Hedgehogs of the Microscopic World

The case load is generally pretty quiet around the UW-Madison Insect Diagnostic Lab during the first few months of the year. There are still the cases of conifer seed bugs or multicolored Asian lady beetles which are the norm for January, February and March. Occasionally I get some interesting specimens (like the cedar tree borer) which emerge from firewood that had been brought in and stored next to the fireplace. My most interesting case lately has been a sample which I was convinced contained small, hairy carpet beetle larvae. I placed the specimen under the microscope to confirm my suspicions, only to be surprised to find that it had a lot more than six legs. With that one glance, the mundane had become the bizarre.  I vaguely recognized the creatures I was staring at through the microscope, as I must have seen them in a book once before.

A few minutes of digging and I had my ID: an unusual type of millipede called a “duff millipede” or “dwarf millipede” from the genus Polyxenus. At only a millimeter or two long, these millipedes are hardly noticeable unless they’re moving against a light colored background. Typically, when I hear of millipedes, I think of the slow-moving, dark-colored, creatures that curl up when disturbed and are frequently associated with moisture and decaying organic matter. When it comes to duff millipedes, you have to take most of your preconceptions about millipedes and throw them out the window. Not only are they incredibly small, but their armature of spines makes them resemble a miniature hedgehog (cuteness, included) or some kind of character from the Muppets.

A “duff millipede” (Polyxenus species). Photo Credit: PJ Liesch

So what do these little spine-balls do anyways? If you dug into the literature, you’d find that they’re often associated with tree bark, leaf litter, or old stone walls and are thought to feed on algae. Other than that, there isn’t much known about them—except for their fascinating defensive mechanisms. Most millipedes rely on chemical defenses to keep predators at bay (including the ability to secrete cyanide).  Some polyxenids take a very different approach and use a physical defense consisting of detachable, barbed spines reminiscent of those novelty “finger trap” toys from childhood. Thomas Eisner and colleagues found that these detachable spines can get hooked on the bodies of would-be predators (such as ants), allowing the millipede to escape while the hungry predator attempts to extricate itself. Like the “finger trap” toys, the more the predator struggles to free itself, the more entangled the barbs become. In some cases, the predators can even die from this entanglement.  [You can read a detailed description of this defensive mechanism here: http://www.pnas.org/content/93/20/10848.full.pdf]

Entanglement by microscopic millipedes: what a way to go. . .