melvyn Yeo has a couple of great pictures of one species teaming up to ambush a much larger ant at http://melvyn-yeo.blogspot.com/2011/10/pseudoscorpion.html#more

    There's also a more recent article on Paratemnoides nidificator that isn't behind a paywall here. :)
    http://www.leci.ib.ufu.br/pdf/tizoeDelclaroInsectSociaux.pdf

  3. scott bredbenner Reply | Permalink

    I have found these little guys in the high arctic in the Kongakut River area of northeastern Alaska. Amazing considering that this area is well below freezing most of the year. An exciting place to visit in the short summer as all the creatures big and small must get busy in a very small window of above freezing temperatures.

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