GEOTROPIC ORIENTATION IN ARTHROPODS : III. THE FIDDLER CRAB UCA

On an inclined surface the fiddler crab Uca pugnax, during sidewise progression, orients upward through an angle theta on the surface. The extent of negatively geotropic orientation (theta) is a rectilinear function of sin alpha, where alpha is the inclination of the surface to the horizontal. This equation differs from that describing the geotropic orientation of various other animals. The difference is traced to the fact that from an initial position with the transverse axis of the body horizontal the crab is required to turn upward to an extent such that the vertical line from its center of gravity pierces the inclined surface within the base of support provided by the legs. This leads to the equation sec theta/tan alpha = const., which is obeyed within the limits of precision of the measurements. This type of control of geotropic orientation represents an extension of the "muscle tension theory," and is in no sense in conflict with this view. The assumptions underlying the analytical expression connecting theta and alpha are verified by the asymmetry in the orientation of male fiddlers, which is shown to be due to the presence of the enlarged chela and which disappears when the claws are removed.