| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Faculty of Life Sciences, University of Vienna, Vienna, Austria
Submitted 10 November 2004; accepted in final form 18 January 2005
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
In insects, ticks, and spiders, thermoreceptors are associated with cuticular sensilla, often in the form of sensory pegs or hairs that are located on the antennae and legs; this simplifies accessibility and identification (Altner and Loftus 1985
; Altner and Prillinger 1980; Loftus 1978; Tichy and Gingl 2001). In most thermoreceptive sensilla, cold cells are found in combination with hygro- or chemoreceptors (Altner and Loftus 1985; Tichy and Gingl 2001). A warm and a cold cell in a single sensillum is the least common combination (Altner and Loftus 1985; Loftus 1978). One explanation is that such unimodal thermoreceptive sensilla are fewer in number than bimodal cold-receptive sensilla; another potential explanation is that the unimodal thermoreceptive sensilla are so delicate that the warm cell inside tends to escape sampling with the usual electrophysiological techniques.
Davis and Sokolove (1975), however, successfully recorded from a warm and a cold cell with the same electrode. They chose the peg-in-pit sensilla on the antennal tip of the mosquito Aedes aegypti for anatomical reasons. The fine structure of these sensilla, which was described in detail by McIver (1973), suggested a thermoreceptive function. Although the combination of two antagonistic thermoreceptors in a single sensillum has subsequently been found on the larva of the cave beetle Speophyes lucidulus (Loftus and Corbière-Tichané 1981) and the tropical bont tick Amblyomma variegatum (Hess and Loftus 1984), the mosquito warm and cold cells remain the most fully documented pair.
One important observation of Davis and Sokolove (1975) concerned the differential sensitivity to rapid step-like changes in ambient temperature. As might be expected, the greater the change in temperature, the greater the magnitude of the response. The curves approximating these functions tended to be steeper where the temperature steps were smaller and to flatten as they became greater, indicating an increase in differential sensitivity as the step size decreases, with due consideration given the sign. Probably sensitivity will increase even further when air temperature changes slowly and continuously. In contrast to rapid step-like temperature changes, gradual changes in air temperature seemed a more natural form of stimulation, especially because the warm and cold cells of other arthropods are known to respond to slow and continuous changes in temperature (Gingl and Tichy 2001).
Contrary to the high sensitivity to slight changes in air temperature originally observed, Davis and Sokolove (1975) reported that infrared radiation from the tungsten filament of a microscope lamp was ineffective in eliciting responses of the warm and cold cells. The failure to respond to such infrared radiation may be explained by structural features of the peg-in-pit sensillum. The peg, which encloses the tips of the thermoreceptive cells, is located at the bottom of a pit and visible only through a small opening at the top (McIver 1973). This reduces the surface area exposed to infrared radiation. The situation is somewhat similar to the peg-shaped sensilla of the cockroach Periplaneta americana and the locust Locusta migratoria. Note that the cold cells innervating these peg-shaped sensilla have been reported to respond to infrared radiation (Gingl and Tichy 2001), although the amount of radiant heat required to modulate the discharge rates was very high in these cases (50–80 mW cm–2). Moreover, small step-like changes in radiant heat were ineffective in eliciting a response, and large steps produced a slow and slight change in the discharge rate; this suggests a slow heat transfer due to the cumulative absorption of thermal energy by the irradiated sensillum. Thus the time constant for the heating process is large for infrared radiation. For this reason, the warm and cold cells of the mosquito will be stimulated with slow and continuous changes in radiant heat, as was done with the warm cell of the tick I. ricinus and the cold cells of the locust L. migratoria and the cockroach P. americana (Gingl and Tichy 2001).
The present study investigates the peripheral neural representation of temperature stimuli in the responses of the warm and cold cell innervating the peg-in-pit sensilla on the antennal tip of A. aegypti, an ecologically and economically important mosquito. We also address the extent to which structural features of these sensilla may be responsible for the relationship between the transfer of convective or radiant heat and the change in sensillum temperature.
|
METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
Eggs of the yellow fever mosquito A. aegypti were provided by the Swiss Tropical Institute in Basel. The eggs, larvae, and pupae were kept in demineralized water; larvae were fed with yeast and Tetramin fish food. Adults were allowed to emerge into a gauze cage, where they had access to sugar water. The container in which the mosquitoes were reared and maintained was kept at 27°C, 80% relative humidity, with a 12-h scotophase period delimited by sudden transitions from dark to light.
Electrophysiology
Preparation and recordings were made according to the method of Davis and Sokolove (1975). Female mosquitoes were anesthetized with CO2 and immobilized on a Plexiglas stand with adhesive tape. Impulses were recorded between two electrolytically sharpened tungsten electrodes, one inserted into the cuticle between the pair of thermoreceptive sensilla at the tip of the antenna and the other just proximal to the sensilla into the same antenna. After amplification, band-pass (0.1–3 kHz) filtered signals were displayed on a storage oscilloscope, passed through a 1401plus AD-converter (Cambridge Electronic Design; 12 bit, 10 kHz) and fed into a PC for on-line recording. Data were stored on a hard disk and analyzed off-line using Spike 2 software (Cambridge Electronic Design).
Convection
Continuous changes in temperature were applied by an air stream emerging at 2 ms–1 from a 7-mm nozzle. The air stream flow rate was controlled by passing it through a mass flow meter (Rotameter). The temperature of the air stream was varied by slowly rising and lowering the temperature of the water-to-air heat exchanger. Single oscillation periods took from 100 to 800 s.
Rapid step-like temperature changes were produced by switching from a constant-temperature air stream moving at 2 ms–1 to another at different temperature and then back to the initial air stream. Electromagnets were used for the switching. The transition from one air stream to the other did not last much longer than the 15–20 ms needed for the electromagnets to substitute one air stream for another. Transition time was measured by a photoelectric sensor.
The temperature of the air stream was measured within ±0.03°C by a small thermistor (250 x 400 µm; Fenwall Electronics, BC 32 L1) 1 mm downstream from the sensillum. When one air stream replaced another, the time course of the temperature of the thermistor (as indicated by the voltage output) resembled an exponential function as the temperature of the second air stream was approached. The time required to cover half the difference in temperature between the two air streams after switching was 
125 ms; temperature values of the second air stream were reached in 400 ms. For rapid temperature changes, the difference in temperature measured just before switching the air streams and 500 ms afterward was taken as step size.
Infrared radiation
Slowly oscillating changes in radiation power at periods between 100 and 800 s were provided by varying the voltage to an Oriel IR (infrared) element (type 6580, wavelength of 1–25 µm). The IR source was placed vertically 40 cm above the preparation. The beam was reflected onto the thermoreceptive sensillum and concentrated into a 1.5-cm spot by a concave first-surface mirror. Radiation power was measured within ±2% by an IR-thermocouple (Omega OS 36) inserted into the IR light spot near the preparation. The output signal of the IR-thermocouple was substituted for temperature in the Stefan-Boltzmann equation (Elsner 1974) to calculate radiation power (for details, see Gingl and Tichy 2001).
Impulse frequency
Impulse frequency (imp s–1) was calculated from running averages of three consecutive 4-s periods (Corbière-Tichané and Loftus 1983). A 4-s period was used rather than the more common 1-s period because the low rate of temperature change was reflected in a slow change in the discharge rate of the cells.
Differential sensitivity and resolving power
The differential sensitivity is the mean change in impulse frequency per unit change in the stimulus magnitude. This quantity is given by the slope of the function that approximates the relation between stimulus parameters and response. Approximation was done with a regression plane using the method of least squares.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
1 µm diameter. Thus only the tip of the peg is visible from outside (Fig. 1). When the electrode was inserted into the wall between the two sensilla, most recordings revealed the activity of both the warm and the cold cell, distinguishable by the amplitude and form of their impulses. For example, a rapid temperature change of an air stream (initially 27°C, then replaced by 1 of 33°C) caused the warm cell to respond with a sharp rise in frequency. The cell also responded to a sudden temperature drop, brought about by the reverse shift in air streams, by ceasing activity for 1 s. The same step-like cooling sharply increased the frequency of the cold cell. Step-like warming interrupted its discharge briefly (Fig. 2).
|
|
40 mW cm–2.
|
) is much smaller than the thermistor (250 x 400 µm), the time course of the temperature within the sensillum is presumably much shorter. Thus there is no correspondence of the discharge rates of the warm and cold cells to the temperature values as indicated by the thermistor. Instantaneous temperature values of the thermistor can only be applied to the thermoreceptive sites when the temperature of the air stream directed at the preparation is constant or the rate of change is low. In such cases, the temperature of both the thermistor and the sensillum can be considered as locked to that of the air stream coursing over them at 2 ms–1, in excess of 105 diameters of the pit opening per second (Gingl and Tichy 2001). Slowly changing convective heat
The warm and cold cells were exposed to continually rising and falling temperatures at low rates between –0.02 and +0.02°Cs–1. An effort was made to produce sinusoidal temperature changes. The obvious advantages were the repetition of measurements under nearly identical stimulus conditions and the possibility to describe phase relationships. In general, the frequency of the warm cell tended to be higher at the higher values of instantaneous temperature and lower at the lower values (Fig. 4A). Conversely, the frequency of the cold cell tended to be higher at the lower values of instantaneous temperature and lower at the higher values (Fig. 5A).
|
|
). Impulse frequency, however, did not depended exclusively on instantaneous temperature. Figures 4A and 5A show a phase difference in the oscillations in impulse frequency, and in instantaneous temperature, impulse frequency leads instantaneous temperature. Not only can the same impulse frequency occur at two different instantaneous temperatures within a given oscillation period, but the same instantaneous temperature can be accompanied successively by two different values of impulse frequency. Because the oscillating frequency in both cells is clearly ahead of the oscillating instantaneous temperature, a second stimulus parameter that is also in advance of instantaneous temperature must influence their responses. The rate of temperature change was the obvious candidate. As is the case for the first differential of instantaneous temperature, the rate of temperature change is necessarily ahead of instantaneous temperature when temperature oscillates (see also Figs. 4A and 5A). Many of the small irregularities in the time course of impulse frequency of the warm and cold cells that could be attributed to random deviations find a correspondence in the time course of the rate of temperature change.
Figures 4A and 5A also clearly demonstrate that impulse frequencies of the warm and the cold cells lag behind the rate of change in temperature. Neither instantaneous temperature nor the rate of temperature change alone can adequately explain the impulse frequency of each cell. Together, however, the two parameters largely explain the variation in impulse frequency. To estimate the double dependence on instantaneous temperature and its rate of temperature change, the impulse frequency of the warm and cold cells was plotted in Figs. 6, A and B, as a function of both parameters. The frequency curves of both cells approached closed curves reminiscent of Lissajous figures in which two oscillating magnitudes are plotted one as a function of the other. The figures underline the preceding conclusions, namely that where a single value of instantaneous temperature accompanies two different values of the rate of temperature change, then two different values of impulse frequency occur, and that the sequence of frequency values is too orderly to simply attribute frequency differences at any given instantaneous temperature to random variation in response.
|
Note, however, that other parameters should not be excluded in explaining the shape of the Lissajous figure. These would involve a gradual change in sensitivity independently of temperature changes. In this case, the closed frequency curve would become increasingly flatter, even spiralling down toward a line where the frequency is zero. An effort was made to repeat oscillating changes in temperature as a control. For a given cell, the series of points at identical oscillations were statistically indistinguishable, without any hint of a systematic reduction in sensitivity.
Not all recordings contained impulses from both the warm and cold cell. Of the 86 units that were tested on 60 animals, 44 were warm cells and 42 cold cells. In eight of these cases, a warm cell and a cold cell could be distinguished easily in the recordings. These 16 cells and 4 additional warm and 4 cold cells, the discharge rates of which continued undiminished after several oscillation cycles, were used for establishing differential sensitivity. The data are summarized in Table 1. The correlations coefficients (r) show a strong linear relationship between impulse frequency, instantaneous temperature and rate of temperature change. For the warm cells, the square of correlation (r2) indicates that in a series of oscillating temperature changes, an average of 94% of the variation in impulse frequency can be explained by a multiple regression; for the cold cell, the value is 92%. When mean r is reduced by its SD, the percentage drops only to 86% for the warm-cell responses and to 82% for the cold-cell responses.
|
Slowly changing radiant heat
All of the 44 warm cells and 42 cold cells studied for their sensitivity to oscillating changes in convective heat were subjected to oscillating changes in radiant heat at rates between +0.5 and –0.5 mW cm–2. The range of radiant heat covered was roughly 35 mW cm–2 between 45 and 80 mW cm–2. Oscillations <45 mW cm–2 hardly affected the discharge rates of the warm and cold cells; values >80 mW cm–2 could not be applied with the technique available.
Figures 4B and 5B show that the impulse frequency of both the warm and cold cells is elicited mainly by instantaneous radiant heat. Frequency values are nearly in step with instantaneous radiant heat but lagged considerably behind the rate of change in radiant heat. Even as oscillation frequency grows, the lag increases. As radiant heat oscillates through a single period, impulse frequency has a single value at the same pair of heat values regardless of its rate of change. A plot of impulse frequency of each cell type with respect to instantaneous radiant heat and its rate of change during a single oscillation illustrates the comparatively strong effect of instantaneous radiant heat (Fig. 6, C and D). From six to eight oscillations were tested on each of the 86 cells. Twelve warm cells and 12 cold cells were used in this study, i.e., those with a firing rate that continued undiminished after at least six oscillations in convective heat and six oscillations in radiant heat. The differential sensitivity of the warm and cold cells to instantaneous radiant heat and its rate of change were determined by the same method used for convective heat.
Table 1 summarizes the data used to determine differential sensitivity of the warm and cold cell for instantaneous radiant heat and its rate of change. The correlation coefficient (r) was 0.94 for the warm cell and 0.95 for the cold cell. The value of r2 indicates that in the warm cell 88% of the variance in frequency can be explained by a multiple regression, in the cold cell, 90%. For the warm cell, the mean sensitivity to instantaneous radiant heat was +0.8 imps–1 per mW cm–2, and the mean sensitivity to the rate of change in radiant heat, +2.6 imps–1 per mW cm–2. Thus an increase in impulse frequency by 1 imps–1 can be elicited either by an increase in instantaneous radiant heat of +1.2 mW cm–2 or by a rate of change in radiant heat of +0.3 mW cm–2 s–1. For the cold cells, the mean sensitivity to instantaneous radiant heat was –0.9 imps–1 per mW cm–2 and to the rate of change in radiant heat –2.4 imps–1 per mW cm–2. Producing a 1-imp/s increase in the cold cell requires either a decrease in instantaneous radiant heat of 1.1 mW cm–2 or a rate of change in radiant heat of –0.4 mW cm–2. A dual dependence of the warm and cold cell still exists, but the responses are largely dependent on instantaneous radiant heat and only slightly influenced by the simultaneous effect of the rate of change in radiant heat. Thus the impulse frequencies of both cells indicate—from instant to instant—the actual radiant heat.
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
Physical factors external to the sensory cells, however, could explain the different sensitivities of the thermoreceptive cells to convective and radiant heat. The distal location of the sensillum at the tip of the antenna enhances contact with the ambient air; the small number of these sensilla and the danger of damage, however, necessitate surrounding them by elevated structures. Each peg is therefore positioned at the bottom of a heavily walled pit. Due to the sunken position, only the sensillum tip is exposed to radiation. Heat transfer by radiation is therefore restricted to a small area of the sensillum surface. The curved surface, on the other hand, enhances the thermal effect of convection. The smaller the radius, the greater the convective heat gain per unit area for a given temperature difference between the sensillum surface and the environment. This may explain the relatively high heat transfer efficiency by convection versus radiation and thereby the high sensitivity to temperature changes provided by convection. The antagonistic responses of the warm and cold cells optimally enhance the contrast of fluctuations in ambient temperature by providing excitatory signals for both increases and decreases in temperature.
Comparison of the responses to rapid and slow changes in ambient temperature
Davis and Sokolove (1975) demonstrated that the differential sensitivity of the warm and cold cells to small step-like temperature changes in air temperature (0.2°C) is about three times higher than to large steps (0.8°C). On average, the differential sensitivity for small step changes was +130 imps–1 per °C for the warm cell and –136 imps–1 per °C for the cold cell. Differential sensitivity was defined as the mean change in impulse frequency for each degree of temperature change. Response functions for the rate of temperature change were not determined. Nevertheless, the time course of the thermistor during such temperature changes indicates a transition time of 1 s (Fig. 2. Davis and Sokolove 1975). A step-like change of 0.2°C might therefore result in a rate of 0.2°C s–1.
In the present study, air temperature was changed at much lower rates than described for the step-like temperature changes (Davis and Sokolove 1975). This affects receptor function. Assuming that the response is linearly related throughout the range of rate of change, then the warm and cold cells should develop lower values during temperature oscillations than during temperature steps. For example, if the differential sensitivity as measured during 0.2°C step changes (for the warm cell, +130 imps–1 per °C; for the cold cell, –136 imps–1 per °C) were to persist during slowly oscillating changes (rate of change between –0.02 and +0.02°C s–1), then the response of the warm cell should decrease to +13.0 imps–1 per °Cs–1, that of the cold cells to –13.6 imps–1 per °Cs–1. But differential sensitivity was +380 imps–1 per °Cs–1 for the warm cell and –470 imps–1 per °Cs–1 for the cold cell. If, on the other hand, the differential sensitivity to the rate of temperature change as measured during slow oscillations were to persist during 0.2°C steps, then impulse frequencies of 1300 imps–1 should develop in the warm cell and 1360 imps–1 in the cold cell. But neither conclusion fits the facts. Such high-frequency values were never observed. The differential sensitivity to the rate of temperature change therefore increases with decreasing value of that rate.
Corresponding effects of convective and radiant heat
The differences between the thermoreceptors responses to temperature stimulation brought about by convective and radiant heat can be compared quantitatively. Although the temperature within the sensillum cannot be measured directly with the methods employed, instantaneous temperature values can be assigned to the receptive sites if the temperature of the air stream directed on the antenna is constant or changing slowly. Under these conditions, the temperature of the thermoreceptive cells is locked to that of the air stream and air stream temperature can be used to calibrate the discharge rates of the warm and cold cells on air temperature. Accordingly, the discharge rates will indicate the amount and rate of change in temperature within the sensillum during exposure to infrared radiation. This approach is used to evaluate the thermal effect of infrared radiation on the peg-in-pit sensilla.
Increasing radiant heat by 1.2 mWcm–2 has the same effect on the warm cell's impulse frequency as increasing air temperature by 0.29°C. Both stimuli increase the impulse frequency by 1 imps–1. Radiation power must therefore increase by 4.1 mW cm–2 to change the temperature in the sensillum by 1°C. In the cold cell, a decrease in radiant heat by 1.1 mW cm–2 is as effective in changing the discharge rate by 1 imps–1 as a temperature decrease of 0.26°C. Thus radiation power must be decreased by 4.2 mW cm–2 to decrease temperature by 1°C. The radiation power required to change temperature by 1°C in both types of cells, sign ignored, is similar.
The same procedure with the rate of change in radiant heat shows the following results. In the warm cell, an increase in impulse frequency by 1 imps–1 can be elicited when increasing radiation power at a rate of 0.3 mW cm–2s–1 or increasing temperature at a rate of 0.002°Cs–1. To increase temperature at a rate of 1°Cs–1 requires an increase in radiation power at a rate of 150 mW cm–2s–1. In the cold cell, a decrease in radiation power of 0.4 mW cm–2s–1 results in the same increase in the discharge rate by 1 imps–1 as a decrease in the rate of change in temperature of 0.002°Cs–1. To decrease temperature at a rate of 1°Cs–1, radiation power has to be decreased at a rate of 200 mW cm–2s–1. The values of changes in radiation power that produce changes in temperature at 1°Cs–1, sign ignored, are again similar. The similarities in the low thermal effect of radiant heat on the warm and cold cells may be explained by what these cells have in common, namely the stimulus transferring structures. Thus the sensillum design attenuates the transfer of radiant heat.
Comparison of the differential sensitivity
Similar investigations have been carried out in the warm cell of a long, tapering hair on the tarsi of the forelegs of the tick I. ricinus, similar in structure to that studied on the tropical bont tick Ambylomma variegatum (Gingl and Tichy 2001; Hess and Loftus 1984), the cold cell of a peg-shaped sensillum at the bottom of a pit on the antennal surface of the locust L. migratoria (Ameismeier and Loftus 1988; Gingl and Tichy 2001; Waldow 1970), and the cold cell of a peg-shaped sensillum projecting from the antennal surface of the cockroach P. americana (Gingl and Tichy 2001; Loftus 1968, 1969; Yokohari 1981). Identical methods of stimulation and evaluation facilitate the comparison with the warm and cold cells of the mosquito A. aegypti. For the comparison, absolute values of the differential sensitivity will be utilized because the sign of the slope is no measure of the thermoreceptor's performance.
The differential sensitivity of the mosquito warm and cold cells to instantaneous values of convective or radiant heat is higher than in the cold cell of the locust, the warm cell of the tick and the cold cell of the cockroach (Table 2, A and B). A different picture, however, is revealed by comparing the change in radiant heat that produces the same change in discharge rate as a 1°C change in convective heat: the cockroach cold cell requires a lower value than the mosquito's warm and cold. The latter require values similar to that of the locust's cold cell but higher than the tick's warm cell (Table 2C). Thus the tick's long, slender hair apparently absorbs and transfers radiant heat less effectively than the peg-in-pit sensillum on the tip of the mosquito's antenna. The peg-in-pit sensilla of the mosquito and the locust, on the other hand, are similar effective in taking up infrared radiation, and they are more effective than the peg-shaped sensillum on the cockroach's antenna.
|
Impulse frequency of the warm and cold cell depends on two parameters of the temperature stimulus, the instantaneous temperature and its rate of change. Individual responses are therefore ambiguous. When mosquitoes fly in an ambient temperature gradient, the warm and cold cells are confronted with minute changes in temperature. Their impulse frequencies vary continually and signal tendencies rather than exact values. The frequency of the warm cell is high when temperature is high, but at a given temperature, frequency is even higher when temperature is also rising. Conversely, the frequency of the cold cell is high when temperature is low and even higher when temperature is also falling. Thus the effect of temperature on the response of both cells is reinforced by low rates of change. Strongly fluctuating frequencies could serve as early warning signals of changes in environmental conditions that could cause a deficit in body temperature. If, on the other hand, impulse frequency fluctuates only slightly or slowly creeps up or down, the cue could well be the fact that impulse frequency begins to change at all. In this case, the mosquito can seek an area where impulse frequency does not change as it flies about.
Another interesting observation concerns the responses to slow changes in radiant heat. When radiant heat alternately rises and falls at low rates, the impulse frequencies of the warm and cold cells vary mainly with the amount of radiant heat and reflect, from instant to instant, a succession of temperatures. High frequencies of the warm cell signal high temperatures due to an influence of radiant heat, and high frequencies of the cold cell correspond to low temperatures due to less radiant heat. Both cells, however, respond only above a certain minimum value of radiant heat. This represents a loss of information. This loss is offset by the fact that the responses represent solely temperature and nothing else. The antagonistic responses of the warm and cold cells will therefore allow the mosquito to explore narrow gradients in infrared radiation from all sides and may thus help in locating an area of specific radiant heat.
How precisely does impulse frequency of both types of cells distinguish infrared stimuli? The question is how great must the difference between two infrared stimuli become before the larger of them can be identified. The precision of distinguishing stimulus magnitudes not only depends on differential sensitivity but also on the reliability of the response. The resolving power may be defined as the number of discrete stimulus steps that the impulse frequency is capable of distinguishing within a stimulus range. To estimate the resolving power of the warm and cold cells, the frequency values from single oscillation periods (Fig. 6, C and D) were plotted against instantaneous radiant power and the best straight line approximated (Fig. 7, A and B). An error in determining the best straight line would only increase deviation and yield lower values for resolving power. Above and below this best straight line, another line was plotted that encloses the deviation of the responses throughout the range (Fig. 7, A and B). Such a band reflects the degree of scatter. The resolving power for the range was determined by drawing the maximum number of steps through the space enclosed by the deviations. For both the warm and the cold cell, this value was 2. Thus within the 55 to 70 mW cm–2 stimulus range, the response of the warm cell or the cold cell can identify the larger of two infrared stimuli when they differ by 8 mW cm–2. Figure 8 shows the spatial infrared gradients produced by a blackbody with different temperatures. As indicated by the slopes, the infrared gradient falls off as the temperature of the infrared source drops. Orientation to an infrared source would become more difficult when the temperature of the infrared source approaches ambient temperature. At an ambient temperature of 25°C, for example, a thermoreceptor with a resolving power of 8 mW cm–2 should be able to signal the direction of an infrared source with 40°C at a short distance of a few centimetres. Besides resolving power, the number of receptor cells providing thermal information is important. The antennal tip bears two peg-in-pit sensilla, each containing a pair of a warm and a cold cell. Combining the signals of both warm cells and both cold cells probably improves the ability to detect the direction of an infrared source. In this case, even a 35°C infrared source might be detected at a close distance as long as a sufficient temperature difference is maintained between the infrared source and the environment.
|
|
|
GRANTS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
|
ACKNOWLEDGMENTS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
|
FOOTNOTES |
|---|
Address for reprint requests and other correspondence: H. Tichy, Institute of Zoology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria (E-mail: harald.tichy{at}univie.ac.at)
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
Altner H and Prillinger L. Ultrastructure of invertebrate chemo-, thermo- and hygroreceptors and its functional significance. Int Rev Cytol 67: 69–139, 1980.
Ameismeier F and Loftus R. Response characteristics of cold cell on the antenna of Locusta migratoria L. J Comp Physiol [A] 163: 507–516, 1988.[CrossRef]
Corbiére-Tichané G and Loftus R. Antennal thermal receptors of the cave beetle Speophyes lucidulus Delar. II. Cold receptor response to slowly changing temperature. J Comp Physiol [A] 153: 343–351, 1983.
Davis EE and Sokolove PG. Temperature responses of antennal receptors of the mosquito, Aedes aegypti. J Comp Physiol 96: 223–236, 1975.[CrossRef]
Elsner WG. Grundlagen der Technischen Thermodynamik. Berlin: Akademie, 1974.
Gingl E and Tichy H. Infrared sensitivity of thermoreceptors. J Comp Physiol [A] 187: 467–475, 2001.[Medline]
Hess E and Loftus R. Warm and cold receptors of two sensilla on the foreleg tarsi of the tropical bont tick Amblyomma variegatum. J Comp Physiol [A] 155: 187–195, 1984.
Loftus R. Response of the antennal cold receptor of Periplaneta americana to rapid temperature changes and to steady temperature. Z Vergl Physiol 59: 413–455, 1968.[CrossRef]
Loftus R. Differential thermal components in the response of the antennal cold receptor of Periplaneta americana to slowly changing temperature. Z Vergl Physiol 63: 415–433, 1969.
Loftus R. Peripheral thermal receptors. In: Sensory Ecology: Reviews and Perspectives, edited by Ali MA. New York: Plenum, 1978, pp. 439–466.
Loftus R and Corbiére-Tichané G. Antennal thermal receptors of the cave beetle Speophyes lucidulus Delar in sensilla with a lamellated dendrite. I. Response to sudden temperature change. J Comp Physiol [A] 143: 443–452, 1981.
McIver S. Fine structure of antennal sensilla coeloconica of culicine mosquitoes. Tissue Cell 5: 105–112, 1973.[Medline]
Tichy H and Gingl E. Problems in hygro- and thermoreception. In: Ecology of Sensing, edited by Barth FG and Schmid A. Berlin: Springer, 2001, pp. 271–287.
Waldow U. Elektrophysiologische Untersuchungen an Feuchte-, Trocken- und Kälterezeptoren auf der Antenne der Wanderheuschrecke Locusta. Z Vergl Physiol 69: 249–283, 1970.[CrossRef]
Yokohari F. The sensillum capitulum, an antennal hygro- and thermoreceptive sensillum of the cockroach, Periplaneta americana L. Cell Tissue Res 216: 525–543, 1981.
This article has been cited by other articles:
|
|
 |
|
  H. Tichy, H. Fischer, and E. Gingl Adaptation as a Mechanism for Gain Control in an Insect Thermoreceptor J Neurophysiol, October 1, 2008; 100(4): 2137 - 2144. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
). B: the pit wall of each sensillum extends above the peg and curves slightly inward, leaving a small opening for communication of the peg with the environment
