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Stefan Schulz
the receiver. The vast majority of pheromones identified so far are releasers. Typical
members of this class are sex pheromones of female night-flying moths, which at·
tract conspecific males over long distances. Further examples are alarm pheromones
of ants or aggregation pheromones of bark beetles. A typical primer is (£)·9-oxodec·
2-enoic acid, the so-called queen substance of the honey bee, which inhibits queen
rearing and diminishes the release of juvenile hormone in worker bees, but also acts
as a releaser, being involved in the retinue response (vander Meer eta!. 1998).
Unlike pheromones, allelochemicals are used for communication between different species. They are divided into kairomones, which are advantageous for the
receiver, allomones, which are advantageous for the emitter, and synomones, which
are useful for both sender and receiver. Allomones can be defensive compounds
given off by a species to deter predators, whereas kairomones can be compounds
that are given off by a prey species and that attract a predator. For example, the alarm
pheromone 6-methyl-5-hepten-2-one (sulcatone) of the meat ant Iridomyrmex purpureus is used by the ant-feeding spider Habronestes bradleyi to find its prey (Allan
et al. 1996). This shows that a compound given off by one species can, at the same
time, be a pheromone (releaser) and a kairomone, depending on the situation in
which it is acting. The volatile compounds given off by many flowers are typical
synomones. They attract pollinating insects, which obtain a reward in the form of
pollen or nectar.
A term also used frequently in recent times is infochemical, which is less clearly
defined. It can be used to denote compounds that are used by an organism to gather
information and that happen to be present in the environment, but that are not pro·
duced by the emitter for a specific reason. All these definitions are not very precise
and sometimes cause confusion when used with different meanings.
The number of arthropod species for which the structures and composition of
pheromones have been elucidated reaches about 1500 (see Table 1). Most of the
work has been carried out on specific insect orders, often driven by the desire of
applying pheromones in pest control (e.g., Lepidoptera). Nevertheless, compared
with the nearly 850000 insect species described so far (Daly eta!. 1998), this number
is only a limited sample (0.17%) and may even be biased against certain orders.
General rules derived therefrom have to be treated with caution, and this is even
more true of pheromones from arachnids.
All semiochemicals used in various biological contexts carry by definition information. To use this chemical channel successfully, an organism must employ compounds that are sufficiently specific to overcome the surrounding chemical noise.
This noise is always present, as in other sensory systems, and consists of com·
pounds given off into the environment by living organisms, decaying material, anthropogenic sources, geochemical processes, and so on. Different strategies might
be used to achieve selective signaling. Generally, specificity can be achieved on the
emitter side by producing a specific signal, or on the receiver side by a selective
receiving mechanism. Mixtures of compounds are most often used to obtain a se·
lective signal. They allow greater flexibility in the composition of the signal, often
by means of less specific compounds that are also employed by other species. In
Stefan Schulz
the receiver. The vast majority of pheromones identified so far are releasers. Typical
members of this class are sex pheromones of female night-flying moths, which at·
tract conspecific males over long distances. Further examples are alarm pheromones
of ants or aggregation pheromones of bark beetles. A typical primer is (£)·9-oxodec·
2-enoic acid, the so-called queen substance of the honey bee, which inhibits queen
rearing and diminishes the release of juvenile hormone in worker bees, but also acts
as a releaser, being involved in the retinue response (vander Meer eta!. 1998).
Unlike pheromones, allelochemicals are used for communication between different species. They are divided into kairomones, which are advantageous for the
receiver, allomones, which are advantageous for the emitter, and synomones, which
are useful for both sender and receiver. Allomones can be defensive compounds
given off by a species to deter predators, whereas kairomones can be compounds
that are given off by a prey species and that attract a predator. For example, the alarm
pheromone 6-methyl-5-hepten-2-one (sulcatone) of the meat ant Iridomyrmex purpureus is used by the ant-feeding spider Habronestes bradleyi to find its prey (Allan
et al. 1996). This shows that a compound given off by one species can, at the same
time, be a pheromone (releaser) and a kairomone, depending on the situation in
which it is acting. The volatile compounds given off by many flowers are typical
synomones. They attract pollinating insects, which obtain a reward in the form of
pollen or nectar.
A term also used frequently in recent times is infochemical, which is less clearly
defined. It can be used to denote compounds that are used by an organism to gather
information and that happen to be present in the environment, but that are not pro·
duced by the emitter for a specific reason. All these definitions are not very precise
and sometimes cause confusion when used with different meanings.
The number of arthropod species for which the structures and composition of
pheromones have been elucidated reaches about 1500 (see Table 1). Most of the
work has been carried out on specific insect orders, often driven by the desire of
applying pheromones in pest control (e.g., Lepidoptera). Nevertheless, compared
with the nearly 850000 insect species described so far (Daly eta!. 1998), this number
is only a limited sample (0.17%) and may even be biased against certain orders.
General rules derived therefrom have to be treated with caution, and this is even
more true of pheromones from arachnids.
All semiochemicals used in various biological contexts carry by definition information. To use this chemical channel successfully, an organism must employ compounds that are sufficiently specific to overcome the surrounding chemical noise.
This noise is always present, as in other sensory systems, and consists of com·
pounds given off into the environment by living organisms, decaying material, anthropogenic sources, geochemical processes, and so on. Different strategies might
be used to achieve selective signaling. Generally, specificity can be achieved on the
emitter side by producing a specific signal, or on the receiver side by a selective
receiving mechanism. Mixtures of compounds are most often used to obtain a se·
lective signal. They allow greater flexibility in the composition of the signal, often
by means of less specific compounds that are also employed by other species. In