Christmas Eve arachnid blogging (and for once it's not a spider)

It is spiders that give the class Arachnida its name (and spiders in turn got their Greek name from Arachne, whose skill at weaving was, alas, matched by her insolence). But there's more to arachnids than just spiders. We've all heard of scorpions, we've all seen harvestmen (though some people mistakenly think them a sort of spider). But there are also orders in the class that relatively few people know. As a special gift, we'll have a look at one of them today.

Meet Allochernes wideri, a pseudoscorpion:

As their name suggests, pseudoscorpions are a lot like real scorpions, only more pseudo. Also, they're tiny. The A. wideri above is just a hair over 2mm. Some pseudoscorpions hitch rides by clinging to the knees of flies.

Pseudoscorpions are known for living in old books, and perhaps you have seen one while perusing your incunabula. I don't own any incunabula myself, and anyway books are not where most of these animals live. If you want to find one, you'd be better off scouring a bird's nest, or some leaf litter, or (in the case of A. wideri) looking beneath the bark of a fallen tree.

These are fascinating creatures, and it is invidious to describe them merely as miniature scorpions devoid of a scorpion's venomous sting. (It is true, though, that within the class Arachnida they are pretty closely related to scorpions, whilst only distantly related to spiders.) For one thing, pseudoscorpions do have venom. They inject it with those alarming-looking claws. You needn't worry about a pseudoscorpion's venom, though; they are far too small to harm you. I have never heard of a pseudoscorpion managing successfully to envenomate a human, much less of a human reacting to the venom. The claws are obviously homologous with the claws of scorpions, and less obviously with the pedipalps of spiders. (The palps are 'mini-legs' just in front of a spider's foremost 'real' legs. Spiders use their palps like hands; males also use them to inseminate their mates.) Like spiders, pseudoscorpions use silk. Unlike spiders, whose silk glands are at the back end, pseudoscorpions emit silk through their 'jaws' (chelicerae, which are in origin the first pair of appendages). They don't make webs like a spider's, but they do weave little tents and sleeping-bags.

Some pseudoscorpions are social. That is, they live in cooperating groups. They are not what zoologists call 'eusocial', like ants and some wasps and bees. In eusocial organisms, the members of an entire nest (or what the Germans charmingly call a Volk, or 'people') subordinate their own reproduction to that of their queen. No, pseudoscorpions have managed to attain only the same level of sociality that, for example, we humans have. (Among mammals, naked mole-rats are the sole known true eusocials). In the first volume of William Hamilton's collected papers, there is a not very high-quality but nonetheless exciting photo of a group of pseudoscorpions hunting an ant. They nip in to paralyse their large and dangerous prey by pinching its extremities; when the ant is subdued, the pseudoscorpions' young will be the first to feed.

Here's another glimpse of our wee friend:

Now, if you'd like to see some truly excellent photos of pseudoscorpions by somebody who (unlike me) has top-grade equipment and knows how to use it, drop in at the American Arachnological Society and have a look at Hans Henderickx's pictures.