Semiochemical: A chemical emitted by a plant or animal that evokes a behavioral or
physiological response in another organism
Allelochemical: When the semiochemical affects an individual of a different species
Pheromone: When the semiochemical affects an individual of the same species
Allomone: An allomone is any chemical substance produced and released by an individual of
one species that affects the behaviour of a member of another species to the benefit of the
originator but not the receiver. Production of allomones is a common form of defense, such as by
plant species against insect herbivores or prey species against predators.
Kairomone: A kairomone is a semiochemical, emitted by an organism, which mediates
interspecific interactions in a way that benefits an individual of another species, which receives
it, without benefitting the emitter. Two main ecological cues are provided by kairomones; they
generally either indicate a food source for the receiver, or give warning of the presence of a
Synomone: benefit both sender and receiver -- such as plant volatiles that attract insect
Hormone: a chemical released by a cell, a gland, or an organ in one part of the body that affects
cells in other parts of the organism
Primer pheromone: Primer pheromones trigger a change of developmental events (in which
they differ from all the other pheromones, which trigger a change in behavior).
Releaser pheromone: cause an alteration in the behavior of the recipient. For example, some
organisms use powerful attractant molecules to attract mates from a distance of two miles or
more. In general, this type of pheromone elicits a rapid response, but is quickly degraded
Chemoreception: the detection of chemical components of the environment.
Gustation: Gustation is analogous to taste. Gustatory stimuli (generally non-volatile
compounds), from foodstuffs or on plant surfaces, are detected in the liquid phase by
chemoreceptors on the mouthparts and feet. Gustatory sensillae require the stimuli to be
present in high concentrations (> 10-5 M thresholds) before chemoreception can occur.
Olfaction: Olfaction is analogous to smell. Olfactory stimuli, such as pheromones and
volatile plant compounds, are detected in the gas phase by chemoreceptors on the
antennae. Olfactory sensillae can detect the stimulus at very low concentrations (10-10 M
Volatility is a property referring to the vapour pressure of a chemical -- that is, the affinity the
chemical has for the gaseous phase at ecologically relevant temperatures. Chemicals that are
highly volatile can be readily sensed in the air.
Stability, in the context of pheromones, refers to the reactivity of a chemical. Pheromones which
do not tend to react or break down are referred to as stable. Stable pheromone signals can be
long-lasting or act over long distances.
Persistence refers to the duration of the pheromone signal. Stable, non-volatile pheromones
which are regularly released produce the most persistent signal.
An arrestant causes an organism to slow down or stop its movements when in contact with
An attractant is a substance which causes the receiving organism to make oriented
movements towards the source of the chemical, whereas a repellent causes an organism to
make movements away from the source.
A stimulant is a substance which causes an organism to make some action, or increase
the frequency of the action -- such as walking, feeding, mating or oviposition --
whereas a deterrent is a substance that prevents or decreases the frequency of these
We’ve already introduced primer and releaser pheromones, and now we can explore these ideas
in greater depth. Remember that primer pheromones are ones that alter or “prime” the
receiver’s physiology for some event, such as bringing about sexual maturity in preparation for
mating. Releaser pheromones are compounds that cause or “release” an immediate behavioral
response in the receiver.
Releaser pheromones can be further categorized into several functional groups:
Aggregation pheromones cause individuals of both sexes to move towards the
source, bringing about a localized increase in population density. Aggregation
pheromones are typically produced by one sex but attractive to both.
Alarm pheromones bring about defensive or escape behaviours.
Epideictic or spacing pheromones cause behaviours which result in increased
spacing between individuals of a species, and are used to reduce intraspecific
competition for a resource.
Recruitment or trail pheromones are most commonly observed in social insects, and
are used to “recruit” nestmates to a food source.
Sex attractants are chemicals which mediate mate location, and are usually released
by one sex (usually female) to “call” to the other. Note that not all sex pheromones are
attractants; some sex pheromones may have a role in courtship, maturation, or mate
acceptance, but the umbrella term “sex pheromone” is often used to refer to sex