Karlson and Butenandt (Nobel Prize 1939) defined pheromones as “substances which are secreted to the outside by an individual of the same species, in which they release a specific reaction, for example, a definite behavior or a developmental process.” They distinguished between pheromones acting via olfaction and those acting via oral or ingestive routes. The former produced immediate releasing responses (e.g., initiating and guiding the flight of the male silk worm moth, Bombyx mori, to the female) and the latter delayed endocrine or reproductive effects, such as the caste-determining substances of many social insects.

(Thanks to Juan Weiss for this reference and its interpretation.)