Photo by cottonmanifesto
Urban species #087: Flat-backed millipede Pseudopolydesmus serratus
Only occasionally in my research do I encounter a phrase as wonderful as "Vast migrating armies of the millipede Pseudopolydesmus serratus in the Dayton, Ohio area." I realize I'm nearly alone in my glee contemplating a vast army of millipedes, but beyond the thrilling mental image it conjures, there is the also the confirmation that, indeed, this is an urban species. While the residents of Dayton were lucky enough to have their armies of millipedes identified to species, I don't have the equipment, training, or inclination to attempt it with our flat-backed millipede. It's fairly safe to say that our millipede is in the order of Polydesmida, and I'd make so bold as to say that it's in the family polydesmidae. Of course, I'm foolhardy enough to make the leap to guess that it's in the genus Pseudopolydesmus, with nothing to go on but a hunch.
Collectively, we can speak of flat-backed millipedes as polydesmids. While I consider myself a veteran rock-overturner, seasoned from childhood, I had never seen one of these animals until I was an adult. The dark gray, round millipedes, those that are more or less cylindrical, those have been familiar to me since I was tiny. Polydesmids, with their flattened body shape and rather redder coloration, are much easier to confuse with centipedes. Close observation shows that they have two pairs of legs per body segment--twice what centipedes have. Why I never encountered them in my childhood near the Connecticut River, but I now find them under nearly every rock, only 80 miles east of where I grew up, remains a mystery to me.
Flat-backed millipedes feed on decaying plant material. In the southern states, and into the tropics, this food source is available year round, and their polydesmids are relatively large animals. Four-inch long specimens are common in Florida and other places. (The millipede in the above picture is less than an inch long.) Many kinds of millipedes produce chemical defenses to deter predators. Polydesmids are distinguished by having some members who produce cyanide compounds to protect themselves.