All forms of life must selectively detect and take in chemicals, and so chemical signaling occurs at many levels in all cells. Hormones operate within an organism, pheromones are signals between conspecifics, and allomones are intended for interspecies communication. In this section we will focus on pheromones.
Two different systems of reception are employed for chemical communication. Airborne and waterborne chemicals received at a distance from their source are detected by olfactory reception, or smell. Other chemicals require contact reception, direct contact of the receiver with the source of the pheromone. Pheromones are usually produced by glands located on the skin, or by ducts within the body that have ducts leading to the body's exterior; the second variety of glands are known as exocrine glands. Mammals produce pheromones in the sudiferous (fluid-producing) gland and the sebaceous (waxy-substance producing) gland. Some mammals have other specialized glands. Insects produce a large variety of pheromones, most notably from their mandibular glands, thoracic glands, and stingers.
Some Important Examples of Chemical Signaling
A good example of a multipurpose pheromone is the Queen substance employed by some eusocial bees. This pheromone motivates and attracts workers, releases swarming, and is a sex pheromone. Absence of queen substance indicates the colony has grown too large (the queen is too far away to smell her) and so workers will build queen cells to rear new queens. An abrupt absence of Queen substance results in emergency queen rearing, since that absence is probably an indication of the queen's death.
Other examples of chemical signaling include alarm pheromones, such as bee stings. Isopentyl acetate, the chemical injected into a sting wound not only serves to wound an enemy, but to alert other bees to danger, and in some case cause swarming. The reason "killer bees" are so deadly is not because they have a more venomous sting, but rather because these bees have a lower threshold for alarm pheromone and so an entire swarm will react and sting the enemy to death. Ants and snakes employ trail pheromones to mark the path to a food source. These chemicals are laid out along a trail, and the next ant will follow the trail by means of contact reception. Many animals, from moths to cats, use pheromones to attract mates.
Visual signals are limited because they require a direct line of sight and lighted conditions, and they only last as long as the sender is signaling. However, studies of communication have overemphasized visual communication, most likely because humans and primates are much more dependent on this type of communication than non-primate animals. We will not spend much time on visual signals because we are already familiar with them from our daily lives. The sender can send a signal by performing a display or by assuming a specific body posture. The receiver views the signal by means of eyes, which the brain translates into a visual image. Visual images are received in real-time, and so are generally dynamic signals.
Visual signals allow for a certain amount of cheating; that is, deceptive signals can lure receivers into responding to the benefit of the sender and the detriment of the receiver. Photuris fireflies are the only predatory species of firefly. By mimicking the female response of the prey species the "femme fatale" Photuris female lures in males, and then preys upon them. Wary males are careful in responding to female displays of their own species for fear of being preyed upon by the Photuris females. In this way, the prey males experience conflicting pressures from natural selection, which demands both individual survival and mating for species survival. This example reveals another problem with visual signals--they are not receiver specific. Any animal can potentially react to the visual signal of any species.