- Levels of methyl farnesoate in the blood and in vitro rates of methyl farnesoate synthesis by the man- dibular organ were investigated to determine whether this compound is related to the differences in morphology and reproductive states of distinct types of male spider crabs described by Homola et al. (1992) in winter populations. Three male types, selected from a summer population, were investigated in detail: ( 1) males with relatively large propoduses (claws) and worn exoskeletons (abraded), (2) males with relatively large propoduses and exoskeletons covered with epicuticle (unabraded), and (3) males with small propoduses and unabraded exoskeletons (small). All males examined had sperm, but abraded males, identical in propodus and body size to unabraded males, had a reproductive system that weighed twice as much. Large-clawed unabraded males had relatively small re- productive systems. Small-clawed males possessed a small reproductive system. Abraded males possessed larger mandibular organs, containing almost twice the total protein, and their man- dibular organs synthesized significantly more methyl far- nesoate in vitro than did the other types of males. Cir- culating levels of methyl farnesoate, in the hemolymph of the abraded males, were more then twice as high as the levels detected in any other type of male.