It is probably safe to say that insects rely more heavily on chemical signals than on any other form of communication. These signals, often called semiochemicals or infochemicals, serve as a form of "language" that helps to mediate interactions between organisms. Insects may be highly sensitive to low concentrations of these chemicals -- in some cases, a few molecules may be enough to elicit a response.
Pros and Cons of Chemical Communication
Not limited by environmental barriers
Effective over distances and around corners
Effective either day or night
Longer lasting than visual or auditory signals
Metabolically "inexpensive" because only small quantities are needed
Low information content (presence/absence)
Not effective in an upwind direction
Semiochemicals can be divided into two groups based on who "sends" a message and who "receives" it:
Pheromones are chemical signals that carry information from one individual to another member of the same species. These include sex attractants, trail marking compounds, alarm substances, and many other intraspecific messages.
Allelochemicals are signals that travel from one animal to some member of a different species. These include defensive signals such as repellents, compounds used to locate suitable host plants, and a vast array of other substances that regulate interspecific behaviors.
Allelochemicals can be further subdivided into three groups based on who "benefits" from the meassage:
Allomones benefit the sender -- such as a repellent, or defensive compound (e. g. cyanide) that deters predation.
Kairomones benefit the receiver -- such as an odor that a parasite uses to find its host.
Synomones benefit both sender and receiver -- such as plant volatiles that attract insect pollinators.
Grape root borer moths (Vitacea polistiformis). Male attracted by female's sex pheromone.
(click the "Play" button for video)
Insects use their sense of taste or smell to detect the presence of semiochemicals. Specialized receptors may be located anywhere on the body, but are especially common on the feet, antennae, palps, and ovipositor (see Chemoreception). The sense of smell (olfaction) is used for remote chemoreception -- detecting semiochemicals with low molecular weight that are volatile enough to become airborne. The sense of taste (gustation) is used for contact chemoreception -- detecting molecules that adhere to a substrate or to the outside of an insect's body.