I walked into my bathroom late one night and was horrified to see something that looked like a tick with scorpion pinchers as large as the rest of its body. It was crawling up the wall. I managed not to scream, and after a moment's reflection, began to get excited. I was looking at an example of that elusive and beneficial order of arachnids, the pseudoscorpions!
Pseudoscorpions look like miniature tailless scorpions. They are related to scorpions, but only distantly, like those third cousins you only ever see at weddings. They use their disproportionately large pinchers to grab invertebrates that are even tinier than they are, including mites and springtails.
There are dozens of species that live in the Northeast. They often live in leaf litter or between layers of bark, but if you've encountered a pseudoscorpion in your bathroom, you were probably seeing Chelifer cancroides, known as the house or book pseudoscorpion.
Despite its fearsome appearance, this species is helpful to have around. It preys on common household pests like book lice, clothes moths and carpet beetles. It’s so small that it can easily go overlooked, especially as it often takes shelter in the cracks and crevices in wood. It often gets into houses on firewood, crawling out from the bark as the logs dry.
The house pseudoscorpion has another method of travel: piggy-backing a ride on a larger insect. It often uses beetles this way, but it can hitch a ride on just about any insect it can grab. Biologists call this phoresis.
It turns out human houses aren't the only homes in which house pseudoscorpions can act as pest managers. They will move into beehives, probably using their phoresis trick on a worker bee, and prey on varroa mites or wax moth larvae, which are a bane to bees and beekeepers.
Researchers are looking into the potential of house pseudoscorpions for biocontrol on these bee parasites; there may have been a symbiotic relationship with bees in the past. It looks like modern beekeeping methods have inadvertently pushed out the pseudoscorpions. Modern hive boxes aren't as friendly a habitat as old boxes, and modern beekeeping methods often involve more pesticides. Ironically, the same chemicals that kill varroa mites, which are also in the arachnid family, kill the beneficial pseudoscorpions.
Pseudoscorpions may keep a low profile, but they've been around a long time. There are fossils of modern-looking pseudoscorpions from 380 million years ago. They started co-habiting with people early, too: they are described by Aristotle, who presumably encountered them in ancient libraries, eating book lice as they still do today.
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