- Email: [email protected]
Pattern generators of the moth flight motor
Camp. Biochem. Physiol., 1972, Vol. 41A, pp.
105to 113.Pergamon Press. Printed in Great Britain
PATTERN GENERATORS OF THE MOTH FLIGHT MOTOR JAMES
L. HANEGAN
Department of Biology, Eastern Washington State College, Cheney, Washington 99004 (Received 28 June 1971)
Abstract-l.
A.C. stimulation of the thoracic ganglia of the moth Hyalophora cecropiu, elicited the warmup pattern of contraction in the flight muscles, whereas d.c. stimulation elicited the flight pattern. 2. Both flight and warmup can be generated in the absence of normal sensory input. 3. The two different patterns of contraction can be most easily described as a function of interneurons or pacemaker cells rather than resulting from interactions between synergistic motor neurons. INTRODUCTION
THE NEUROPHYSIOLOGICAL basis of insect flight has been studied extensively
by Weis-Fogh (1956), WI1 son (1961), Wilson & Weis-Fogh (1962), Waldron (1967a), Wilson & Waldron (1968) and others. The work was primarily performed on the desert locust, Shistocerca gregari, which was a neurogenic flight system. The basic pattern of flight in this insect is alternating bursts of activity in the dorsal-longitudinal muscles (wing depressors) and dorsoventral muscles (wing elevators). From recordings of the electrical activity in the two muscle groups while the insect was in flight (Wilson, 1961; Waldron, 1967a, b) and from neuromimes (electronic equivalents of patches of neural membranes) a model was described which could account for the basic flight pattern. Wilson (1961) and Wilson & Waldron (1968) postulated that the rhythmic bursting of each muscle group could be generated by positive interactions of the synergistic motor neurons. The cessation of bursts is due to accumulated refractoriness of the motor neurons, and the property of alternation of bursts in the two muscle groups is due to reciprocal inhibition. This model adequately describes the flight pattern; however, it remains possible that the flight pattern generators may be interneurons rather than the motor neurons. In certain nocturnal moths and possibly butterflies, flight is preceded by warmup in which the flight muscles contract synchronously (Dorsett, 1962; Moran & Ewer, 1966; Kammer, 1968, 1970; and others). This process of “shivering” increases the thoracic temperature to a level which is optimal for flight when the ambient temperature is low. The frequency of contractions during warmup differs from the flight frequency in many cases and the shift from warmup to flight can be gradual or occur abruptly (Kammer, 1968). In Cecropia moths, the transition 105
106
JAMES L. HANECAN
of warmup the thoracic In these account for describe the contraction
to flight has been shown to be dependent upon the temperature of ganglia (Hanegan SC Heath, 1970a). insects, the flight pattern generator must have special properties to the warmup pattern. This investigation is an attempt to further moth flight system and to account for the two different patterns of observed in the flight muscles. MATERIALS
AND
METHODS
The Hyalophora cecropiu moths used in these experiments were raised from eggs on wild cherry trees covered with nylon nets. The animals pupated in August 1969, and have subsequently been kept at 5°C in diapause. The muscle potentials from the indirect flight muscles were recorded using fine copper wires insulated to the tip. These potentials were recorded differentially and amplified with Tektronics 122 preamplifiers. The signals were then displayed on a Tektronics 502A dual-beam oscilloscope and photographed for permanent records with a Cossor Instrument’s Camera. The thoracic ganglia was stimulated with direct current using fine silver wires. The stimulating electrodes were inserted through an apodeme which projects inward from the ventral side of the animal between the pro- and pterothoracic ganglia. A detailed account of the methods for recording the muscle potentials and stimulation of the ganglia has been reported previously (Hanegan & Heath, 1970a). RESULTS
Normal and lesioned animals Figure 1 shows the normal phasing of the warmup pattern and the transition In warmup, the DLM and DVM muscles to the flight pattern of contraction. contract in synchrony and in the flight pattern these two muscle groups exhibit alternating bursts of activity. The frequency of warmup is 17/set and flight is
The upper record shows the tranFIG. 1. Normal warmup and flight patterns. sition of warmup (left side of record) to the flight pattern. Upper trace is from the DLM (dorsal-longitudinal muscles) and the lower trace is a DVM (dorsoventral muscle).
MOTH
FLIGHT
MOTOR
107
4-8 bursts/set. Figure 2a is a record of flight in which the sensory nerve to the wing has been cut. The basic pattern of alternating activity in the two muscle groups is present; however, the frequency is reduced. Figure 2b is also a record of muscle activity occurring during flight. In this animal the connectives between the brain and the thoracic ganglia have been cut. The frequency of bursts (3+sec) is slightly reduced yet remains within the range exhibited by normal animals (4-8/set). The burst length is extended and the number of spikes within the burst is more variable than those seen in a normal animal.
200
msec
FIG. 2. a. Record of an animal in flight with the sensory nerve to the wing base cut. b. Flight record from an animal in which the connectives from the brain to the thoracic ganglia have been cut. In both records the upper trace is the DLM and the lower trace is the DVM.
A.C. stimulation In Fig. 3, the thoracic ganglia is being stimulated at 17/set with a square wave of 2-S V and 10 msec duration. On the left side of the upper record the pattern appears to be a normal warmup. Bursts of activity begin to appear in the DLM muscles at the flight frequency in the middle of the record. In the lower record, which is a continuation of the upper one, bursts in the DLM have become more evident and to a lesser extent can be seen in the DVM muscles. Even with the bursts of activity at the flight frequency, the stimuli at the warmup frequency continue to evoke synchronous contractions. Figure 4 is a record of an animal in flight which is being stimulated with a square wave of 2.5 V and 10 msec duration at 17/set. It can be seen that a.c. stimulation at the warmup frequency does not override the flight response and “drive” the animal in the warmup pattern. Once the flight pattern has been initiated, subsequent stimulation at the warmup frequency had no effect. The animal did not phase the flight pattern to the incoming stimulation, suggesting that the flight pattern inhibits the warmup pattern.
108
JAMESL. HANECAN
(2)
..‘...
.
.
.
.
.
.
.
.
200
.
.
.
.
.
.
.
.
.
.
.
m set
FIG. 3. Stimulation of the thoracic ganglia at 17/set. Note on the left side of record 1 the normal warmup pattern. Bursts of activity at the flight frequency appear in the middle of the record (circled). Both the warmup pattern and the flight pattern are evident in record 2. Upper trace DLM, lower trace DVM. Dots represent
FIG. 4.
the stimulus
frequency.
Record of the DLM (upper trace) and DVM (lower trace of an animal in flight. The ganglia are being stimulated at 17/set (dots).
D.C. stimulation Figure Sa is a record of a quiescent moth which was stimulated with d.c. current of 10 V and approximately 2 mA. The thoracic temperature of this moth was 25°C. It can be seen that the animal exhibited the normal flight pattern when stimulated and then shifted into the warmup pattern on the right side of the record. Figure 5b is a record of another animal similarly stimulated with d.c. current. In this
MOTH FLIGHT MOTOR
109
animal there is a longer time lag between the initiation of the stimulus and the appearance of the flight pattern. Note the random firing of the DVM muscle group when the DLM began bursting in the flight pattern. In this record the flight pattern of contraction died out after a few seconds and did not revert to the warmup pattern. In Fig. 5c the DVM muscles were firing in a normal warmup pattern. The DLM group was randomly firing with respect to the DVM’s. This random firing could be due to damage of the ganglia by the stimulating electrodes. When the d.c. current was turned on (at the arrow) the warmup pattern in the DVM’s was inhibited but did not switch over to the flight pattern. If the ganglia was damaged it may not have been capable of producing the flight pattern with d.c. stimulation. The d.c. stimulation could have caused direct inhibition on the motor neurons or possibly inhibited the warmup pattern through excitation of the flight pattern generators.
200 msec 5. a. Stimulation of the thoracic ganglia with d.c. current (arrow). The quiescent animal initiates a normal flight response which switches to warmup on the right side of the record. b. Similar record in which the animal did not revert to warmup. The flight response stopped when stimulation was removed. c. Stimulation with d.c. current (arrow) while the animal was in warmup. The flight response was not initiated, however, note the inhibition of the warmup pattern. FIG.
Two pattern generators In 2 animals, what appeared to be flight activity in the muscles was recorded; however, no apparent movement of the wings was observed. The DVM muscles
110
JAMESL. HANEGAN
exhibit bursts in the normal flight pattern but there is no activity in the DLM muscle group on the left side of the record (Fig. 6a). The DLM gradually increases in activity and the normal pattern of flight is evident at the right side of the record. In Fig. 6b the normal burst pattern seen in flight is evident in the DLM muscle group, but the bursting pattern in the DVM muscles is random. These records indicate that there may be separate flight pattern generators for the two muscle groups which are loosely coupled.
200 tnsec
FIG. 6. a. Recording of the DLM (upper trace) and DVM (lower trace) when there was no visible flight response. Note the bursting in the DVM at the flight frequency while the DLM remains silent. b. Similar recording in another animal. In this record the DLM is bursting at the Aight frequency and the DVM is randomly activated. DISCUSSION
Wilson (1961) established in locusts that the flight pattern can be generated in the thoracic ganglia in the absence of sensory input. This has been shown to be true for H. cecropia. When the connectives between the brain and the thoracic ganglia were cut, the animal can be stimulated to initiate a warmup period and is able to make the transition to flight. The flight frequency is reduced and the number of spikes within a burst in both the DLM and DVM muscle groups fluctuates more than in a normal animal. When the wing hinge is cut, severing the sensory nerves, the basic flight pattern of alternating contractions remains; however, the frequency is also reduced in this case. Cutting the sensory nerves or the connectives from the brain caused an alternation in frequency but does not disrupt the basic pattern of flight or warmup. Aiternating current stimulation at a frequency of 17jsec will trigger a warmup response in which the DLM and DVM’s contract synchronously at the stimulation frequency. A.C. stimulation at frequencies of 5-30/set also cause a warmup response; however, the synchronous contractions are not phased to the stimulus frequency but contract at the normal warmup frequency of 13-17jsec. This
MOTH FLIGHT MOTOR
111
indicates that a.c. stimulation turns on the warmup response by stimulating the warmup pattern generator, and that the stimulus frequency can be synchronized with the inherent warmup frequency. In Fig. 3 the a.c. stimulation may be acting directly on the motor neurons rather than on the warmup pattern generator. The synchronous contractions of the DLM and DVM muscles generate heat and cause the temperatute of the thoracic capsule to increase. The increased temperature of the thoracic ganglia can initiate the transition of warmup to flight (Hanegan & Heath, 1970a). In a normal animal once the flight pattern has begun, the synchronous contractions of warmup were never observed, presumably through inhibition of the warmup pattern by the flight pattern generator. If the a.c. stimulation were acting on the motor neurons directly the inhibition by the flight pattern on the warmup pattern generator would be bypassed and both patterns would be observed; the warmup pattern resulting from direct stimulation of the motor neurons and the flight pattern resulting from the temperature effects on the thoracic ganglia. This direct effect of a.c. stimulation on the motor neurons was observed in only one animal. In all other animals tested the stimulation activated the warmup pattern generator which either shifted to the flight pattern if continued for a sufficient period of time to allow for thoracic heating (3”C/min) or stopped within a few wing beats after the stimulation was removed. If the stimulation was begun after a normal transition to flight has occurred (Fig. 4) there was no apparent effect on the flight pattern. Synchronous contractions were not observed and the flight pattern was not inhibited by stimulation at the warmup frequency. Stimulation of the thoracic ganglia with d.c. current caused the initiation of the flight pattern without previous activity. The flight pattern was initiated even when the thoracic temperature was below the ~~rnurn required for flight in a normal animal (3438°C; Hanegan & Heath, 1970b). Moran & Ewer (1966) found similar results in both sphingid and saturniid moths. If the prothoracic ganglion was stimulated with low current levels, 0.08 mA and 0.5 V, the wings moved in the pattern resembling warmup. With stronger stimulation, O-15 mA and l-0 V, the wings flapped as in flight. Direct current stimuIaion of the pterothoracic ganglion resulted in flapping of the wings and never warmup, independent of the stimulus strength. They postulated that the warmup pattern generator was located in the prothoracic ganglion and the flight pattern generator was in the pterothoracic ganglia, In the experiments reported on here the electrodes were inserted through an apodeme which projects between the two thoracic ganglia. Since the animal was not opened to expose the ganglia, it was impossible to determine if the electrodes were primarily stimulating the pro-, pterothoracic or both ganglia simultaneously. The intensity of the stimulus required to elicit the flight response was considerably higher than that required by Moran and Ewer (1966). Presumably the electrodes may not have been in direct contact with the ganglia but merely in the hemolymph surrounding it. Hanegan & Heath (1970a) have reported that the generation of the flight pattern is dependent upon the temperature of the thoracic ganglia. They were able to turn
112
JAMESL. HANECAN
on and off the flight pattern by heating or cooling the ganglia without substantially altering the temperature of the thoracic capsule. Their experiments and the ones reported here indicate that a non-specific stimulus, either thermal or d.c. current, activates the flight pattern generator which is physiologically separate from the warmup pattern generator. These two different modes of stimulation may initiate positive interactions among the synergistic motor neurons which by themselves are capable of producing the flight pattern as suggested by Wilson (1961) and Wilson & Waldron (1968); or they may cause the release of inhibition on oscillatory pacemaker cells which generate the flight pattern. This latter interpretation appears to be more reasonable since a.c. stimulation driving the motor neurons in a warmup pattern did not prevent the simultaneous initiation of the flight pattern (Fig. 3). When the thoracic temperature had increased to the critical set-point the flight pattern was switched on and both patterns were recorded. Waldron (1967b) found that sensory input which depressed elevator activity did not affect rhythmic bursting of the depressor motor neurons, even after all elevator motor activity ceased. She therefore postulated that inhibitory interactions between ~tagonistic motor neurons was not required to produce the rhythmic bursting pattern. This was first suggested by Wilson (1964) who reasoned that the pause between the bursts of antagonistic units indicates that the depressors do not inhibit the elevators and vice versa. The cessation of the bursts is due to accumulated refractoriness of the motor neurons. The reciprocal inhibition of the two antagonistic flight pattern generators in the model of Hanegan & Heath (1970a) would not appear to be necessary for the generation of rhythmic burst of activity. Indeed it was found that reciprocal inhibition was not necessary for rhythmic bursting (Fig. 6). In one animal, the DLM burst in a normal flight pattern while the DVM remained silent. In the second record the DVM burst rhythmically while the DLM was randomly active. However, the pattern of alternation of bursts in the antagonistic muscle groups can only be accounted for by reciprocal inhibition, otherwise during the transition of warmup to flight the antagonistic muscle groups could begin bursting simultaneously at the flight frequency. There is evidence of inhibition in the system but not specifically between the In Fig. 5c, the animai is not responding antagonistic flight pattern generators. normally in that the DVM is firing randomly with respect to the DVM during warmup. This could be due to damage of the ganglia when the electrodes were inserted into the animal. When the ganglia were stimulated with d.c. current, which in other animals elicited a flight pattern of motor output, the warmup pattern was inhibited as seen by the decreased activity of the DLM muscle group. Further stimulation with d.c. current did not elicit the flight pattern. REFERENCES DORSETTD. A. (1962) Preparation for flight by hawk-moths. r. exp. Biol. 39, 579-588. HANEGANJ. L. & HEATHJ_ E. (1970a) Temperature dependence of the neural control of the mother flight system. J. exp. BioE. 53, 629-639.
MOTHFLIGHTMOTOR
113
HANEGANJ. L. & HEATH J. E. (1970b) Mechanisms for the control of body temperature in the moth, Hyalophora cecmpia. J. exp. Biol. 53, 349-362. RAMMERA. E. (1968) Motor patterns during flight and warm-up in Lepidoptera. J, exp. Biol. 48, 88-109. RAMMERA. E. (1970) Thoracic temperature, shivering, and flight in the monarch butterfly, Danaus plexippus (L.). 2. vergl. Physiol. 68, 334-344. MORANV. C. & EWER D. W. (1966) Observations on certain characteristics of the flight motor of sphingid and satumiid moths. J. Insect Physiol. 12, 457-463. WALDRONI. (1967a) Mechanisms for the production of the motor output pattern in flying locusts. J. exp. Biol. 41, 201-212. WALDRONI. (1967b) Neural mechanism by which controlling inputs influence motor output, studied in flying locusts. y. exp. Biol. 47, 213-228. WEIS-FOGH T. (1956) Biology and physics of locust flight-IV. Notes on sensory mechanisms in locust flight. Phil. Trans. R. Sot. Lond. B 329, 553-584. WI-ON D. M. (1961) The central nervous control of flight in a locust. g. exp. Biol. 38, 471-490. WILSON D. M. (1964) Relative refractoriness and patterned discharge of locust flight motor neurons. J. exp. Biol. 41, 191-205. WILSON D. M. (1966) Central nervous mechanisms for the generation of rhythmic behavior in Arthropods. Sot. exp. Biol. Symp. 20, 199-228. WILSOND. M. & WALDRONI. (1968) Models for the generation of the motor output pattern in flying locusts. Proc. I.E.E.E. 5, 1058-1064. WILSON D. M. & WEIS-FOGH T. (1962) Patterned activity of coordinated motor units, studied in flying locusts. r. exp. Biol. 39, 643-667. Key Word Index-Insect warmup.
flight; Hyalophora
cecropia; insect CNS; pattern generators;
105to 113.Pergamon Press. Printed in Great Britain
PATTERN GENERATORS OF THE MOTH FLIGHT MOTOR JAMES
L. HANEGAN
Department of Biology, Eastern Washington State College, Cheney, Washington 99004 (Received 28 June 1971)
Abstract-l.
A.C. stimulation of the thoracic ganglia of the moth Hyalophora cecropiu, elicited the warmup pattern of contraction in the flight muscles, whereas d.c. stimulation elicited the flight pattern. 2. Both flight and warmup can be generated in the absence of normal sensory input. 3. The two different patterns of contraction can be most easily described as a function of interneurons or pacemaker cells rather than resulting from interactions between synergistic motor neurons. INTRODUCTION
THE NEUROPHYSIOLOGICAL basis of insect flight has been studied extensively
by Weis-Fogh (1956), WI1 son (1961), Wilson & Weis-Fogh (1962), Waldron (1967a), Wilson & Waldron (1968) and others. The work was primarily performed on the desert locust, Shistocerca gregari, which was a neurogenic flight system. The basic pattern of flight in this insect is alternating bursts of activity in the dorsal-longitudinal muscles (wing depressors) and dorsoventral muscles (wing elevators). From recordings of the electrical activity in the two muscle groups while the insect was in flight (Wilson, 1961; Waldron, 1967a, b) and from neuromimes (electronic equivalents of patches of neural membranes) a model was described which could account for the basic flight pattern. Wilson (1961) and Wilson & Waldron (1968) postulated that the rhythmic bursting of each muscle group could be generated by positive interactions of the synergistic motor neurons. The cessation of bursts is due to accumulated refractoriness of the motor neurons, and the property of alternation of bursts in the two muscle groups is due to reciprocal inhibition. This model adequately describes the flight pattern; however, it remains possible that the flight pattern generators may be interneurons rather than the motor neurons. In certain nocturnal moths and possibly butterflies, flight is preceded by warmup in which the flight muscles contract synchronously (Dorsett, 1962; Moran & Ewer, 1966; Kammer, 1968, 1970; and others). This process of “shivering” increases the thoracic temperature to a level which is optimal for flight when the ambient temperature is low. The frequency of contractions during warmup differs from the flight frequency in many cases and the shift from warmup to flight can be gradual or occur abruptly (Kammer, 1968). In Cecropia moths, the transition 105
106
JAMES L. HANECAN
of warmup the thoracic In these account for describe the contraction
to flight has been shown to be dependent upon the temperature of ganglia (Hanegan SC Heath, 1970a). insects, the flight pattern generator must have special properties to the warmup pattern. This investigation is an attempt to further moth flight system and to account for the two different patterns of observed in the flight muscles. MATERIALS
AND
METHODS
The Hyalophora cecropiu moths used in these experiments were raised from eggs on wild cherry trees covered with nylon nets. The animals pupated in August 1969, and have subsequently been kept at 5°C in diapause. The muscle potentials from the indirect flight muscles were recorded using fine copper wires insulated to the tip. These potentials were recorded differentially and amplified with Tektronics 122 preamplifiers. The signals were then displayed on a Tektronics 502A dual-beam oscilloscope and photographed for permanent records with a Cossor Instrument’s Camera. The thoracic ganglia was stimulated with direct current using fine silver wires. The stimulating electrodes were inserted through an apodeme which projects inward from the ventral side of the animal between the pro- and pterothoracic ganglia. A detailed account of the methods for recording the muscle potentials and stimulation of the ganglia has been reported previously (Hanegan & Heath, 1970a). RESULTS
Normal and lesioned animals Figure 1 shows the normal phasing of the warmup pattern and the transition In warmup, the DLM and DVM muscles to the flight pattern of contraction. contract in synchrony and in the flight pattern these two muscle groups exhibit alternating bursts of activity. The frequency of warmup is 17/set and flight is
The upper record shows the tranFIG. 1. Normal warmup and flight patterns. sition of warmup (left side of record) to the flight pattern. Upper trace is from the DLM (dorsal-longitudinal muscles) and the lower trace is a DVM (dorsoventral muscle).
MOTH
FLIGHT
MOTOR
107
4-8 bursts/set. Figure 2a is a record of flight in which the sensory nerve to the wing has been cut. The basic pattern of alternating activity in the two muscle groups is present; however, the frequency is reduced. Figure 2b is also a record of muscle activity occurring during flight. In this animal the connectives between the brain and the thoracic ganglia have been cut. The frequency of bursts (3+sec) is slightly reduced yet remains within the range exhibited by normal animals (4-8/set). The burst length is extended and the number of spikes within the burst is more variable than those seen in a normal animal.
200
msec
FIG. 2. a. Record of an animal in flight with the sensory nerve to the wing base cut. b. Flight record from an animal in which the connectives from the brain to the thoracic ganglia have been cut. In both records the upper trace is the DLM and the lower trace is the DVM.
A.C. stimulation In Fig. 3, the thoracic ganglia is being stimulated at 17/set with a square wave of 2-S V and 10 msec duration. On the left side of the upper record the pattern appears to be a normal warmup. Bursts of activity begin to appear in the DLM muscles at the flight frequency in the middle of the record. In the lower record, which is a continuation of the upper one, bursts in the DLM have become more evident and to a lesser extent can be seen in the DVM muscles. Even with the bursts of activity at the flight frequency, the stimuli at the warmup frequency continue to evoke synchronous contractions. Figure 4 is a record of an animal in flight which is being stimulated with a square wave of 2.5 V and 10 msec duration at 17/set. It can be seen that a.c. stimulation at the warmup frequency does not override the flight response and “drive” the animal in the warmup pattern. Once the flight pattern has been initiated, subsequent stimulation at the warmup frequency had no effect. The animal did not phase the flight pattern to the incoming stimulation, suggesting that the flight pattern inhibits the warmup pattern.
108
JAMESL. HANECAN
(2)
..‘...
.
.
.
.
.
.
.
.
200
.
.
.
.
.
.
.
.
.
.
.
m set
FIG. 3. Stimulation of the thoracic ganglia at 17/set. Note on the left side of record 1 the normal warmup pattern. Bursts of activity at the flight frequency appear in the middle of the record (circled). Both the warmup pattern and the flight pattern are evident in record 2. Upper trace DLM, lower trace DVM. Dots represent
FIG. 4.
the stimulus
frequency.
Record of the DLM (upper trace) and DVM (lower trace of an animal in flight. The ganglia are being stimulated at 17/set (dots).
D.C. stimulation Figure Sa is a record of a quiescent moth which was stimulated with d.c. current of 10 V and approximately 2 mA. The thoracic temperature of this moth was 25°C. It can be seen that the animal exhibited the normal flight pattern when stimulated and then shifted into the warmup pattern on the right side of the record. Figure 5b is a record of another animal similarly stimulated with d.c. current. In this
MOTH FLIGHT MOTOR
109
animal there is a longer time lag between the initiation of the stimulus and the appearance of the flight pattern. Note the random firing of the DVM muscle group when the DLM began bursting in the flight pattern. In this record the flight pattern of contraction died out after a few seconds and did not revert to the warmup pattern. In Fig. 5c the DVM muscles were firing in a normal warmup pattern. The DLM group was randomly firing with respect to the DVM’s. This random firing could be due to damage of the ganglia by the stimulating electrodes. When the d.c. current was turned on (at the arrow) the warmup pattern in the DVM’s was inhibited but did not switch over to the flight pattern. If the ganglia was damaged it may not have been capable of producing the flight pattern with d.c. stimulation. The d.c. stimulation could have caused direct inhibition on the motor neurons or possibly inhibited the warmup pattern through excitation of the flight pattern generators.
200 msec 5. a. Stimulation of the thoracic ganglia with d.c. current (arrow). The quiescent animal initiates a normal flight response which switches to warmup on the right side of the record. b. Similar record in which the animal did not revert to warmup. The flight response stopped when stimulation was removed. c. Stimulation with d.c. current (arrow) while the animal was in warmup. The flight response was not initiated, however, note the inhibition of the warmup pattern. FIG.
Two pattern generators In 2 animals, what appeared to be flight activity in the muscles was recorded; however, no apparent movement of the wings was observed. The DVM muscles
110
JAMESL. HANEGAN
exhibit bursts in the normal flight pattern but there is no activity in the DLM muscle group on the left side of the record (Fig. 6a). The DLM gradually increases in activity and the normal pattern of flight is evident at the right side of the record. In Fig. 6b the normal burst pattern seen in flight is evident in the DLM muscle group, but the bursting pattern in the DVM muscles is random. These records indicate that there may be separate flight pattern generators for the two muscle groups which are loosely coupled.
200 tnsec
FIG. 6. a. Recording of the DLM (upper trace) and DVM (lower trace) when there was no visible flight response. Note the bursting in the DVM at the flight frequency while the DLM remains silent. b. Similar recording in another animal. In this record the DLM is bursting at the Aight frequency and the DVM is randomly activated. DISCUSSION
Wilson (1961) established in locusts that the flight pattern can be generated in the thoracic ganglia in the absence of sensory input. This has been shown to be true for H. cecropia. When the connectives between the brain and the thoracic ganglia were cut, the animal can be stimulated to initiate a warmup period and is able to make the transition to flight. The flight frequency is reduced and the number of spikes within a burst in both the DLM and DVM muscle groups fluctuates more than in a normal animal. When the wing hinge is cut, severing the sensory nerves, the basic flight pattern of alternating contractions remains; however, the frequency is also reduced in this case. Cutting the sensory nerves or the connectives from the brain caused an alternation in frequency but does not disrupt the basic pattern of flight or warmup. Aiternating current stimulation at a frequency of 17jsec will trigger a warmup response in which the DLM and DVM’s contract synchronously at the stimulation frequency. A.C. stimulation at frequencies of 5-30/set also cause a warmup response; however, the synchronous contractions are not phased to the stimulus frequency but contract at the normal warmup frequency of 13-17jsec. This
MOTH FLIGHT MOTOR
111
indicates that a.c. stimulation turns on the warmup response by stimulating the warmup pattern generator, and that the stimulus frequency can be synchronized with the inherent warmup frequency. In Fig. 3 the a.c. stimulation may be acting directly on the motor neurons rather than on the warmup pattern generator. The synchronous contractions of the DLM and DVM muscles generate heat and cause the temperatute of the thoracic capsule to increase. The increased temperature of the thoracic ganglia can initiate the transition of warmup to flight (Hanegan & Heath, 1970a). In a normal animal once the flight pattern has begun, the synchronous contractions of warmup were never observed, presumably through inhibition of the warmup pattern by the flight pattern generator. If the a.c. stimulation were acting on the motor neurons directly the inhibition by the flight pattern on the warmup pattern generator would be bypassed and both patterns would be observed; the warmup pattern resulting from direct stimulation of the motor neurons and the flight pattern resulting from the temperature effects on the thoracic ganglia. This direct effect of a.c. stimulation on the motor neurons was observed in only one animal. In all other animals tested the stimulation activated the warmup pattern generator which either shifted to the flight pattern if continued for a sufficient period of time to allow for thoracic heating (3”C/min) or stopped within a few wing beats after the stimulation was removed. If the stimulation was begun after a normal transition to flight has occurred (Fig. 4) there was no apparent effect on the flight pattern. Synchronous contractions were not observed and the flight pattern was not inhibited by stimulation at the warmup frequency. Stimulation of the thoracic ganglia with d.c. current caused the initiation of the flight pattern without previous activity. The flight pattern was initiated even when the thoracic temperature was below the ~~rnurn required for flight in a normal animal (3438°C; Hanegan & Heath, 1970b). Moran & Ewer (1966) found similar results in both sphingid and saturniid moths. If the prothoracic ganglion was stimulated with low current levels, 0.08 mA and 0.5 V, the wings moved in the pattern resembling warmup. With stronger stimulation, O-15 mA and l-0 V, the wings flapped as in flight. Direct current stimuIaion of the pterothoracic ganglion resulted in flapping of the wings and never warmup, independent of the stimulus strength. They postulated that the warmup pattern generator was located in the prothoracic ganglion and the flight pattern generator was in the pterothoracic ganglia, In the experiments reported on here the electrodes were inserted through an apodeme which projects between the two thoracic ganglia. Since the animal was not opened to expose the ganglia, it was impossible to determine if the electrodes were primarily stimulating the pro-, pterothoracic or both ganglia simultaneously. The intensity of the stimulus required to elicit the flight response was considerably higher than that required by Moran and Ewer (1966). Presumably the electrodes may not have been in direct contact with the ganglia but merely in the hemolymph surrounding it. Hanegan & Heath (1970a) have reported that the generation of the flight pattern is dependent upon the temperature of the thoracic ganglia. They were able to turn
112
JAMESL. HANECAN
on and off the flight pattern by heating or cooling the ganglia without substantially altering the temperature of the thoracic capsule. Their experiments and the ones reported here indicate that a non-specific stimulus, either thermal or d.c. current, activates the flight pattern generator which is physiologically separate from the warmup pattern generator. These two different modes of stimulation may initiate positive interactions among the synergistic motor neurons which by themselves are capable of producing the flight pattern as suggested by Wilson (1961) and Wilson & Waldron (1968); or they may cause the release of inhibition on oscillatory pacemaker cells which generate the flight pattern. This latter interpretation appears to be more reasonable since a.c. stimulation driving the motor neurons in a warmup pattern did not prevent the simultaneous initiation of the flight pattern (Fig. 3). When the thoracic temperature had increased to the critical set-point the flight pattern was switched on and both patterns were recorded. Waldron (1967b) found that sensory input which depressed elevator activity did not affect rhythmic bursting of the depressor motor neurons, even after all elevator motor activity ceased. She therefore postulated that inhibitory interactions between ~tagonistic motor neurons was not required to produce the rhythmic bursting pattern. This was first suggested by Wilson (1964) who reasoned that the pause between the bursts of antagonistic units indicates that the depressors do not inhibit the elevators and vice versa. The cessation of the bursts is due to accumulated refractoriness of the motor neurons. The reciprocal inhibition of the two antagonistic flight pattern generators in the model of Hanegan & Heath (1970a) would not appear to be necessary for the generation of rhythmic burst of activity. Indeed it was found that reciprocal inhibition was not necessary for rhythmic bursting (Fig. 6). In one animal, the DLM burst in a normal flight pattern while the DVM remained silent. In the second record the DVM burst rhythmically while the DLM was randomly active. However, the pattern of alternation of bursts in the antagonistic muscle groups can only be accounted for by reciprocal inhibition, otherwise during the transition of warmup to flight the antagonistic muscle groups could begin bursting simultaneously at the flight frequency. There is evidence of inhibition in the system but not specifically between the In Fig. 5c, the animai is not responding antagonistic flight pattern generators. normally in that the DVM is firing randomly with respect to the DVM during warmup. This could be due to damage of the ganglia when the electrodes were inserted into the animal. When the ganglia were stimulated with d.c. current, which in other animals elicited a flight pattern of motor output, the warmup pattern was inhibited as seen by the decreased activity of the DLM muscle group. Further stimulation with d.c. current did not elicit the flight pattern. REFERENCES DORSETTD. A. (1962) Preparation for flight by hawk-moths. r. exp. Biol. 39, 579-588. HANEGANJ. L. & HEATHJ_ E. (1970a) Temperature dependence of the neural control of the mother flight system. J. exp. BioE. 53, 629-639.
MOTHFLIGHTMOTOR
113
HANEGANJ. L. & HEATH J. E. (1970b) Mechanisms for the control of body temperature in the moth, Hyalophora cecmpia. J. exp. Biol. 53, 349-362. RAMMERA. E. (1968) Motor patterns during flight and warm-up in Lepidoptera. J, exp. Biol. 48, 88-109. RAMMERA. E. (1970) Thoracic temperature, shivering, and flight in the monarch butterfly, Danaus plexippus (L.). 2. vergl. Physiol. 68, 334-344. MORANV. C. & EWER D. W. (1966) Observations on certain characteristics of the flight motor of sphingid and satumiid moths. J. Insect Physiol. 12, 457-463. WALDRONI. (1967a) Mechanisms for the production of the motor output pattern in flying locusts. J. exp. Biol. 41, 201-212. WALDRONI. (1967b) Neural mechanism by which controlling inputs influence motor output, studied in flying locusts. y. exp. Biol. 47, 213-228. WEIS-FOGH T. (1956) Biology and physics of locust flight-IV. Notes on sensory mechanisms in locust flight. Phil. Trans. R. Sot. Lond. B 329, 553-584. WI-ON D. M. (1961) The central nervous control of flight in a locust. g. exp. Biol. 38, 471-490. WILSON D. M. (1964) Relative refractoriness and patterned discharge of locust flight motor neurons. J. exp. Biol. 41, 191-205. WILSON D. M. (1966) Central nervous mechanisms for the generation of rhythmic behavior in Arthropods. Sot. exp. Biol. Symp. 20, 199-228. WILSOND. M. & WALDRONI. (1968) Models for the generation of the motor output pattern in flying locusts. Proc. I.E.E.E. 5, 1058-1064. WILSON D. M. & WEIS-FOGH T. (1962) Patterned activity of coordinated motor units, studied in flying locusts. r. exp. Biol. 39, 643-667. Key Word Index-Insect warmup.
flight; Hyalophora
cecropia; insect CNS; pattern generators;