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 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’.
Hooke's 'land-crab' (pseudoscorpion) from Micrographia (1665)
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:
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 confusumwithout 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.