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Chemical Communication

 

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.

Pheromones vs. Allelochemicals

Semiochemicals can be divided into two groups based on who “sends” a message and who “receives” it:

  1. 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.
  2. 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.

Pros and Cons of Chemical Communication

Advantages:

  • 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

Disadvantages:

  • Low information content (presence/absence)
  • Not effective in an upwind direction

Allelochemicals can be further subdivided into three groups based on who “benefits” from the message:

  1. Allomones benefit the sender — such as a repellent, or defensive compound (e. g. cyanide) that deters predation.
  2. Kairomones benefit the receiver — such as an odor that a parasite uses to find its host.
  3. Synomones benefit both sender and receiver — such as plant volatiles that attract insect pollinators.

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 Chemoreceptors).  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.

Can you identify which type of signal (pheromone, allomone, kairomone, or synomone) is involved in each of the following cases?

A parasitic fly, Euclytia flavahe, is attracted to the odor of its host.

Ants are able to recognize their nestmates by the taste and smell of hydrocarbons (lipids) found on the surface of the cuticle.

Caterpillars that feed on plants sometimes unwittingly induce those plants to release a defensive compound that attracts parasitoids of the caterpillars.

Caterpillars of Lycaena arion produce a chemical that attracts ants and elicits care-giving behavior. When carried into the ant’s nest and placed among the brood, these caterpillars will eat the ant larvae.

Grape root borer moths
(Vitacea polistiformis)

Male attracted by female’s
sex pheromone.

(click “Play” button for video)

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