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Harald Tichy and Ewald Gingl
body temperature and water content, temperature and humidity serve as ultimate
ecological factors as well. However, the proximate and ultimate factors governing
preference or avoidance responses to temperature and humidity must not always
be the same. Nonthermal and nonhumidity cues may provide the ultimate ecological value of a temperature or humidity response; some examples are habitat selection, interspecific niche differentiation, predator avoidance, prey occurrence, escape reactions, and migrations (die!, seasonal, spawning), abundance of food, absence of predators and competitors, light intensity, or depth in soil. The associations between proximate orientation and ultimate ecological factors conferring
adaptive value on preference or avoidance behavior immediately lead to an awkward paradox. It is obviously desirable to separate the physical state of an animars environment from its physiological responses to that environment; but unless the nature of the response is recognized and understood, the components of
the thermal and humid environment cannot be specified in a physiologically
meaningful way. A way to resolve this circular problem is by assessing the sensitivity of thermo- and hygroreceptive sensory cells to temperature and humidity
stimuli occurring in the animal's biotope. A comparison of the differentiation of
hygro- and thermoreceptive sense organs in animals with different ecology may
give information on their adaptation to serve that kind of life and the knowledge of
how certain structures of the sense organs vary because of different functional
demands.
The number of animals which have been shown to respond behaviorally to temperature and humidity stimulation is much greater than the number in which hygro- and thermoreceptors have been identified electrophysiologically and investigated by electronmicroscopic techniques. Most of our knowledge on hygro- and
thermoreception is based on studies of a few insects and a spider (reviewed by
Loftus 1978; Altner and Loftus 1985; Tichy and Loftus 1996; Steinbrecht 1998;
Yokohari 1999). The reasons are that in arthropods hygro- and thermoreceptors
can be localized by removing body appendages, and, in addition, they occur together in the same cuticular sensillum. Due to their superficial position they are
acessible to the electrophysiological experiment.
This chapter brings together three lines of study. The first of these is on the fine
structure of hygro- and thermoreceptive sensilla and the second on their electrophysiological properties. The third line refers to ideas about mechanisms of improving sensitivity to temperature stimuli and to the transduction of humidity
stimuli into action potentials.
2 Identification of Hygro- and Thermo receptive
Sensory Cells
The experimental procedure used in most electrophysiological studies on hygroand thermoreceptive sensilla is summarized in Fig. l (Waldow 1970; Loftus 1976;
Yokohari and Tateda 1976; Yokohari 1978; Tichy 1987; Tichy and Loftus 1990).
The insect is immobilized and the antenna is held securely (Fig. lA). One electrode is inserted lengthwise into the tip of the antenna (Fig. lA), and the other into
Harald Tichy and Ewald Gingl
body temperature and water content, temperature and humidity serve as ultimate
ecological factors as well. However, the proximate and ultimate factors governing
preference or avoidance responses to temperature and humidity must not always
be the same. Nonthermal and nonhumidity cues may provide the ultimate ecological value of a temperature or humidity response; some examples are habitat selection, interspecific niche differentiation, predator avoidance, prey occurrence, escape reactions, and migrations (die!, seasonal, spawning), abundance of food, absence of predators and competitors, light intensity, or depth in soil. The associations between proximate orientation and ultimate ecological factors conferring
adaptive value on preference or avoidance behavior immediately lead to an awkward paradox. It is obviously desirable to separate the physical state of an animars environment from its physiological responses to that environment; but unless the nature of the response is recognized and understood, the components of
the thermal and humid environment cannot be specified in a physiologically
meaningful way. A way to resolve this circular problem is by assessing the sensitivity of thermo- and hygroreceptive sensory cells to temperature and humidity
stimuli occurring in the animal's biotope. A comparison of the differentiation of
hygro- and thermoreceptive sense organs in animals with different ecology may
give information on their adaptation to serve that kind of life and the knowledge of
how certain structures of the sense organs vary because of different functional
demands.
The number of animals which have been shown to respond behaviorally to temperature and humidity stimulation is much greater than the number in which hygro- and thermoreceptors have been identified electrophysiologically and investigated by electronmicroscopic techniques. Most of our knowledge on hygro- and
thermoreception is based on studies of a few insects and a spider (reviewed by
Loftus 1978; Altner and Loftus 1985; Tichy and Loftus 1996; Steinbrecht 1998;
Yokohari 1999). The reasons are that in arthropods hygro- and thermoreceptors
can be localized by removing body appendages, and, in addition, they occur together in the same cuticular sensillum. Due to their superficial position they are
acessible to the electrophysiological experiment.
This chapter brings together three lines of study. The first of these is on the fine
structure of hygro- and thermoreceptive sensilla and the second on their electrophysiological properties. The third line refers to ideas about mechanisms of improving sensitivity to temperature stimuli and to the transduction of humidity
stimuli into action potentials.
2 Identification of Hygro- and Thermo receptive
Sensory Cells
The experimental procedure used in most electrophysiological studies on hygroand thermoreceptive sensilla is summarized in Fig. l (Waldow 1970; Loftus 1976;
Yokohari and Tateda 1976; Yokohari 1978; Tichy 1987; Tichy and Loftus 1990).
The insect is immobilized and the antenna is held securely (Fig. lA). One electrode is inserted lengthwise into the tip of the antenna (Fig. lA), and the other into