Pheromones are any chemical signal used to communicate between the members of a species. The existence of pheromones has been studied most thoroughly with respect to insects, but more complex species likely also possess pheromones. Their existence in humans has not been conclusively proved. If pheromone-excreting organs still exist in more complex species, they may be merely vestigal, as members of such species have developed more sophisticated ways to communicate.
As a concrete example of a known pheromones, bees use isopentyl acetate as an alarm signal. "Guard bees" are capable of raising their abdomen and emitting pheromones, beating their wings to transmit the chemical further. In this way, an entire hive of bees can be made quickly aware of an incoming threat, allowing them to act in concert. When bees sting a target, high concentrations of pheromones are deposited along with the venom, encouraging other bees to aid in the attack, stinging the same place. The effects can be devastating.
Ants use pheromone trails to navigate to and from food sources and the nest. If the trail is cut off by a pheromone-free material, for example chalk, the ants will stop and either wander randomly or head in the reverse direction. Before the advent of eyes, pheromones could be used reliably for insect navigation and a variety of signalling.
Charlatans have advertised sales of human pheromones. They have claimed that there exist distinct pheromones for members of each gender which are used to signal each other sexually. This is a myth. Evidence for the existence of active human pheromones is doubtful. In one case, analysis of a product advertised as human pheromone ended up being canine pheremone.
The first pheromone was identified by German scientists in 1956. They worked for over 20 years to isolate it. It was a powerful sexual pheremone used by silkworm moths. Because animals like humans have such complicated interactions with members of their own species, it is difficult to create control groups in experiments designed to detect pheromones. For this reason it could be a while before the presence or absence of active human pheromones is known conclusively.