Link: http://www.straightdope.com/mailbag/mspiderhydraulic.html
In today’s Staff Report, “Do Spiders Have Hydraulic Legs?” Doug claims that arthropods don’t have opposing muscle groups like humans do. That’s true for some arthropods, but by no means all. Arthopods without opposing muscle groups are in the minority. I’ll use crustaceans as an example, because that’s the group I’m personally most familiar with.
The legs and claws of crabs and lobsters, for instance, have six joints. Of those, one joint has an unpaired muscle (the reductor, which connects to the joint between the basi-ischium and the merus). But that joint isn’t really used for locomotion: it seems to be more involved in breaking off the leg when the animal is getting hassled (autotomy), perhaps allowing it to escape.
The other five joints typically have perfectly good opposing muscles. The pincher (dactyl) of the claw is moved two muscles called the opener and closer, for instance.
When you get into the muscles that move the joints near the body, things get very complicated. There are several muscles that originate and insert at different points on those joints, and it’s still not clear why the animal has so many different muscles when the movement is restricted by the joint, which acts as a simple hinge.
I think one of the other arthopod groups which largely uses hydraulics and has unopposed muscles in their legs are pycnogonids, a.k.a. sea spiders. But they’re just generally weird.
Zen Faulkes
Department of Biology
University of Texas - Pan American
References
Antonsen, B.L. & Paul, D.H. 2000. The leg depressor and levator muscles in the squat lobster Munida quadrispina (Galatheidae) and the crayfish Procambarus clarkii (Astacidae) have multiple heads with potentially different functions. Brain, Behavior and Evolution 56: 63-85.
Faulkes, Z. & Paul, D.H. 1997. A map of the distal leg motor neurons in the thoracic ganglia of four decapod crustacean species. Brain, Behavior and Evolution 49(3): 162-178.
Lochhead, J.H. 1961. Locomotion. In: Waterman, T. H., Physiology of Crustacea II, pp. 313-364. New York: Academic Press.