footloose

I don’t spend a lot of time thinking about millipedes, even though there are an estimated 80,000 species on earth (only 10,000 named), with about 1,400 in the U.S. As a gardener and someone who is fond of peeking under rotting logs, I certainly encounter millipedes.  Although these are small and non-descript, I recognize their important role as decomposers and their contribution to the nutrient cycle. And, well, that’s about the extent of my relationship with class Diplopoda.

Be that as it may, I could hardly ignore this whopper, nearly 20 centimeters, crossing my path in the Sierra Cubitas mountains in Cuba.  Despite its 110 or so pairs of legs (name notwithstanding, no millipede has more than 750 legs) it had lost its grip on rain-slicked rocks and keeled over like an ungainly double-bottomed tanker truck.

A prod with a stick to nudge it upright caused it to immediately secrete droplets of liquid from pores on its sides.  Nearly all millipedes have defensive secretions, and this one was excreting surprisingly copious amounts just from this good-intentioned provocation.

After walking away, I sighed.  I knew that once I returned home, I would be unable to resist trying to put a name to this mega-millipede, and learn more about its haunts and habits.

Knowing virtually nothing about millipedes (caught flat-footed, so to speak), I had to first familiarize myself with the general types of millipedes: flat, plated types, bristly bark dwellers, ones that roll themselves up into balls, and an array of cylindrical kinds.  The Cuban one was clearly a member of the latter. I further narrowed it down to large tropical cylindrical millipedes in the superorder Juliformia.  Eventually I concluded this millipede was in the order Spirobolida, and the family Rhinocricidae, which contains genera that might be familiar to millipede fans in North America, including the species Narceus americanus, or American Giant Millipede. There are nine genera of Rhinocricidae in the West Indies, five in Cuba.  I’m pretty sure this is in the genus Rhinocricus, but that’s only a semi-educated guess.

As for the secretions, the chemical composition varies across taxonomic groups.  Spirobolida produce benzoquinones, noxious chemicals similar to what is sprayed by bombadier beetles. Aside from their ability to dissuade predators, benzoquinones can be pretty irritating to humans, causing mild burns and/or skin discoloration.  The glands that discharge the secretions, known by the vividly descriptive term, “repugnatorial glands,” don’t merely weep, but allow some tropical species to actually squirt their arsenal two to three feet.

To warn potential predators of their distasteful discharge, some millipedes exhibit bright (aposematic) coloration. It is thought that the bioluminescence of some species, such as the Motyxia (order Polydesmida) of California, which produce cyanide, serves this same function.

Some predators don’t heed the caution.  Monkeys have been observed rubbing millipedes on their fur.  It turns out that the monkeys utilize the benzoquinones emitted by the millipedes as an insect repellent. Coatis, relatives of raccoons, actually use the distinctive odor of benzoquinones to locate millipedes. The coatis neutralize the toxin by vigorously rolling the millipedes on the ground.  Then they eat them.

By the time I had learned all this, my Diplopoda curiosity had been sated.  My fact-finding had consumed more hours than was prudent for me to spend.  That’s often the case when I indulge my familiar compulsion to understand still another aspect of the natural world. Perhaps what I’ve uncovered will never serve me later in life.  Or maybe this story has legs…

More resources:

• Gordon’s Millipede page. It was at this site that I was fascinated by various Diplopoda minutiae that I could not work into this post, including the fact that millipedes spend “…a great deal of time cleaning and polishing all the various parts of their bodies. They have special brush like group of hairs on their 2nd or 3rd pair of legs which are used in cleaning the antennae. They are also noted for being meticulous about cleaning the gonopods after sex.”
• Millipedes at BugGuide.net
• Field Museum of Natural History (Chicago) Milli-PEET project
• Perez-Asso, A. R. 1998. Three new genera of millipeds of the family Rhinocricidae (Diplopoda: Spirobolida) from Cuba. Carribean Jrl. Science 34:84-91.
• Weldon P. J., Aldrich, J. R., Klun, J. A., Oliver,  J. E., and M. Debboun. 2003. Benzoquinones from millipedes deter mosquitoes and elicit self-anointing in capuchin monkeys (Cebus spp.).  Naturwissenschaften. 90:301-304.
• Zito, M., Evans, S., and P. J. Weldon. 2003. Owl monkeys (Aotus spp.) self-anoint with plants and millipedes. Folia Primatol 74:159-161.