1. Matings in two commonly co-occurring, limnetic species of the genus Brachionus, B. calyciflorus and B. angularis, are completely species-specific. Males initiate mating reactions only when they happen to swim head-on into females of their own species. In these reactions the male maintains contact with the female and simultaneously moves round and round her body until either he loses contact and swims away or he copulates. Females play an entirely passive role throughout the mating process.
2. The male mating reaction is the manifestation of an innate reflex response which is triggered when certain coronal chemoreceptors make contact with, or are exposed to, a sudden, marked increase in concentration of a species-specific chemical substance located only in the females of the species.
3. This substance, which is continually released into the environment by growing females and is extractable from crushed females with water, is: dialysable, ether-insoluble, heat-stable, acid-stable, base-labile, resistant to periodate oxidation, adsorbed on charcoal, amphoteric, and unaffected by trypsin, pepsin, and carboxypeptidase.