Definition of Terms

Compounds involved in chemical communication are collectively called semiochemicals (Figure 1). This umbrella term integrates both pheromones, which mediate communication between members of the same species, and allelochemicals, which denote chemicals used for communication between different species. According to their function, pheromones are further subdivided into primers, which change the receiver's physiology (e.g., endocrine or reproductive system in animals), and releasers, which elicit an immediate behavioral response in the receiver. The distinction into primer and releaser pheromones is often blurred, because the same pheromone can have both effects. Furthermore, hormones can modulate neuronal memories for odors themselves and thus have long-term effects on the behavior. These difficulties can be circumvented by viewing primer and releaser pheromones as early or later actors on a temporal continuum of physiological and behavioral effects on the organism. Beyond, pheromones are classified according to their function, such as sex, social, alarm, recruitment pheromones, etc.

In contrast to pheromones, the classification of allelo-chemicals is usually based on a cost-benefit framework: kairomones benefit the receiver only, allomones benefit the emitter only, and synomones benefit both sender and receiver (Figure 1). An allomone, for example, can be a toxic compound, which is used by a species to deter a predator, whereas a typical kairomone is a substance,

Semiochemicals

Pheromone (intraspecific)

Allelochemical (interspecific)

Primer

Releaser

Allomone

Kairomone

Synomone

Figure 1 The taxonomy of chemical communication including the relevant categories and some examples.

which is either actively or passively released by a prey organism and used by a predator to localize its prey. A classical example for synomones is floral odors involved in pollinator attraction: the pollinator benefits from the nectar - pollen and the plant benefits from the transfer of pollen - fertilization. The compounds used for communication are not necessarily produced by the target organism of the receiver, but can result from the activity of associated organisms such as bacteria or fungi.

The classification of a particular compound, however, depends on the ecological context in which it is being considered: the same component can serve multiple purposes. The alarm pheromone 6-methyl-5-hepten-2-one of the meat ant Iridomyrmex purpureus, for example, is used by the ant-feeding spider Habronestes bradleyi to locate its prey. Thus, the compound that serves in one species as a priming pheromone acts at the same time as a kairomone in another one.

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