The Mechanism of Extension in the Legs of Spiders

The absence of extensor muscles in the legs of spiders presents the problem of accounting for the quick extensor movements characteristic of these animals. The theory that the "elasticity" of the interarticular membrane might be the means whereby this is accomplished has been shown to be untenable. Experimental evidence has been presented showing that extension is intimately associated with changes in the volume of blood in the spider leg, i.e., with changes in the internal fluid pressure in the leg. Certain structural peculiarities in the joints have been described, but the evidence is insufficient to permit evaluation of their possible roles in extension.