A characteristic feature of onychophorans is the presence of crural glands in males. These exocrine glands open on the ventral surface at the base of the legs. Their restriction to males suggests a sexual function. Histochemical staining of the glands indicates that their secretion has both a lipid and a protein component. Behavioural bioassays show that the secretion acts as a pheromone attractive to conspecific females. The production of a sex pheromone in onychophorans is discussed in relation to their ecology and phylogeny.