- Two forms of the large-clawed male spider crab, Labinia emarginata , from the same molt stage, were examined to determine possible roles of ecdysteroids and juvenoids (methyl farnesoate) during maturation of the reproductive system and mating behavior. These forms are distinguished by the condition of the epicuticle covering the exoskeleton, abraded (worn away) and unabraded (intact), and mating behavior, active and inactive, respectively. Methyl farnesoate (MF) levels in the blood are high during active reproduction (abraded males)—mating, mate guarding and male competition; but, when the animals are in reproductive diapause (unabraded males), these levels are low. Ecdysteroids are found primarily in the testes of both male types. However, the amounts detected are highest in the non-reproductive unabraded males, and lowest in the actively mating abraded ones. It appears that ecdysteroids may function early in maturation in unabraded animals by making the gonad competent, while MF may be functioning in mature males with developed reproductive systems and exhibiting mating behavior.