Problems in Hygro- and Thermoreception
275
upward discharge frequency (F) increases. Arrow pointing downward discharge frequency
(F) decreases. Slash, value remains constant. Read as follows, e.g., line 2: when T increases
and Pw is constant, Hr decreases. Then the discharge frequency of the cold cell decreases,
the discharge frequency of the moist cell decreases, and the discharge frequency of the dry
cell does not change
The cell could also be acting as a cold cell in its responses to changes in Pw.
Changes in Pw of a hygroscopic surface would be expected to produce changes in
T. When Pw is reduced some of the water bound to the surface will evaporate;
evaporation of water will cool the sensillum. When Pw increases, the binding of
water will cause warming. A decrease in Pw will increase impulse frequency of a
cold cell (Fig. IF 4) and an increase in Pw will decrease it in a cold cell (Fig. IF
3). The responses of the cell to changes in Pw can be interpreted as responses to
changes in enthalpic temperature. This cell is a cold cell.
The third cell exhibits an increase in F when Pw is decreased (Fig. 1 F 4) and a
decrease in F when Pw is increased (Fig. IF 3). These changes in impulse frequency are in the same direction as those of the cold cell. However, the response
of the cold cell is transitory, lasting less than a second. The response of the third
cell to the same stimulus rises rapidly to a maximum and then slowly levels off
during time periods of many seconds. This response is opposite to that of the
moist cells. The third cell is therefore designated as a dry cell.
The cold, the moist and the dry cells respond clearly to changes in Pw, the cold
and the moist cells to changes in T as well. These responses are important because
insects are generally exposed to with changes in both T and Pw as they range
about. Hr is changed by both T and Pw, and the cold and the moist cell respond to
both. However, they respond differently to both parameters (Fig. IF). Thus the
ambiguity in the response could be eliminated by taking into consideration the responses of all three cells. However, next to nothing is known about central processing of sensory information on ambient humidity and temperature.
3 Thermoreceptor Sensitivity: Significance of Dendritic
Membrane Area and Dendritic Tip Position
Although the number of arthropod hygro- and thermoreceptive sensilla examined
for both their fine structure and physiology is quite small, considerable structural
differences are emerging which have a bearing on the response to temperature.
Before discussing structure-function relationships, the internal structures of insect
and spider sensilla must be elucidated (forreview see Tichy and Loftus 1996).
Hygro-thermoreceptive sensilla occur as two morphological types. Those with
an unperforated cuticular wall form type 1 (Figs. 2A-C, 3A-E); they occur in insects. Type 2 sensilla have a pore opening at the tip and are known from a spider
(Figs. 2D-F, 3F). Both types of sensilla contain an antagonistic pair of hygroreceptors together with a thermoreceptor. The thermoreceptor is a cold cell in insects and a warm cell in the spider.
275
upward discharge frequency (F) increases. Arrow pointing downward discharge frequency
(F) decreases. Slash, value remains constant. Read as follows, e.g., line 2: when T increases
and Pw is constant, Hr decreases. Then the discharge frequency of the cold cell decreases,
the discharge frequency of the moist cell decreases, and the discharge frequency of the dry
cell does not change
The cell could also be acting as a cold cell in its responses to changes in Pw.
Changes in Pw of a hygroscopic surface would be expected to produce changes in
T. When Pw is reduced some of the water bound to the surface will evaporate;
evaporation of water will cool the sensillum. When Pw increases, the binding of
water will cause warming. A decrease in Pw will increase impulse frequency of a
cold cell (Fig. IF 4) and an increase in Pw will decrease it in a cold cell (Fig. IF
3). The responses of the cell to changes in Pw can be interpreted as responses to
changes in enthalpic temperature. This cell is a cold cell.
The third cell exhibits an increase in F when Pw is decreased (Fig. 1 F 4) and a
decrease in F when Pw is increased (Fig. IF 3). These changes in impulse frequency are in the same direction as those of the cold cell. However, the response
of the cold cell is transitory, lasting less than a second. The response of the third
cell to the same stimulus rises rapidly to a maximum and then slowly levels off
during time periods of many seconds. This response is opposite to that of the
moist cells. The third cell is therefore designated as a dry cell.
The cold, the moist and the dry cells respond clearly to changes in Pw, the cold
and the moist cells to changes in T as well. These responses are important because
insects are generally exposed to with changes in both T and Pw as they range
about. Hr is changed by both T and Pw, and the cold and the moist cell respond to
both. However, they respond differently to both parameters (Fig. IF). Thus the
ambiguity in the response could be eliminated by taking into consideration the responses of all three cells. However, next to nothing is known about central processing of sensory information on ambient humidity and temperature.
3 Thermoreceptor Sensitivity: Significance of Dendritic
Membrane Area and Dendritic Tip Position
Although the number of arthropod hygro- and thermoreceptive sensilla examined
for both their fine structure and physiology is quite small, considerable structural
differences are emerging which have a bearing on the response to temperature.
Before discussing structure-function relationships, the internal structures of insect
and spider sensilla must be elucidated (forreview see Tichy and Loftus 1996).
Hygro-thermoreceptive sensilla occur as two morphological types. Those with
an unperforated cuticular wall form type 1 (Figs. 2A-C, 3A-E); they occur in insects. Type 2 sensilla have a pore opening at the tip and are known from a spider
(Figs. 2D-F, 3F). Both types of sensilla contain an antagonistic pair of hygroreceptors together with a thermoreceptor. The thermoreceptor is a cold cell in insects and a warm cell in the spider.