VI.l Problems in Hygro- and Thermoreception
Harald Tichy and Ewald Gingl
Institut for Zoology, University ofVienna, 1090 Vienna, Althanstrasse 14, Austria
Abstract
Hygroreceptors have been found only in insects and a spider. They associate in
antagonistic pairs of a moist and a dry cell in the same sensilla with a thermoreceptor. In insect hygro-thermoreceptive sensilla all of the three cells respond to
changes in humidity and two of the three cells respond in addition to changes in
temperature. Assigning the individual cells to a particular modality thus becomes a
problem. Structural features ofthermoreceptors have a bearing on their sensitivity.
The size of the dendritic membrane area is related to the sensitivity to slowly
changing and steady temperatures. It may determine the number of molecular receptors which in turn sets signal-to-noise ratio. Sensitivity to rapid changes in
temperature seems to be increased by positioning the dendritic tips above the surface of the body wall. This is believed to reduce damping of heat transfer. Structural diversity among insect and spider hygro-thermoreceptive sensilla suggests
different transduction mechanisms. Three models for hygroreception are discussed. In mechanical hygrometers activity is initiated by swelling or shrinking m
hygroscopic sensillum structures, whereas in psychrometers the degree of cooling
due to evaporation is used to measure humidity. In electrochemical hygrometers
humidity affects electrolyte concentration outside the dendrites.
Key words Hygro- and thermoreceptor identification, structure-function relationships, humidity transduction models
1 Introduction
The environment is to a large extent determined by the average climate and its
seasonal variations. Superimposed is the capricious weather with frequent shifts in
air temperature, precipitation in the form of rain or snow, and solar radiation. The
ranges of temperature and humidity which are found even within restricted habitats are considerable. Animals are continually challenged to regulate and maintain
functional capacities as ambient temperature and humidity change. It is therefore
not surprising that, in addition to metabolic adaptations, most animals show active
temperature and humidity preference or avoidance responses that enhance or optimize their chances of survival and reproduction. In these animals, the existence
ofhygro- and thermoreceptors is very likely. When an animal exhibits a locomotor response to temperature or humidity stimulation, temperature and humidity
serve as proximate factors in orientation. In the case of behavioral regulation of
Harald Tichy and Ewald Gingl
Institut for Zoology, University ofVienna, 1090 Vienna, Althanstrasse 14, Austria
Abstract
Hygroreceptors have been found only in insects and a spider. They associate in
antagonistic pairs of a moist and a dry cell in the same sensilla with a thermoreceptor. In insect hygro-thermoreceptive sensilla all of the three cells respond to
changes in humidity and two of the three cells respond in addition to changes in
temperature. Assigning the individual cells to a particular modality thus becomes a
problem. Structural features ofthermoreceptors have a bearing on their sensitivity.
The size of the dendritic membrane area is related to the sensitivity to slowly
changing and steady temperatures. It may determine the number of molecular receptors which in turn sets signal-to-noise ratio. Sensitivity to rapid changes in
temperature seems to be increased by positioning the dendritic tips above the surface of the body wall. This is believed to reduce damping of heat transfer. Structural diversity among insect and spider hygro-thermoreceptive sensilla suggests
different transduction mechanisms. Three models for hygroreception are discussed. In mechanical hygrometers activity is initiated by swelling or shrinking m
hygroscopic sensillum structures, whereas in psychrometers the degree of cooling
due to evaporation is used to measure humidity. In electrochemical hygrometers
humidity affects electrolyte concentration outside the dendrites.
Key words Hygro- and thermoreceptor identification, structure-function relationships, humidity transduction models
1 Introduction
The environment is to a large extent determined by the average climate and its
seasonal variations. Superimposed is the capricious weather with frequent shifts in
air temperature, precipitation in the form of rain or snow, and solar radiation. The
ranges of temperature and humidity which are found even within restricted habitats are considerable. Animals are continually challenged to regulate and maintain
functional capacities as ambient temperature and humidity change. It is therefore
not surprising that, in addition to metabolic adaptations, most animals show active
temperature and humidity preference or avoidance responses that enhance or optimize their chances of survival and reproduction. In these animals, the existence
ofhygro- and thermoreceptors is very likely. When an animal exhibits a locomotor response to temperature or humidity stimulation, temperature and humidity
serve as proximate factors in orientation. In the case of behavioral regulation of