Problems in Hygro- and Thermoreception
283
4.2 Psychrometer Model
Psychrometers measure the humidity (or the dryness) of the air by means of
evaporative temperature depression. To determine the depression, two separate
temperature measurements in ambient air are needed, one by a thermometer with a
dry surface and one by a thermometer with a wet surface. The dry-surface thermometer shows the air temperature. The wet-surface thermometer indicates a
lower value by reason of the cooling effect of water evaporating from its surface.
The temperature depression is the difference between the two readings. As absolute humidity decreases and temperature rises, the greater the temperature depression and the greater the power of the air to desiccate. Together, evaporation cooling and air temperature are a quite precise measure for humidity, both absolute
and relative. While the hygroreceptors in the mechanical hygrometer model are
mechanoreceptors and the movement of water can be in either direction, out of the
air into the sensillum (inward) or the reverse (outward), in the psychrometer
model the hygroreceptors are thermoreceptors and the water moves only towards
the outside. Only evaporation occurs. Water from inside will reach the surface
where it evaporates and causes cooling of the sensillum. The question is, where is
the reservoir of water that is evaporated? Interestingly, in the bombycid moth it
was found that after prolonged exposure to dry air the dendrites shorten considerably (Steinbrecht and Muller 1991 ). This is suggested to be due to evaporation
of water to the air. The water may come from the receptor lymph cavity, the dendrites themselves, and the cells surrounding them.
4.3 Electrochemical Hygrometer Model
In the case of the tarsal organ of the spider the electrochemical hygrometer model
has much to offer. It views the concentration of electrolytes in the thin layer of
lymph just inside the sheath and immediately surrounding the dendrites as varying
with humidity. The drier the air, the greater the evaporation rate of water out of
the lymph and the greater the change in electrolyte concentration. The concentration in tum would affect the potential across the dendritic membranes and thereby
govern the responses of the hygroreceptors. As with the psychrometer model, a
hygroscopic material would appear unnecessary. Rather lymph is viewed here as
moving in one direction only, slowly towards the outside, where its water content
is exposed to controlled evaporation in ambient air. The electrochemical hygrometer shares evaporation as a constitutive feature with the psychrometer
model. Concentration of the electrolytes is affected by the humidity of the air. The
drier the air, the greater the evaporation rate of water out of the lymph and the
greater the change in electrolyte concentration.
Is the electrolyte concentration model feasible for insects? It might be, provided
their structures permit a slight and controlled flow of lymph along the dendrites
towards the outside where humidity could affect the concentration of its electrolytes. None of the sensilla in Fig. 3A-E displays a large, "open" apical pore like
the sensilla of the spider tarsal organ. Many insects probably encounter drier air
283
4.2 Psychrometer Model
Psychrometers measure the humidity (or the dryness) of the air by means of
evaporative temperature depression. To determine the depression, two separate
temperature measurements in ambient air are needed, one by a thermometer with a
dry surface and one by a thermometer with a wet surface. The dry-surface thermometer shows the air temperature. The wet-surface thermometer indicates a
lower value by reason of the cooling effect of water evaporating from its surface.
The temperature depression is the difference between the two readings. As absolute humidity decreases and temperature rises, the greater the temperature depression and the greater the power of the air to desiccate. Together, evaporation cooling and air temperature are a quite precise measure for humidity, both absolute
and relative. While the hygroreceptors in the mechanical hygrometer model are
mechanoreceptors and the movement of water can be in either direction, out of the
air into the sensillum (inward) or the reverse (outward), in the psychrometer
model the hygroreceptors are thermoreceptors and the water moves only towards
the outside. Only evaporation occurs. Water from inside will reach the surface
where it evaporates and causes cooling of the sensillum. The question is, where is
the reservoir of water that is evaporated? Interestingly, in the bombycid moth it
was found that after prolonged exposure to dry air the dendrites shorten considerably (Steinbrecht and Muller 1991 ). This is suggested to be due to evaporation
of water to the air. The water may come from the receptor lymph cavity, the dendrites themselves, and the cells surrounding them.
4.3 Electrochemical Hygrometer Model
In the case of the tarsal organ of the spider the electrochemical hygrometer model
has much to offer. It views the concentration of electrolytes in the thin layer of
lymph just inside the sheath and immediately surrounding the dendrites as varying
with humidity. The drier the air, the greater the evaporation rate of water out of
the lymph and the greater the change in electrolyte concentration. The concentration in tum would affect the potential across the dendritic membranes and thereby
govern the responses of the hygroreceptors. As with the psychrometer model, a
hygroscopic material would appear unnecessary. Rather lymph is viewed here as
moving in one direction only, slowly towards the outside, where its water content
is exposed to controlled evaporation in ambient air. The electrochemical hygrometer shares evaporation as a constitutive feature with the psychrometer
model. Concentration of the electrolytes is affected by the humidity of the air. The
drier the air, the greater the evaporation rate of water out of the lymph and the
greater the change in electrolyte concentration.
Is the electrolyte concentration model feasible for insects? It might be, provided
their structures permit a slight and controlled flow of lymph along the dendrites
towards the outside where humidity could affect the concentration of its electrolytes. None of the sensilla in Fig. 3A-E displays a large, "open" apical pore like
the sensilla of the spider tarsal organ. Many insects probably encounter drier air