Pheromones are a chemical substance or mixture of substances produced by the exocrine glands which trigger physiological or behavioural reactions between individuals in the same species.
Pheromones are chemical smell signals that act in molecular doses at great distances, and are a way that insects communicate. Several types are recognised: sexual, flight, epideictic, or spatial, gregarious, alarm, social regulation, etc. Some trigger specific behaviour such as cycloalexy (defensive circling ) which is seen in Chrysomelidae amongst others.
These were the first to have been studied in most detail, and have led to practical applications in agriculture. "Pheromone traps" are used which contain an attractive synthetic analogue of a natural female scavenger pheromone and a system to trap the males. They are currently used to combat certain Lepidoptera, either as an agricultural warning system, or to produce sexual confusion.
The chemoreceptors are mostly antennae with sensory or sentient organs.