A specialized type of chemical signal used for communication among conspecifics (individuals of the same species) is called a pheromone. Individual animals secrete pheromone molecules into the environment, triggering behavioral responses in other individuals of the species. Pheromones may communicate alarm signals, mark food trails, or define territories, among many other uses. Their function in mating and mate attraction especially illustrates the remarkable sensitivity of chemosensory systems, and the silkworm moth Bombyx mori is an extensively studied example.

To attract a mate, the female silkworm moth releases a pheromone called bombykol from a gland at the tip of her abdomen (Figure 45.4A). The male silkworm moth has about 10,000 receptors for this molecule on each of his feathery antennae (Figure 45.4B). A single molecule of bombykol may be sufficient to generate action potentials in the antennal nerve, which transmits the signal to the male’s CNS. The extreme sensitivity of the male bombykol receptors ensures that the sexual message sent by a female moth is likely to reach any male within a huge downwind area. The rate of firing in the male’s sensory nerves is proportional to the concentration of bombykol in the air, so by flying upwind he can follow an airborne concentration gradient to “home in” on the signaling female.

Question

Bombykol has the function of attracting mates from a distance, so it has to diffuse widely. A territory-marking pheromone serves to indicate a specific place and therefore should not diffuse widely, so it would have a lower diffusion coefficient.