What is a pheromone?

The exact definition of a pheromone is an issue that is still debated. A deceptively simple definition is that it is an externally secreted chemical that sends information from one organism to other members of the same species. But organisms secrete an abundance of chemicals. What makes one chemical an odor and another a pheromone? Many say that a chemical must elicit a behavioral response in other organisms to be considered a pheromone. Others say that it must be detected by the vomeronasal organ, a specialized system similar to the olfactory system, found in many animals. Still others say that both must be true. The clearest examples of pheromones are found in insects, where a chemical secreted by the female when she is ready to mate can attract the male of the species from great distances.