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Some cold receptors in larvae of three Lepidoptera species
J. Insect Phytiof., 1967, Vol. 13, pp. 821 to 826. Pergamm Press Ltd.
Rimted in Great Brituin
SOME COLD RECEPTORS IN LARVAE THREE LEPIDOPTERA SPECIES
OF
L. M. SCHOONHOVEN Department of Entomology, Agricultural University, Wageningen, The Netherlands (Received 23 December 1966)
Abstract-Caterpillars of three different species have three peripheral nerve cells in their third antenna1 segment, which can be stimulated by decreasing temperatures. Increasing temperatures exert inhibitory effects on their spontaneous activity. Probably not more than one cell with this characteristic response is located in the maxillary palp. INTRODUCTION
LITTLE information is available on the physiology of temperature receptors in insects. A rough location of temperature senses was obtained in a number of species by behavioural studies, indicating that in most cases the antennae contain such receptors (WIGGLESWORTHand GILLJXT, 1934; ROTH, 1951; HERAN, 1952; GEBHARDT,1953 ; MAKINGS,1964). In some species temperature reception has been found to be located on the maxillary palpi as well (GEBHARJIT, 1953), on the tarsi (LEES, 1948), or on other parts of the body (HERTJXR,1953). Physiological informatemperature reception is still more restricted. HODG~ON (1956), HODGSONand ROEDER(1956), and GILLARY (1966) found an influence of temperature upon the reaction of chemoreceptors on the labellum of the blowfly. Here the number of spike potentials in some of these receptors decreased when the temperature was raised, while in others it increased. KERKUT and TAYLOR (1957) and DETHIER and ARAB (1958) observed temperature effects on the spike frequency in leg nerves of the cockroach and the blowfly, respectively. The impulse frequency in mechanoreceptors of honey bees is positively correlated with the temperature (Tm, 1963). Also changes in the spontaneous activity within the central nervous system have been recorded, induced by changes of temperature (KERKUT and TAYLOR, 1958). LACHER (1964) presented electrophysiological evidence for the presence of temperature receptors on the antennae of the honey bee. In the present study responses to temperature changes are described from some peripheral nerve cells in the larvae of three Lepidoptera. tion about
MATERIAL AND METHODS Most experiments were performed on last-instar larvae of Dendrolimus pini L. (Lasiocampidae), but Philosamiu Cynthia tini Boisd. (Saturniidae) and Protopmce sexta (Johan.) (Sphingidae) showed essentially the same reactions. The nerve cells 53
821
822
L.M.
SCHOONHOVEN
which have been investigated are located in the second segment of the maxillary palp and the third segment of the antenna (Fig. 1). Movements of the head appendages during the experiments were prevented by cutting the head and
FIG. 1. Ventral view of a caterpillar head. A, third antenna1 segment; P, second segment of maxillary palp.
damaging the nervous system. The recordings from the maxillary palp were made by introducing a tungsten electrode (uninsulated, 1~ tip) into the membrane at the base of the most distal segment. The electrode was moved in several directions until action potentials were recorded. The recordings of the last antenna1 segment were made by inserting it at the base of the largest sensillum basiconicum and pushing the electrode a small distance towards the base of the antenna. Action potentials of several cells were often recorded, and, by moving the electrode, temperature sensitive receptors could be selected. The electrode was connected to an oscilloscope (Tektronix 502) via an a.c. preamplifier (Tektronix 123). Temperature stimuli were presented in two ways. A metal rod 4 mm wide, heated electrically or by flame, or cooled by dipping into a salt-ice mixture or into dry ice, was brought within 3 to 10 mm from the organ. In an alternative method a flow of air from a compressed air system was led via an activated charcoal filter into a plastic tube. This tube subsequently was wound a few times inside a Dewar vessel filled with dry ice. The end was directed towards the preparation. Hot air was obtained by leading the compressed air into a wash-bottle containing hot water. The heated air was saturated and brought to the preparation via a plastic tube. The outlet of the plastic tube was about 10 cm from the insect. The airflow was constant. Beginning and end of stimulation were brought about by changing the direction of the outlet of the tube. When an airstream was used, its temperature was measured by a point-shaped copper-constantan thermocouple, made of wire 125 ~1 wide, mounted about 1 mm from the receptors, and d.c. connected to the second oscilloscope input. When the metal rod was used, the periods of stimulation were indicated on the film records by a change in the second trace of the oscilloscope controlled by a footswitch. Room temperatures fluctuated between 20 and 24°C.
SOME COLD RECEPTORS IN LARVAE OF THREt LEPIDOPlBRA SPECIFS
823
RESULTS
Some nerve cella, located in the maxUary palpi and in the antennae, show a clear reaponae to sudden changes in temperature. Lowering the temperature results in an increase of frequency of their spontaneous or resting activity, whereas an increase in temperature suppresses this activity. Reactions of a maxillary palp cell to a cold as well as to a hot metal rod are shown in Fig. 2. An increase in
A
C
FIG. 2. Reactions of a peripheral nerve cell in the maxillary palp of D. pinito the presence of a cold rod (A), a hot rod (B), and an airstream at room temperature (C). The period of stimulation is marked by the second horizontal trace in each record. The calibration symbol indicates 0.2 mV and 0.2 sec. activity can be observed also when air from a plastic wash-bottle (at room temperature) is blown over the preparation (Fig. 2C). This is presumably due to increased evaporation, resulting in a drop of temperature. In the maxillary palp only one cell at a time reacts in this way, although this segment contains between nineteen and twenty-four nerve cell bodies (SCHOONHOVEN and DETHIER, 1966). Thus probably only one (or a few at the most) shows the characteristic responses. A stream of air does not stimulate the other cells innervating olfactory receptors, but may depress their activity. Activity from several cells at one time is usually monitored when the electrode is moved into the third antenna1 segment, which is innervated by twelve neurons. A few cells, probably not more than three, highly sensitive to temperature can be found here. The record of Fig. 3 shows the stimulation of some cells by the presence of a cold metal rod, while one cell, represented by the smallest spike, does
FIG. 3. Stimulation by a cold rod of some peripheral nerve cells in the antenna of
D. pini. The second line is a continuation of the first line. The calibration symbol indicates 0.5 mV and O-2 sec.
824
L.M.
SCHOONHOVEN
not respond. It is not known whether the former cells can be stimulated by odours. The cells which are not stimulated by a decrease of temperature have been shown to be olfactory receptors. Some of them are not affected by a stream of pure air at room temperature; in others the background activity is slightly decreased. Increase of temperature increases the spontaneous activity somewhat, whereas a temperature decrease shows the opposite effect. Blowing air of different temperatures over the preparation has the advantage that the temperature of the air passing the receptors can be measured with reasonable accuracy by a thermocouple. The results of such an experiment, in which the total frequency of spikes originating in three cells is related to change in temperature, are shown in Fig. 4. This impulse frequency pattern shows a striking
0
,
2
2
4
I
6
7
II
SECONDS
FIG. 4. The total spike frequency in three peripheral nerve cells of the antenna of D. pini in relation with a decreasing temperature (upper trace). The ordinate of the temperature curve indicates the number of degrees Celsius below room temperature (= 22°C).
similarity to the reactions observed in a group of human cold receptors during brief cooling periods (HENSEL and BOMAN, 1960). A phasic response can be seen, followed by a phase of adaptation. During the phasic reaction the spike frequency (as calculated from six experiments) increased 150 to 300 per cent within half a second for each degree Celsius temperature decrease. When the temperature is raised again after a short exposure to cool air, some cells may cease to fire for several seconds, different cells showing different recovery periods. The occurrence of post-excitatory inhibitions of different durations conforms to the characteristics of vertebrate thermoreceptors (DODT, 1952).
SOME COLD RECEPTORS IN LARVAE OF THREE LEPIDOPTERA SPECIES
825
Increasing the temperature has the reverse effect on the spike frequency, though the frequency change is less spectacular than with cooled air. When the temperature drops back to its original level, the spike frequency not only goes back to its normal level, but shows a considerable amount of ‘overshoot’, which gradually levels out again. Thus these cells appear to react more strongly to a decrease in temperature than to an increase. However, a hot metal rod, kept close enough to the preparation, may depress completely all activity present. DISCUSSION
When thermoreception as a possible function of the cells described is to be considered, it must be kept in mind that so far no behavioural evidence is available. Although the sensitivity to temperature changes (which in our experiments were only of short duration) appears to be fairly high, this does not necessarily determine the cells as temperature receptors, as MURRAY(1962) has pointed out. However, the striking fact that in the maxillary palp only one or maybe a very few cells out of about twenty show the typical responses and in the antenna only about three cells out of a total number of about thirty, assigns these cells to a physiologically different type and suggests that they are concerned with sending information about ambient temperatures to the central nervous system. The quickness of the adaptation (Fig. 4) may indicate that their function is to register changes in temperature rather than to record constant environmental temperatures. Also the fact that many insects do have temperature receptors on the antennae (and some also on the maxillae) supports the idea that we are dealing here with true thermoreceptors, although the larval structures in this case possess only very few sense cells as compared with those of the insect species on which behavioural evidence was obtained. Theoretically, a sense cell may very well serve a dual purpose and transfer information about temperature as well as, for instance, the chemical environment or mechanical stimulation (MURRAY,1962; HODGSON,1965). The central nervous system could then decode this information in combination with the input from other receptors. Thus, assuming the existence of true temperature receptors, there is no need for the presence of special, morphologically distinct structures. However, one may speculate that the small sensillum styloconicum, found on the third antenna1 segment next to the three sensilla basiconica and which is innervated by three nerve cells (SCHOONHO~ENand DETHIER, 1966), might house the temperature sense. Because of the fact that in the maxillary palp we never hit more than one temperature-sensitive cell at the same time, one or more of the four small sensilla campaniformia (each innervated by one cell) seem to be a more likely location for a temperature receptor than the sensilla basiconica, which are usually innervated by more than one cell. In conclusion it can be said that, although their exact location is still uncertain, the fact that the temperature characteristics of the cells discussed differ so remarkably from the other receptor cells present suggests strongly that they do play a r6le in informing the caterpillar about temperature conditions around the head region.
826
L. M. SCHOONHOVEN
Acktwwk&ements-The author is indebted to Dr. V. G. DETHIER, Dr. R. W. MU-Y, and Dr. J. DE WILDE for reading and helpful discussion of the manuscript and to Mr. J. E. MOORHOUSE for correcting the English text. REFERENCES DETHIERV. G. and ARABY. M. (1958) Effect of temperature on the contact chemoreceptors of the blowfly. J. Insect Physiol. 2, 153-161. DODTE. (1952) The behaviour of thermoreceptors at low and high temperatures with special reference to Ebbecke’s temperature phenomena. Acta physiol. stand. 27, 295-314. GEBHARDTH. (1953) Die Lage der wichtigsten Thermorezeptoren bei einigen Insekten. Zool. Jb. (Allg.) 63, 558-592. GILLARY H. L. (1966) Stimulation of the salt receptor of the blowfly-II. Temperature. J. gen. Physiol. 50, 351-357. HENSEL H. and BOMANK. K. A. (1960) Afferent impulses in cutaneous sensory nerves in human subjects. J. Neurophysiol. 23, 564-568. HERAN H. (1952) Untersuchungen iiber den Temperatursinn der Honigbiene (Apis mellifica) unter besonderer Beriicksichtigung der Wahrnehmung strahlender Wilrme. Z. vergl. Physiol. 34, 179-206. HERTERK. (1953) De-r Temperatursinn der Insehten. Duncker und Humblot, Berlin. HODGSONE. S. (1956) Temperature sensitivity of primary chemoreceptors of insects. Anat. Rec. 125,560-561. HODGSONE. S. (1965) The chemical senses and changing viewpoints in sensory physiology. V&points in Biology 4,83-124. HODGSONE. S. and ROEDERK. D. (1956) Electrophysiological studies of arthropod chemoreception-I. General properties of the labellar chemoreceptors of Diptera. g. cell. camp. Physiol. 48, 51-76. KERKUT G. A. and TAYLOR B. J. R. (1957) A temperature receptor in the tarsus of the cockroach, Periplaneta americana. J. exp. Biol. 34486-493. KERKUTG. A. and TAYLORB. J. R. (1958) The effect of temperature changes on the activity of poikilotherms. Behaviour 13, 259-279. LACHER V. (1964) Elektrophysiologische Untersuchungen an einzelnen Rezeptoren fiir Geruch, Kohlendioxyd, Luftfeuchtigkeit und Temperatur auf den Antennen der Arbeitsbiene und der Drohne (Apis mellifia L.) Z. vergl. Physiol. 48, 587-623. LEES A. D. (1948) The sensory physiology of the sheep tick &odes ricinus L. J. exp. Biol. 25, 145-207. MAKINGSP. (1964) ‘Slifer’s patches’ and the thermal sense in Acrididae (Orthoptera). J. exp. Biol. 41,473497. MURRAYR. W. (1962) Temperature receptors. Ado. camp. Physiol. Biochem. 1, 117-175. ROTH L. M. (1951) Loci of sensory end-organs used by mosquitoes (Aedes aegypti (L.) and Anopheles quadrimaculatus Say) in receiving host stimuli. Ann. ent. Sot. Am. 44, 59-74. SCHOONHOVEN L. M. and DETHI~RV. G. (1966) Sensory aspects of host-plant discrimination by lepidopterous larvae. Archs n&rl. Zool. 16,497~530. THURM U. (1963) Die Beziehungen zwischen mechanischen Reizgrossen und stationaren Erregungzust%-iden bei Borstenfeldsensillen von Biene. Z. vergl. Physiol. 46, 351-382. WIGGI.ESWORTH V. B. and GILLETT J. D. (1934) The function of the antennae in Rhodnius p~olixus (Hemiptera) and the mechanism of orientation to the host. r. exp. Bill. 11, 120-139.
Rimted in Great Brituin
SOME COLD RECEPTORS IN LARVAE THREE LEPIDOPTERA SPECIES
OF
L. M. SCHOONHOVEN Department of Entomology, Agricultural University, Wageningen, The Netherlands (Received 23 December 1966)
Abstract-Caterpillars of three different species have three peripheral nerve cells in their third antenna1 segment, which can be stimulated by decreasing temperatures. Increasing temperatures exert inhibitory effects on their spontaneous activity. Probably not more than one cell with this characteristic response is located in the maxillary palp. INTRODUCTION
LITTLE information is available on the physiology of temperature receptors in insects. A rough location of temperature senses was obtained in a number of species by behavioural studies, indicating that in most cases the antennae contain such receptors (WIGGLESWORTHand GILLJXT, 1934; ROTH, 1951; HERAN, 1952; GEBHARDT,1953 ; MAKINGS,1964). In some species temperature reception has been found to be located on the maxillary palpi as well (GEBHARJIT, 1953), on the tarsi (LEES, 1948), or on other parts of the body (HERTJXR,1953). Physiological informatemperature reception is still more restricted. HODG~ON (1956), HODGSONand ROEDER(1956), and GILLARY (1966) found an influence of temperature upon the reaction of chemoreceptors on the labellum of the blowfly. Here the number of spike potentials in some of these receptors decreased when the temperature was raised, while in others it increased. KERKUT and TAYLOR (1957) and DETHIER and ARAB (1958) observed temperature effects on the spike frequency in leg nerves of the cockroach and the blowfly, respectively. The impulse frequency in mechanoreceptors of honey bees is positively correlated with the temperature (Tm, 1963). Also changes in the spontaneous activity within the central nervous system have been recorded, induced by changes of temperature (KERKUT and TAYLOR, 1958). LACHER (1964) presented electrophysiological evidence for the presence of temperature receptors on the antennae of the honey bee. In the present study responses to temperature changes are described from some peripheral nerve cells in the larvae of three Lepidoptera. tion about
MATERIAL AND METHODS Most experiments were performed on last-instar larvae of Dendrolimus pini L. (Lasiocampidae), but Philosamiu Cynthia tini Boisd. (Saturniidae) and Protopmce sexta (Johan.) (Sphingidae) showed essentially the same reactions. The nerve cells 53
821
822
L.M.
SCHOONHOVEN
which have been investigated are located in the second segment of the maxillary palp and the third segment of the antenna (Fig. 1). Movements of the head appendages during the experiments were prevented by cutting the head and
FIG. 1. Ventral view of a caterpillar head. A, third antenna1 segment; P, second segment of maxillary palp.
damaging the nervous system. The recordings from the maxillary palp were made by introducing a tungsten electrode (uninsulated, 1~ tip) into the membrane at the base of the most distal segment. The electrode was moved in several directions until action potentials were recorded. The recordings of the last antenna1 segment were made by inserting it at the base of the largest sensillum basiconicum and pushing the electrode a small distance towards the base of the antenna. Action potentials of several cells were often recorded, and, by moving the electrode, temperature sensitive receptors could be selected. The electrode was connected to an oscilloscope (Tektronix 502) via an a.c. preamplifier (Tektronix 123). Temperature stimuli were presented in two ways. A metal rod 4 mm wide, heated electrically or by flame, or cooled by dipping into a salt-ice mixture or into dry ice, was brought within 3 to 10 mm from the organ. In an alternative method a flow of air from a compressed air system was led via an activated charcoal filter into a plastic tube. This tube subsequently was wound a few times inside a Dewar vessel filled with dry ice. The end was directed towards the preparation. Hot air was obtained by leading the compressed air into a wash-bottle containing hot water. The heated air was saturated and brought to the preparation via a plastic tube. The outlet of the plastic tube was about 10 cm from the insect. The airflow was constant. Beginning and end of stimulation were brought about by changing the direction of the outlet of the tube. When an airstream was used, its temperature was measured by a point-shaped copper-constantan thermocouple, made of wire 125 ~1 wide, mounted about 1 mm from the receptors, and d.c. connected to the second oscilloscope input. When the metal rod was used, the periods of stimulation were indicated on the film records by a change in the second trace of the oscilloscope controlled by a footswitch. Room temperatures fluctuated between 20 and 24°C.
SOME COLD RECEPTORS IN LARVAE OF THREt LEPIDOPlBRA SPECIFS
823
RESULTS
Some nerve cella, located in the maxUary palpi and in the antennae, show a clear reaponae to sudden changes in temperature. Lowering the temperature results in an increase of frequency of their spontaneous or resting activity, whereas an increase in temperature suppresses this activity. Reactions of a maxillary palp cell to a cold as well as to a hot metal rod are shown in Fig. 2. An increase in
A
C
FIG. 2. Reactions of a peripheral nerve cell in the maxillary palp of D. pinito the presence of a cold rod (A), a hot rod (B), and an airstream at room temperature (C). The period of stimulation is marked by the second horizontal trace in each record. The calibration symbol indicates 0.2 mV and 0.2 sec. activity can be observed also when air from a plastic wash-bottle (at room temperature) is blown over the preparation (Fig. 2C). This is presumably due to increased evaporation, resulting in a drop of temperature. In the maxillary palp only one cell at a time reacts in this way, although this segment contains between nineteen and twenty-four nerve cell bodies (SCHOONHOVEN and DETHIER, 1966). Thus probably only one (or a few at the most) shows the characteristic responses. A stream of air does not stimulate the other cells innervating olfactory receptors, but may depress their activity. Activity from several cells at one time is usually monitored when the electrode is moved into the third antenna1 segment, which is innervated by twelve neurons. A few cells, probably not more than three, highly sensitive to temperature can be found here. The record of Fig. 3 shows the stimulation of some cells by the presence of a cold metal rod, while one cell, represented by the smallest spike, does
FIG. 3. Stimulation by a cold rod of some peripheral nerve cells in the antenna of
D. pini. The second line is a continuation of the first line. The calibration symbol indicates 0.5 mV and O-2 sec.
824
L.M.
SCHOONHOVEN
not respond. It is not known whether the former cells can be stimulated by odours. The cells which are not stimulated by a decrease of temperature have been shown to be olfactory receptors. Some of them are not affected by a stream of pure air at room temperature; in others the background activity is slightly decreased. Increase of temperature increases the spontaneous activity somewhat, whereas a temperature decrease shows the opposite effect. Blowing air of different temperatures over the preparation has the advantage that the temperature of the air passing the receptors can be measured with reasonable accuracy by a thermocouple. The results of such an experiment, in which the total frequency of spikes originating in three cells is related to change in temperature, are shown in Fig. 4. This impulse frequency pattern shows a striking
0
,
2
2
4
I
6
7
II
SECONDS
FIG. 4. The total spike frequency in three peripheral nerve cells of the antenna of D. pini in relation with a decreasing temperature (upper trace). The ordinate of the temperature curve indicates the number of degrees Celsius below room temperature (= 22°C).
similarity to the reactions observed in a group of human cold receptors during brief cooling periods (HENSEL and BOMAN, 1960). A phasic response can be seen, followed by a phase of adaptation. During the phasic reaction the spike frequency (as calculated from six experiments) increased 150 to 300 per cent within half a second for each degree Celsius temperature decrease. When the temperature is raised again after a short exposure to cool air, some cells may cease to fire for several seconds, different cells showing different recovery periods. The occurrence of post-excitatory inhibitions of different durations conforms to the characteristics of vertebrate thermoreceptors (DODT, 1952).
SOME COLD RECEPTORS IN LARVAE OF THREE LEPIDOPTERA SPECIES
825
Increasing the temperature has the reverse effect on the spike frequency, though the frequency change is less spectacular than with cooled air. When the temperature drops back to its original level, the spike frequency not only goes back to its normal level, but shows a considerable amount of ‘overshoot’, which gradually levels out again. Thus these cells appear to react more strongly to a decrease in temperature than to an increase. However, a hot metal rod, kept close enough to the preparation, may depress completely all activity present. DISCUSSION
When thermoreception as a possible function of the cells described is to be considered, it must be kept in mind that so far no behavioural evidence is available. Although the sensitivity to temperature changes (which in our experiments were only of short duration) appears to be fairly high, this does not necessarily determine the cells as temperature receptors, as MURRAY(1962) has pointed out. However, the striking fact that in the maxillary palp only one or maybe a very few cells out of about twenty show the typical responses and in the antenna only about three cells out of a total number of about thirty, assigns these cells to a physiologically different type and suggests that they are concerned with sending information about ambient temperatures to the central nervous system. The quickness of the adaptation (Fig. 4) may indicate that their function is to register changes in temperature rather than to record constant environmental temperatures. Also the fact that many insects do have temperature receptors on the antennae (and some also on the maxillae) supports the idea that we are dealing here with true thermoreceptors, although the larval structures in this case possess only very few sense cells as compared with those of the insect species on which behavioural evidence was obtained. Theoretically, a sense cell may very well serve a dual purpose and transfer information about temperature as well as, for instance, the chemical environment or mechanical stimulation (MURRAY,1962; HODGSON,1965). The central nervous system could then decode this information in combination with the input from other receptors. Thus, assuming the existence of true temperature receptors, there is no need for the presence of special, morphologically distinct structures. However, one may speculate that the small sensillum styloconicum, found on the third antenna1 segment next to the three sensilla basiconica and which is innervated by three nerve cells (SCHOONHO~ENand DETHIER, 1966), might house the temperature sense. Because of the fact that in the maxillary palp we never hit more than one temperature-sensitive cell at the same time, one or more of the four small sensilla campaniformia (each innervated by one cell) seem to be a more likely location for a temperature receptor than the sensilla basiconica, which are usually innervated by more than one cell. In conclusion it can be said that, although their exact location is still uncertain, the fact that the temperature characteristics of the cells discussed differ so remarkably from the other receptor cells present suggests strongly that they do play a r6le in informing the caterpillar about temperature conditions around the head region.
826
L. M. SCHOONHOVEN
Acktwwk&ements-The author is indebted to Dr. V. G. DETHIER, Dr. R. W. MU-Y, and Dr. J. DE WILDE for reading and helpful discussion of the manuscript and to Mr. J. E. MOORHOUSE for correcting the English text. REFERENCES DETHIERV. G. and ARABY. M. (1958) Effect of temperature on the contact chemoreceptors of the blowfly. J. Insect Physiol. 2, 153-161. DODTE. (1952) The behaviour of thermoreceptors at low and high temperatures with special reference to Ebbecke’s temperature phenomena. Acta physiol. stand. 27, 295-314. GEBHARDTH. (1953) Die Lage der wichtigsten Thermorezeptoren bei einigen Insekten. Zool. Jb. (Allg.) 63, 558-592. GILLARY H. L. (1966) Stimulation of the salt receptor of the blowfly-II. Temperature. J. gen. Physiol. 50, 351-357. HENSEL H. and BOMANK. K. A. (1960) Afferent impulses in cutaneous sensory nerves in human subjects. J. Neurophysiol. 23, 564-568. HERAN H. (1952) Untersuchungen iiber den Temperatursinn der Honigbiene (Apis mellifica) unter besonderer Beriicksichtigung der Wahrnehmung strahlender Wilrme. Z. vergl. Physiol. 34, 179-206. HERTERK. (1953) De-r Temperatursinn der Insehten. Duncker und Humblot, Berlin. HODGSONE. S. (1956) Temperature sensitivity of primary chemoreceptors of insects. Anat. Rec. 125,560-561. HODGSONE. S. (1965) The chemical senses and changing viewpoints in sensory physiology. V&points in Biology 4,83-124. HODGSONE. S. and ROEDERK. D. (1956) Electrophysiological studies of arthropod chemoreception-I. General properties of the labellar chemoreceptors of Diptera. g. cell. camp. Physiol. 48, 51-76. KERKUT G. A. and TAYLOR B. J. R. (1957) A temperature receptor in the tarsus of the cockroach, Periplaneta americana. J. exp. Biol. 34486-493. KERKUTG. A. and TAYLORB. J. R. (1958) The effect of temperature changes on the activity of poikilotherms. Behaviour 13, 259-279. LACHER V. (1964) Elektrophysiologische Untersuchungen an einzelnen Rezeptoren fiir Geruch, Kohlendioxyd, Luftfeuchtigkeit und Temperatur auf den Antennen der Arbeitsbiene und der Drohne (Apis mellifia L.) Z. vergl. Physiol. 48, 587-623. LEES A. D. (1948) The sensory physiology of the sheep tick &odes ricinus L. J. exp. Biol. 25, 145-207. MAKINGSP. (1964) ‘Slifer’s patches’ and the thermal sense in Acrididae (Orthoptera). J. exp. Biol. 41,473497. MURRAYR. W. (1962) Temperature receptors. Ado. camp. Physiol. Biochem. 1, 117-175. ROTH L. M. (1951) Loci of sensory end-organs used by mosquitoes (Aedes aegypti (L.) and Anopheles quadrimaculatus Say) in receiving host stimuli. Ann. ent. Sot. Am. 44, 59-74. SCHOONHOVEN L. M. and DETHI~RV. G. (1966) Sensory aspects of host-plant discrimination by lepidopterous larvae. Archs n&rl. Zool. 16,497~530. THURM U. (1963) Die Beziehungen zwischen mechanischen Reizgrossen und stationaren Erregungzust%-iden bei Borstenfeldsensillen von Biene. Z. vergl. Physiol. 46, 351-382. WIGGI.ESWORTH V. B. and GILLETT J. D. (1934) The function of the antennae in Rhodnius p~olixus (Hemiptera) and the mechanism of orientation to the host. r. exp. Bill. 11, 120-139.