3.2
Chemical Signals
Some carnivorous plants emit characteristic scents from their trap leaves that can be
detected by humans [10]. The flower-like scents of Drosophyllum lusitanicum [23]
and Sarracenia spp. [74] and the honey-like sweet scents of Drosera indica and the
related species Drosera finlaysoniana [75] and Drosera toyoakensis (Tagawa, personal observation) are particularly strong. The chemicals secreted from the trap
leaves are mainly terpenoids, benzenoids, and aliphatic compounds, which are the
same chemicals as are usually secreted from the flowers and fruits of angiosperms
[76]. Therefore, it has been considered that the chemicals that are secreted from the
trap leaves function to attract insects in the same way as those secreted from the
flowers and fruits.
The relationship between the quantity of secreted chemicals and the efficiency of
prey trapping has been investigated for some carnivorous plant species. Dionaea
muscipula has been shown to secrete more than 60 kinds of chemical substances
from the trap leaves, including terpenes, benzenoids, and aliphatics, most of which
are also secreted from the flowers and fruits [77]. In one experiment, the odor of
Dionaea muscipula effectively attracted starved but not non-starved Drosophila
melanogaster under light conditions, suggesting that this carnivorous plant attracts
insects through “food smell mimicry” [77]. Sarracenia flava and S. leucophylla
secrete 47 kinds of chemical substances, most of which are also secreted from the
flowers and fruits, and the amount of secreted chemicals per unit time was found to
be positively correlated with the number of attracted blowflies [78]. In Nepenthes
rafflesiana var. typica, which has ontogenetic pitcher dimorphism (see Sect. 2), the
composition and quantity of chemicals produced differ between the ground and
aerial pitchers, with the ground pitchers secreting some aliphatics and terpenoids but
being poor in benzenoids and the aerial pitchers secreting larger amounts of odors
and a larger spectrum of volatiles, including some terpenoids and benzenoids that are
usually found in flower scents [42]. In accordance with the scent profiles of these
pitchers, the aerial pitchers have been shown to trap larger numbers of more diverse
prey, including generalist pollinators (e.g., Lepidoptera, Thysanoptera, and Coleoptera), than the lower pitchers, suggesting that the upper pitchers effectively attract
flower-visiting insects by emitting chemical substances that are similar to the flowers
[42]. To confirm the fitness consequences of trapping flower-visiting insects, additional studies are required on the pollinator fauna of focal carnivorous plants – if the
focal plants attract and trap insects that usually visit the flowers but are not pollinators, this will have a positive effect on their fitness.
If pollinator trapping negatively affects the fitness of a carnivorous plant, it will
be adaptive to secrete different chemical substances from the trap leaves and flowers
to attract (or repel) different insect species [12]. El-Sayed et al. [20] showed that in
Drosera auriculata, which produces flowers adjacent to the trap leaves, the trap
leaves are made unattractive to pollinators through the emission of typical chemical
substances. Furthermore, there is no overlap between the chemical substances that
are emitted from the flowers and trap leaves, with the flowers mainly emitting 2
0 -
aminoacetophenone (34% of the total chemicals produced) and 2-phenylethanol
784
K. Tagawa
Chemical Signals
Some carnivorous plants emit characteristic scents from their trap leaves that can be
detected by humans [10]. The flower-like scents of Drosophyllum lusitanicum [23]
and Sarracenia spp. [74] and the honey-like sweet scents of Drosera indica and the
related species Drosera finlaysoniana [75] and Drosera toyoakensis (Tagawa, personal observation) are particularly strong. The chemicals secreted from the trap
leaves are mainly terpenoids, benzenoids, and aliphatic compounds, which are the
same chemicals as are usually secreted from the flowers and fruits of angiosperms
[76]. Therefore, it has been considered that the chemicals that are secreted from the
trap leaves function to attract insects in the same way as those secreted from the
flowers and fruits.
The relationship between the quantity of secreted chemicals and the efficiency of
prey trapping has been investigated for some carnivorous plant species. Dionaea
muscipula has been shown to secrete more than 60 kinds of chemical substances
from the trap leaves, including terpenes, benzenoids, and aliphatics, most of which
are also secreted from the flowers and fruits [77]. In one experiment, the odor of
Dionaea muscipula effectively attracted starved but not non-starved Drosophila
melanogaster under light conditions, suggesting that this carnivorous plant attracts
insects through “food smell mimicry” [77]. Sarracenia flava and S. leucophylla
secrete 47 kinds of chemical substances, most of which are also secreted from the
flowers and fruits, and the amount of secreted chemicals per unit time was found to
be positively correlated with the number of attracted blowflies [78]. In Nepenthes
rafflesiana var. typica, which has ontogenetic pitcher dimorphism (see Sect. 2), the
composition and quantity of chemicals produced differ between the ground and
aerial pitchers, with the ground pitchers secreting some aliphatics and terpenoids but
being poor in benzenoids and the aerial pitchers secreting larger amounts of odors
and a larger spectrum of volatiles, including some terpenoids and benzenoids that are
usually found in flower scents [42]. In accordance with the scent profiles of these
pitchers, the aerial pitchers have been shown to trap larger numbers of more diverse
prey, including generalist pollinators (e.g., Lepidoptera, Thysanoptera, and Coleoptera), than the lower pitchers, suggesting that the upper pitchers effectively attract
flower-visiting insects by emitting chemical substances that are similar to the flowers
[42]. To confirm the fitness consequences of trapping flower-visiting insects, additional studies are required on the pollinator fauna of focal carnivorous plants – if the
focal plants attract and trap insects that usually visit the flowers but are not pollinators, this will have a positive effect on their fitness.
If pollinator trapping negatively affects the fitness of a carnivorous plant, it will
be adaptive to secrete different chemical substances from the trap leaves and flowers
to attract (or repel) different insect species [12]. El-Sayed et al. [20] showed that in
Drosera auriculata, which produces flowers adjacent to the trap leaves, the trap
leaves are made unattractive to pollinators through the emission of typical chemical
substances. Furthermore, there is no overlap between the chemical substances that
are emitted from the flowers and trap leaves, with the flowers mainly emitting 2
0 -
aminoacetophenone (34% of the total chemicals produced) and 2-phenylethanol
784
K. Tagawa