The extremely large millipede known as the iron worm (Narceus armericanus/annularis) is one of the more spectacular arthropods in our area. This animal is often about 3 inches in length and crawls slowly along the ground. When disturbed, it curls into a tight spiral (head at the center), and usually also secretes a smelly liquid from pores on the sides of its segments. Like other millipedes, it is a scavenger, has two pairs of legs per segment, and does not bite or have venom. Centipedes, which are similarly shaped but are actually placed in a different class of arthropods, are fast moving predators with venomous fangs. They have only one pair of legs per segment, and cannot curl up tightly like a millipede can.
Identification of millipedes is difficult and the double species name for this one reflects that confusion. In the past, this genus, which occurs throughout much of the eastern part of the U.S., has been split into at least a dozen species. More recent research has shown that they are probably all accommodated within just two species, but those are not clearly defined yet.
Other kinds of millipedes in our area are much smaller, usually topping out at around an inch or less in length, so there is no mistaking this one. In spite of the similarity in appearance of most millipedes and the fact that there don't seem to be a large number of species, they are as diverse as insects or arachnids, and several different orders are represented here in central Texas.