Behaviour modifying chemicals are called semiochemicals and pheromones fall in the same category. Pheromones are chemicals which are used for communication among the members of the same species. They form the chemical language of a species. For instance, attractants are pheromones which bind the members of a colony of social insects.
Kairomones are chemicals used to communicate with members of other species. For example, parasites use the host smell for searching and preys use predators’ smell to escape away.
All social insects release alarm pheromones when their colony is attacked. This triggers defensive behaviour in the other members of the colony. Deers emit a special scent when they are attacked by a tiger. Similarly, skin cells of many species of fishes release alarm pheromones when they are damaged by the teeth of predator. The Canadian skunk (Mephitis) emits an extremely foul smelling secretion from the anal glands which not only repels the predator but also sends alarm to other members of species. Snails, sea urchins and earthworms also release alarm pheromones when injured.
Ants and termites release a complex of several pheromones from mandibular glands, anal glands and Dufour’s glands to send alarm and confuse the attacker at the same time. J.W.S. Bradshaw, R. Baker & P.W. Howse of University of Southampton, England isolated 33 volatile components from the head of worker ants, 4 of which were specific alarm pheromones. Alarm pheromones of ants spread to a distance of 6 cm in 13 seconds. Pheromones that spread to long distances elicit alerting behaviour in other members of colony, while short range pheromones serve as attacking and biting behaviour releasers.
When honey bees sting, their sting along with its glands is left on the skin of victim and keeps pumping venom into its body. The sting as well as the dying bee releases alarm and distress pheromones from Nassanoff’s glands to attract and stimulate the other bees to attack the intruder.
Pheromone perceiving structures are located on the antennae of insects in various shapes and sizes and are always connected to the nerve.