Hydraulic arthopods

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. :slight_smile:

Zen Faulkes
Department of Biology
University of Texas - Pan American


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.

I’m not sure whether to put this in the same thread or start a new one, but I’m wondering how much of the spiders ability to jump “25 times it’s length” stems from its “remarkably efficient” system of hydraulics, and how much is just the result of a scaling argument. (Muscle strength depends on cross sectional area, and scales like length squared, but mass depends on volume and scales like length cubed – so smaller things are stronger relative to their weight.)

Y’know, I gotta say, the bug columns are among my favorites. “Hydraulic spiders,” indeed.

(Yes, I know spiders aren’t bugs, and not all insects are “true bugs,” etc. And the esteemed Zen atop takes issue with certain of the specifics. Still a great column.)

Actually, in looking over zfaulkes’s post, my curiosity is piqued by the reference to sea spiders. I know I could probably google up some more information, but it’s so much more entertaining to read it here: how are they so weird, exactly?

Sea spiders (incl. Horseshoe crabs IIRC) resemble crustaceans, but are taxonimically more closely related to arachnids - essentially, the evolutionary branching event that separates the ancestors of crustaceans from the ancestors of arachnids is further back in time than the branching event that separates the ancestors of sea spiders from the ancestors of land spiders.

The overly broad reference to the musculature of arthropods was an error inserted during editing. The article has been corrected.

Imagine a critter with eight legs… and no body!

Sounds very much like a horror show refuge, doesn’t it? Okay, they actually do have a body, but it’s so small in relation to the legs that it just doesn’t look like much. They just look like someone’s imaginary creation rather than a real organism.

They also have some other oddball features that are mainly interesting specialists, like that their body is not segmented (most arthropods are); they have an unusual proboscis; and so on.


Myers P. 2001. Pycnogonida (On-line), Animal Diversity Web. Accessed September 29, 2004 at ADW: Pycnogonida: INFORMATION

In a discussion of spider leg hydaulics, Vogel gives the impression that the hydraulic system would not be expected to be as effective as antagonistic muscles. He writes, “The [hydraulic leg] system works well enough so one doesn’t notice that it’s quite different from the scheme used by insects; some spiders (salticids) are even good jumpers. Perhaps the hydraulic arragement permits larger flexor muscles that can grapple with larger prey – spiders are preminently predators on creatures of around their own size” (pg. 274).

Elsewhere in the book (pg. 291), Vogel lists maximum acceleration for a jumping spider at 51 metres / second. Generally, smaller animals have higher maximum accelerations; an estimate of 15 m/s for a human (standing start), 40 for a fast start in a trout, and 2000 m/s for a rat flea, to name a few competitors. But squid, which are bigger than spiders, are listed as a max acceleration of 330 m/s for the tentacles.

I personally suspect scaling factors are the major explanation.


Vogel, S. 1988. Life’s Devices. Princeton University Press: Princeton.

Zen Faulkes
Department of Biology
University of Texas - Pan American

Nitpick: You probably mean 51 meters/second[sup]2[/sup]. m/s is a measure of speed or velocity, not of acceleration. And given that a human can walk around (with considerable difficulty, admittedly) in 3 gs or so, I suspect that the figure of 15 m/s[sup]2[/sup] for humans is a little low, but it would be in the right ballpark.

Correct. That was a mistake on my part, not Vogel’s. (The book actually wrote it as m x s [sup]-2[/sup], which means the same thing but is less intuitive – at least for me).

Zen Faulkes
University of Texas - Pan American