Although claws broke at many different locations along the pollex, most breaks fell either towards the base (0.2-0.4 of standardized pollex length), near the proximal margin of the tanned cuticle (the black ends to the fingers of many brachyuran crab claws), or towards the tip (0.6-0.8 of standardized pollex length, [ILLUSTRATION FOR FIGURE 2A OMITTED]).
The fracture plane, however, was not always perpendicular to the upper pollex surface (i.e., parallel to the load vector).
Under natural conditions, claws may fail mechanically in ways other than the breakage of the pollex that we examined in this study.
Although breaking force did not vary along the pollex, the ultimate stress (force per unit area, [[Sigma].sub.b]) - a measure of the ability of claw cuticular material to resist failure in tension - did [ILLUSTRATION FOR FIGURE 2B OMITTED].
oregonensis claws have a much larger area of black cuticle at the pollex tip (G.
The indirect method we used, based on linearly elastic theory applied to cantilevered beams (Young, 1989), assumes (I) that the pollex, or fixed finger, of the claw exhibited a constant cross-section along its length; (II) that the cuticle is isotropic and homogeneous; (III) that shape variation along the length of the pollex, such as caused by teeth, does not create local points of stress concentration; and (IV) that tensile strength was less than compressive strength (i.e., that the cuticle failed in tension).
Autotransplantation of a combination of distal and proximal claw segments (dactyl + pollex + ischium) into eye sockets regenerated claws or clawlike structures in four cases (26.7% of surviving crabs, n = 15).
Autotransplantation of proximal segments of claws (ischium + merus/carpus joint) or distal parts of claws (dactyl + pollex) regenerated claws or clawlike structures (11 cases, 21.2% of surviving crabs, n = 52).
Autotransplantation of single digits of claws (dactyl or pollex) regenerated tapered projections, characterized by the lack of a joint, in four cases (16% of surviving crabs, n = 25; [ILLUSTRATION FOR FIGURES 2G, H OMITTED]).
One mode - regeneration of a complete claw following ecdysis - can occur in autotransplantation of both digits of the claw, the proximal segments of claws (ischium + merus/carpus joint), or a combination of distal and proximal segments of claws (dactyl + pollex + ischium).
The second mode of regeneration occurred in autotransplantation of a combination of distal and proximal claw tissues (dactyl + pollex + ischium) or of both claw digits (dactyl + pollex), but not in autotransplantation of the proximal claw segments (ischium + merus/carpus joint).
The two claws with host-site handedness resulted from contralateral autotransplantation of a combination of distal and proximal claw tissues (dactyl + pollex + ischium; [ILLUSTRATION FOR FIGURE 2A OMITTED]), or proximal claw tissues (ischium + menus/carpus joint).