240
Stefan Schulz
a low vapor pressure. Most salts are, for example, not volatile, but apolar compounds
with a relatively high molecular weight may still be volatile enough to be used in
chemical communication systems.
Arguably, the most volatile pheromone component identified so far is ethyl acetate, which is a constituent of the male sex pheromone of the mediterranean fruit
fly, Ceratitis capitata (Jang et a!. 1994). Its boiling point of noc and the vapor
pressure of 76 Torr at room temperature indicate that the substance easily evaporates. If a reservoir (e.g., a gland or surface) containing 1 is exposed, most of the
molecules will evaporate immediately, giving a short signal of high intensity. A
high mean emission rate of the pheromone, up to 11.4 ng min- 1 per fly, has been
measured (Flath et a!. 1993). The flies assemble in small groups or leks for calling, and within this group, every male calls only for short periods. This may reflect the fast evaporation of the compound. To maintain a steady evaporation of
a substance, either a biosynthetic process that matches the evaporation rate of the
pheromone (which must be very fast in the case of 1), or a reservoir that contains
the pheromone is required. Such a reservoir, e.g., a gland, can be filled either by a
continuous biosynthetic process or by uptake with food, when food components are
used as pheromones. The range of a very volatile signal is relatively small, because
the high diffusion coefficient of small molecules leads to rapid dilution in air below
the threshold level of a given receptor. In addition, the concentration of low molecular weight compounds in the atmosphere is relatively high (Kesselmeier and Staudt
1999), because of their high volatility. The possible structural variability rises exponentially with an increasing number of atoms in the molecule. Both effects favor
larger molecules for signal transmission.
If the vapor pressure decreases with increasing size or polarity of the molecule,
the evaporation rate is reduced, and the signal can be given off for a longer period of
time, because the time needed for biosynthesis or translocation now is a less limiting
factor. Larger molecules also diffuse more slowly, and thus the signal stays longer
in the environment. At the other end of the vapor pressure scale of pheromones is
nonacosane (2), which is used as a pheromone component by the solitary digger bee,
Andrena nigroaenea (Schiestl et al. 1999).1t has a vapor pressure of 1.33 X 1 o-4 Torr
at 105°C (calculated to be 5.0 x 10- 10 Torr at room temperature!) and a boiling point
of714°C (Piacente eta!. 1991). Such compounds will be given off in small amounts
only. They might be suitable to be carried openly all the time, e.g., on the cuticle.
The loss of material by evaporation is so low that it can be substituted without the
need for accelerated biosynthesis. On the other hand, the low concentration in the air
requires highly sensitive receptors and prohibits action over long distances, because
of diffusional dilution. The active range of 2 has been estimated to be about 0.3 m
(M. Ayasse, pers. comm.). Many flies use cuticular pheromones, which evaporate
very slowly (Howard 1993; Nelson and Blomquist 1995), depending on their size.
As an example, the housefly Musca domestica carries its sex pheromone, (Z)-9tricosene (3), openly within its normal cuticular lipids (Carlson et a!. 1971 ).
The well-known moth sex pheromones given off by females to attract males are
of intermediate volatility. About 80-90% of the species investigated so far use long
Stefan Schulz
a low vapor pressure. Most salts are, for example, not volatile, but apolar compounds
with a relatively high molecular weight may still be volatile enough to be used in
chemical communication systems.
Arguably, the most volatile pheromone component identified so far is ethyl acetate, which is a constituent of the male sex pheromone of the mediterranean fruit
fly, Ceratitis capitata (Jang et a!. 1994). Its boiling point of noc and the vapor
pressure of 76 Torr at room temperature indicate that the substance easily evaporates. If a reservoir (e.g., a gland or surface) containing 1 is exposed, most of the
molecules will evaporate immediately, giving a short signal of high intensity. A
high mean emission rate of the pheromone, up to 11.4 ng min- 1 per fly, has been
measured (Flath et a!. 1993). The flies assemble in small groups or leks for calling, and within this group, every male calls only for short periods. This may reflect the fast evaporation of the compound. To maintain a steady evaporation of
a substance, either a biosynthetic process that matches the evaporation rate of the
pheromone (which must be very fast in the case of 1), or a reservoir that contains
the pheromone is required. Such a reservoir, e.g., a gland, can be filled either by a
continuous biosynthetic process or by uptake with food, when food components are
used as pheromones. The range of a very volatile signal is relatively small, because
the high diffusion coefficient of small molecules leads to rapid dilution in air below
the threshold level of a given receptor. In addition, the concentration of low molecular weight compounds in the atmosphere is relatively high (Kesselmeier and Staudt
1999), because of their high volatility. The possible structural variability rises exponentially with an increasing number of atoms in the molecule. Both effects favor
larger molecules for signal transmission.
If the vapor pressure decreases with increasing size or polarity of the molecule,
the evaporation rate is reduced, and the signal can be given off for a longer period of
time, because the time needed for biosynthesis or translocation now is a less limiting
factor. Larger molecules also diffuse more slowly, and thus the signal stays longer
in the environment. At the other end of the vapor pressure scale of pheromones is
nonacosane (2), which is used as a pheromone component by the solitary digger bee,
Andrena nigroaenea (Schiestl et al. 1999).1t has a vapor pressure of 1.33 X 1 o-4 Torr
at 105°C (calculated to be 5.0 x 10- 10 Torr at room temperature!) and a boiling point
of714°C (Piacente eta!. 1991). Such compounds will be given off in small amounts
only. They might be suitable to be carried openly all the time, e.g., on the cuticle.
The loss of material by evaporation is so low that it can be substituted without the
need for accelerated biosynthesis. On the other hand, the low concentration in the air
requires highly sensitive receptors and prohibits action over long distances, because
of diffusional dilution. The active range of 2 has been estimated to be about 0.3 m
(M. Ayasse, pers. comm.). Many flies use cuticular pheromones, which evaporate
very slowly (Howard 1993; Nelson and Blomquist 1995), depending on their size.
As an example, the housefly Musca domestica carries its sex pheromone, (Z)-9tricosene (3), openly within its normal cuticular lipids (Carlson et a!. 1971 ).
The well-known moth sex pheromones given off by females to attract males are
of intermediate volatility. About 80-90% of the species investigated so far use long