by Joseph DeSisto
Time for some centipedes! Recently I’ve been looking at a lot of centipedes in the family Lithobiidae, and in particular the striking modifications of some of the males, and I thought I’d share them here.
For those of you who don’t spend your free time studying centipedes, they are arthropods with 15 or more pairs of legs and venomous fangs. I say “fangs,” but they are technically a highly modified pair of legs, positioned beneath the head, that centipede biologists may refer to as poison claws, forcipules, maxillipeds, prehensors, prehensorial feet, forcipular telopodites, toxicognaths (Bonato et al. 2010) … you get the picture. Fangs. They look like this, when viewed from below:
The fangs are the things that look like fangs. Pretty cool, huh?
Lithobiid centipedes belong to the order Lithobiomorpha, which includes centipedes with flattened bodies, spiracles (breathing holes) on the sides of their body, and 15 pairs of legs as adults. They are also show anamorphic development, which means the young add legs as they grow until they reach the final 15. Those in the family Lithobiidae have spines or spurs on their legs, which are helpful in identification.
Without a microscope, all lithobiids look pretty much the same. Some are bigger than others, but here in the eastern U.S. none exceed an inch or so. But on a smaller scale, the diversity in body form is fantastic, and one of the reasons they are probably my favorite family of centipedes.
To tell the difference between males and females, we need to move our view to the rear end of the centipede, and look at it again from beneath. While males are pretty nondescript in this regard, the females have a set of gonopod claws, which they use to manipulate the eggs they lay. A female centipede can lay an egg and then carry it around with her until she finds a suitable place to leave it, then use her legs and claws to coat the egg in dirt for camouflage .
But in a few North American lithobiids, the males make themselves known by other means. This is most obvious in the last two pairs of legs, which may be highly modified into strange, contorted forms. Here’s an example, viewed from the side:
Pearsobius is a poorly known genus from Virginia and North Carolina (Causey 1942). The specimen above is unidentified. There will be a later post devoted just to Pearsobius, but for now, let’s look at more pictures! Pictures are great.
These centipedes are about half an inch in length, but the “spike” on the femur of the last pair of legs is visible even without a microscope. Although impressive, the purpose of these structures is unclear. My best guess is that the females use them to recognize males of the same species. While butterflies might use colors to achieve this effect, and birds might use songs, female centipedes live their lives in leaf litter and soil, where sight are of little use, and they can’t hear. So in an area where multiple centipede species might roam the same patch of leaf litter, a female needs something she can feel to avoid getting friendly with a male of a different species.
Not all leg modifications are so striking. Here is the 15th pair of legs on a male Paitobius zinus, also from Virginia.
Not as cool as a massive spike, but the modification here (the long indent on one of the segments) is still enough to make Paitobius distinguishable from other lithobiid genera. However, in Paitobius zinus, this is not the most striking male modification. Uniquely in this species, the male and female fangs/forcipules are different. The female’s forcipules are normal, and look pretty much like the ones from earlier (on Bothropolys). The male’s however … well, they look like this:
So far, P. zinus is the only species known to have modified male forcipules, and nobody knows why they have these. Long, narrow fangs could be an adaptation to extracting prey from narrow spaces (like in the woodlouse-eating spider, Dysdera crocata) … but why aren’t they found in females? Usually when we find a structure that is present in one sex but not the other, the function is related to reproduction. But as far as we know, the only thing forcipules are used for is killing prey.
Crabill (1960) was the first to write about this phenomenon in P. zinus, and since then, not a single person has bothered to study it. Why? Because despite being totally and undeniably awesome, centipedes are hard, and barely anyone studies them. I am currently planning a summer collecting trip to Virginia, though, and while I’m there I’ll see if I can learn anything. I have no idea why this species is so strange, but whatever reason there is, I bet it’s amazing.
A big thank you is owed to Dr. Bill Shear at Hampden-Sydney College in Virginia, who kindly sent me the specimens of Pearsobius and Paitobius, which he collected.
Bonato, L., G. Edgecombe, J. Lewis, A. Minelli, L. Pereira, R. Shelley, and M. Zapparoli. 2010. A common terminology for the external anatomy of centipedes (Chilopoda). ZooKeys 69: 17-51.
Causey, N.B. 1942. New lithobiid centipedes from North Carolina. Journal of the Elisha Mitchell Scientific Society 58: 79-83.
Crabill Jr., R.E. 1960. A remarkable form of sexual dimorphism in a centipede (Chilopoda: Lithobiomorpha: Lithobiidae). Entomological News 71: 156-161.