by Joseph DeSisto
This post started out titled “My Favorite Centipede Genus,” but that could never last. I have too many favorites. Right now, Theatops is my 6th, but things can always shift around. Theatops has no official common name, but in the spirit of this blog, I’m going to make one up right now: the forcep centipedes. I’ll explain why.
Forcep centipedes are not especially diverse, with only six species known worldwide, four of which are strictly North American. They are, however, large and impressive, and reasonably common through much of their range. A week or so ago Derek Hennen sent me a speciment of Theatops posticus, one of two species found in the eastern U.S.:
A Theatops posticus from Ohio, sent to me by Derek Hennen. Photo by Joseph DeSisto.
Modified hind legs are common in centipedes, especially in the scolopendromorphs (centipedes with 21 or 23 pairs of legs). A paper published today in ZooKeys reviewed the uses for hind legs in the family Scolopendridae (Kronmüller and Lewis 2015), which includes some of the most impressive centipedes, i.e., the foot-long giants in the genus Scolopendra.
Their conclusions were that the hind legs of scolopendrids have a wide variety of uses, the least of which is walking. In Scolopendra, the legs are covered in short spines and are used to capture prey, guide courtship, and grapple with predators. After an intimidating display, a Scolopendra can grab onto you with its hind legs, then rear its whole body around to inject venom with its fangs — which are themselves modified legs.
Figure 2 from the Kronmüller and Lewis paper, showing uses for the spiny modified legs of Scolopendra. The threat display (A and B) would be quite intimidating coming from a centipede that can reach 12 inches or more in length. From Kronmüller and Lewis (2015), licensed under CC BY 4.0.
Hind legs can also mimic antennae, so that predators like birds are tricked into attacking the less vulnerable rear end of the centipede. Attackers are then met with an unpleasant surprise when the real head makes its move! This strategy is most obvious in Scolopendra heros arizonensis, where the trunk is orange but the first and last few segments are jet black. But many non-scolopendrids also have rear legs that resemble antennae, especially the scutigeromorphs or “house centipedes,” whose legs readily detach if they are caught.
Scolopendra heros arizonensis, from the deserts of the southwestern U.S. and Mexico. The “mimicry” between the front and rear segments might confuse predators into attacking the wrong end. Photo by Aaron Goodwin, licensed under CC BY-ND-NC 1.0.
Scolopendrids can also use their legs to climb, even on the ceilings of caves. Scolopendra gigantea, a South American giant that can reach 12 or more inches in length, has been observed hanging from its rear legs in Venezuelan caves and snatching bats out of the air (Molinari et al. 2005). This phenomenon was even featured in the first episode of David Attenborough’s Life in the Undergrowth television series.
Then there’s the flag-tailed Alipes, which uses its hind legs to make a hissing noise … really you should just read the paper. It’s a peer-reviewed journal article, but it is publicly accessible here and, for the most part, readable by a non-expert.
Miscellaneous scolopendrid rear legs, modified in different ways. The flag-tailed centipede Alipes (C) is probably the strangest, and its legs can be rubbed together to make a loud hissing noise. Photo from Kronmüller and Lewis (2015), licensed under CC BY 4.0.
All of this was a very long way of saying that Theatops isn’t really that special … probably. Actually, Theatops was not covered in this paper, since it belongs not to Scolopendridae but to the less-known family Plutoniumidae. And yes, that’s the real name.
In the East we have two species: T. posticus and T. spinicaudus. The main difference between the two is that spinicaudus has a pretty large spine on the last pair of legs, while posticus does not. So, I here propose we call posticus the smooth-tailed forcep centipede and spinicaudus the spiny-tailed forcep centipede.
Smooth-tails are found through most of the eastern U.S., from Connecticut south to Florida and west to eastern Texas (Shelley 2002). A separate population of smooth-tails is found in the West (along with two other Theatops species), in western Arizona and southern California and Nevada, but this may represent a separate species.
Spiny-tails, meanwhile, are abundant but geographically restricted: there are two populations, one in the southern Appalachians and one in the Ozarks (Shelley 2002). Apparently this species prefers mountain habitats, since spinicaudus is not found in the area between the two mountain ranges.*
The forcep-like hind legs of the smooth-tailed forcep centipede, Theatops posticus. Photo by Joseph DeSisto.
*[Actually, this is why I’m looking for Theatops specimens in the first place — to obtain genetic data so I can figure out how/when they came to inhabit their bizarre geographic ranges.]
The modified hind legs immediately make me think of an earwig, and they appear to be useful for grabbing things — but what are these centipedes grabbing? To start, they certainly do grab the forceps I used to pick them up during my May trip to the southern Appalachians. If you try and pick up one of these centipedes, they will simultaneously grab you with their legs and inject venom with their fangs.
But then, why attack with non-venomous hind legs, when forcep centipedes can and do use their venom-injecting fangs? Perhaps the hind legs give the centipede leverage with which to inflict a longer-lasting, more painful bite. It’s possible, and scolopendrids certainly do it, but I think there may be more to this story.
The business end of Theatops posticus — the fangs beneath the head pack a healthy dose of venom, used to dispatch prey and make predators cry. Photo by Joseph DeSisto.
The whole last segment of a forcep centipede’s body, including the legs, is heavily protected with extra-thick cuticle. The exoskeleton on the rear legs and segment is about as thick as that around the fangs at the front of the body, which is usually the most heavily protected part of a centipede. I think that Theatops species might use their hind legs to attack dangerous prey — spiders, or maybe even other centipedes. In this way, a forcep centipede can inflict its deadly bite only after the prey has been subdued, so avoiding injury.
It’s only a hypothesis, and one that requires some testing. I have tried feeding smaller centipedes to Theatops, who attacked and fed with gusto, but never used involved their rear legs in the process. So for now, we really don’t know why forcep centipedes have such strange hind legs. All we know is that they are strange, oddly captivating, and will likely remain that way for a long time.
A couple of things. First, I want to thank Derek Hennen, a Masters student at the University of Arkansas, for sending me the specimen photographed here. Actually, this is just one of many centipedes he has sent me! He also has his own blog, Normal Biology, featuring insects, millipedes, and even the occasional centipede.
On July 19 I will be speaking about centipedes, millipedes, and why they’re amazing at the Schoodic Research and Education Center in Acadia National Park (Maine)! This is a public program, part of the bioblitz going on that same weekend in Acadia. If you’re in the area, stop by! More information is available here.
Kronmüller, C. and J.G. Lewis. 2015. On the function of the ultimate legs of some Scolopendridae (Chilopoda, Scolopendromorpha). ZooKeys 510: 269-278.
Molinari, J., E.E. Gutiérrez, A.A. De Ascenção, J.M. Nassar, A. Arends, R.J. Marquez. 2005. Predation by giant centipedes, Scolopendra gigantea, on three species of bats in a Venezuelan cave. Caribbean Journal of Science 41(2): 340-6.
Shelley, R. 2002. A synopsis of the North American centipedes of the order Scolopendromorpha (Chilopoda). Memoir of the Virginia Museum of Natural History 5: 1-108.