When most people hear the word "animal", they probably think first of birds and mammals, with reptiles, amphibians, and fish possibly coming to mind as afterthoughts. Though these vertebrates (animals with backbones) are indeed important, invertebrates (animals without backbones) greatly outnumber the vertebrates by any measure. Not only are there far more species of invertebrates than vertebrates, but invertebrates also exceed vertebrates in sheer number of individuals and in total biomass.

Of the invertebrates that live on land, the most diverse and abundant are the arthropods (the phylum that includes the insects, crustaceans, arachnids, and others), and of these arthropods the insects are the most diverse by far. Some of these terrestrial arthropods are very familiar to most people, including some of the insects (though only a small proportion of the insects are familiar to non-biologists), as well as some sorts of spiders, millipedes, centipedes, scorpions, and a few others. However, there are many groups of arthropods that are much less well known to the general public, including entire arthropod classes and orders. Though most people have never heard of them, many of these more obscure arthropods are actually quite common.

Such is the case for psuedoscorpions, the subject of this month's photos. Pseudoscorpions are diminutive predators of small invertebrates. Their large claws (technically, chelate pedipalps) make them resemble true scorpions, to which they are related, but they are different in that they lack the large upcurved tails and venomous stingers characteristic of scorpions (most species of pseudoscorpions do have venom glands used to subdue prey, but rather than delivering the venom via a large stinger on the tail, the venom is released from the claw-like pedipalps). Also, while scorpions can grow to be quite large, pseudoscorpions are downright tiny, as can be seen in the photos below.

Pseudoscorpion with a dime for scale. Psedoscorpions seldom exceed 5 mm in length. Click here or on photo for a larger view.

The same pseudoscorpion on the author's finger. This is a very tiny animal!

As you can see, pseudoscorpions are distinctly different from any of the other arthropods. Not only are they not scorpions, but they are clearly not insects either, since they do not have the three body segments, six legs, and two antennae characteristic of insects. Though they are related to and resemble spiders (psedoscorpions and spiders are both arachnids), they are also not spiders, since spiders lack claws and differ in several other respects. Nor are they mites, or ticks, or centipedes, or millipedes, or members of any of the other numerous groups of arthropods. Pseudoscorpions belong to their own unique group of arthropods, the order Pseudoscorpiones.

Though unknown to most people, pseudoscorpions are abundant and widespread, even in regions with cold winters such as Minnesota. They live in leaf litter (dead leaves decomposing on the soil surface), under stones, on tree bark, between the boards of buildings, in caves, and other places. Some species even hitch rides by clinging to the bodies of large insects. The pseudoscorpion shown here was brought to me by a student in my general biology class. Apparently, this pseudoscorpion had wandered into the home of this student's parents, where it was found in the bathroom on the shower curtain. Most people would have overlooked this tiny creature, but just days before we had been discussing arthropods in class, which included a brief discussion of psuedoscorpions. Consequently, this minute pseudoscorpion caught my student's eye, and I am very grateful that she took the time to bring it to class for all of us to see.

So, the next time you see a tiny creature crawling across your floor, take a closer look. You might be surprised by what you see!

Move mouse over photo for closer view, and click photo for a much larger view. (The color difference between this and the previous photos is due to a change in lighting. These are all photos of the same animal)

Though menacing looking when viewed at high magnification, pseudoscorpions are quite harmless to all but their tiny prey.