J. Exp. Biol. (1969), 50, 363-373 3 6 3 With 2 plates and 4 text-figures
Printed in Great Britain
LOCUST WIND RECEPTORS
III. CONTRIBUTION TO FLIGHT INITIATION AND LIFT CONTROL*
BY JEFFREY M. CAMHIf
Biological Laboratories, Harvard University
(Received 17 June 1968)
INTRODUCTION
The previous paper showed that interneurone responses of four different types can be recorded from axons in the locust cervical connective upon wind stimulation of the facial setae (Camhi, 1969 b). Of these four interneurone categories, the' wind-indicator' and the 'acceleration' cells showed bilaterally symmetrical responses, producing about equal numbers of impulses for wind directed toward the head from either side. Since these two types of interneurone are unable to discriminate between wind from the left and wind from the right, it seems reasonable that any flight manoeuvres resulting from their activity would occur about the pitch axis; therefore, turns, yaws and rolls presumably are not controlled by these symmetrically responding cells.
Weis-Fogh (1949, 1950) demonstrated two bilaterally symmetrical flight reflexes of tethered locusts: (1) initiation, and (2) maintenance of wing beating, each resulting from wind stimulation of the facial setae. The previous paper showed that the flight-maintenance reflex probably results from the activity of wind-indicator interneurones. Flight initiation, consisting of the opening and incipient beating of the wings at wind onset, has not been studied in detail. Evidence presented here will show that the flight initiation following normal free jumps probably does not result from wind-receptor stimulation, or from loss of tarsal contact.
A third vertical flight control, lift regulation, is shown to be controlled in part by facial wind-receptor stimulation.
MATERIAL AND METHODS
As in the previous papers all experiments were performed on male Schistocerca gregaria Forsk., pkasis gregaria, between 2 and 6 weeks following final moult. The
wind-tunnel construction was described earlier (Camhi, 1969 a).
(1) Methods for studying flight initiation
For studying the start of a locust's flight I photographed the insect with a Bolex H 16 cine" camera at 64 frames/sec, as he leaped from a rough-surfaced platform. Photographing a rapid-sweep stop-watch served to check the camera's frame rate. For further observations of flight initiation I arranged a tethered locust, having all legs and antennae amputated, in front of a black card mounted just above the screen of
364 JEFFREY M. CAMHI
a dual-beam oscilloscope (Tektronix 502A). The card was visible to the oscilloscope camera, but the locust was not. A Leitz slide projector focused a 1 mm. diameter spot of light onto the extreme posterior tip of the closed forewings. At the instant the wings made the slightest move in the direction of opening, they ceased interrupting the light beam which therefore flashed a spot instantaneously on to the black card. The geometry was such that the spot flashed on to the card was in perfect vertical alignment with the two oscilloscope traces, which were spots, the sweep trigger having been turned off.
The oscilloscope displayed the instantaneous wind velocity, registered by a Flow Corporation 55A1 hot-wire anemometer. The anemometer's active wire (200 fi dia-meter) remained fixed about 3 mm. above the facial setae. A Grass C4 kymograph camera recorded on continuously moving film at 1 m./sec. the oscilloscope wind trace and the projected spot. The distance on the film between the signal marking wind onset and the first appearance of the projected spot gave the latency of the wing-opening response. I shall describe later a modification of this method which was also useful.
(2) Method for studying the regulation of for erring Hoist
Slight modification of a technique used by Gettrup & Wilson (1964) allowed deter-mination of forewing twist as a function of wind velocity flowing past the head. I positioned in front of the oscilloscope screen a tethered locust, having all legs and antennae amputated and the wounds sealed with wax to prevent bleeding. Each fore-wing bore a narrow marker of light-weight paper glued across the fore-wing chord. The locust wore on the anterior and lateral margins of the prothoracic cuticle a shield of aluminium foil, firmly attached with wax. Probing measurements with the anemo-meter showed that the shield, if properly shaped, effectively prevented the wind from flowing over any part of the body other than the head. It also prevented the wind from coursing into the wing path. Certain of these shapes effectively allow flow over the facial setae to be laminar.
With the room darkened, a 35 mm. Nikon camera viewed the oscilloscope screen and the locust. Single or repetitive flashes of a strobe light (General Radio Corporation Strobotac) mounted above the locust allowed unblurred images of the moving wing and its marker to be photographed without illuminating the oscilloscope screen.
It was difficult to make a locust fly for long periods at low wind speeds. Occasionally, however, certain individuals would perform well at low air speeds or in still air when I gradually reduced the velocity from the normal flight speed. After reducing the wind to a particular velocity, I waited at least 1 min. before taking the photograph. Opening the camera shutter, set for a 1 sec. exposure, triggered a 1 sec. sweep of the oscilloscope beams. One beam displayed the instantaneous wind velocity recorded by the anemo-meter probe just above the setae. The other beam, connected to the output of the strobe light, marked the instant of each strobe flash.
Locust wind receptors. Ill 365
Using this technique different photographs showed the wing in different positions within the stroke cycle. Only those records showing the wing in the mid-downstroke were of interest, since it is at this instant that the wing generates most power (Jensen, 1956; Gettrup & Wilson, 1964). As the results will indicate, differentiating between mid-upstroke and mid-downstroke wing images was accomplished easily on the basis of the wing-twist angle.
RESULTS
(1) Wing opening in tethered and free flight
The first series of experiments compared the latency from wind onset to wing openings for locusts under different conditions: first, with the insect creating its relative wind by jumping freely into flight, and secondly, with the insect tethered and exposed to wind flow from the tunnel. A similarity between the two latencies would lend support to the view that in natural flight the onset or acceleration of relative wind created by the take-off leap promotes reflex wing opening.
Free jumps. A locust, when placed on the launching platform, would usually explore the area for several minutes and then remain quiescent for a long while. Occasionally he would jump, usually several seconds after bringing the hindlegs into kicking posi-tion, allowing the photographer to capture the jump sequence on film. Reluctant individuals would usually jump in response to slight hand movements or gentle prodding. I photographed over 40 take-off sequences of six locusts.
Plate 1 shows the frame sequences of five typical take-offs. Sequences a and b show spontaneous jumps, while those of c-e were induced by hand motions or prodding. Approximately 15 m./sec. separates the action recorded in each frame. In each of the jump sequences, some degree of wing opening has ensued within 15 msec, of the onset
of the take-off.
Wind stimulation of tethered locusts. In the first paper of this series (Camhi, 1969 a) I showed that the conduction time of the wind-receptor neurones to the suboeso-phageal ganglion ranges between 7 and 14 msec. It seemed unlikely, therefore, that these receptors could mediate wing opening within 15 msec, of the onset of the jump wind. The point was examined, however, in the following series of experiments.
I arranged a tethered locust as described in Part 1 of the Methods section. I then accelerated the wind at different rates ranging from 100 to 600 m./sec.2, terminating at velocities between 2-5 and 4-5 m./sec. These ranges encompassed all the values observed in records of free jumps, such as those shown in PI. 1. I recorded over 150 such flight initiations of six animals.
Only velocities greater than 2-5 m./sec. provoked wing opening. The latency was shorter for faster accelerations within the range studied.
Text-fig, i shows the records on continuously moving film of two typical flight-initiation responses to wind onset. The latency to the beginning of wing opening is about 45 msec. The fastest response ever recorded was 35 msec. Thus the latency is two to three times as great as that observed in flight initiated by a free jump.
Tarsal release of tethered locusts. A second way of stimulating tethered locusts to fly is by removing contact between the tarsi and the substratum, thereby discontinuing a flight-inhibition reflex (Fraenkel, 1932; Weis-Fogh, 1956A). It seemed possible, therefore, that on a take-off jump the wings might open reflexively in response to loss of contact inhibition. Careful scrutiny of the jump sequences of PI. 1 reveals that normally the wings begin to open before the tarsi appear to have left the ground. However, since it is difficult to determine from such photographs the exact instant a leg loses contact, I determined experimentally the latency between tarsal release and wing opening for a tethered locust. For tarsal release to control wing opening in free flight, this latency would have to be no more than about 15 msec.
50 msec.
Text-fig. 1. Latency measurements, wind onset to wing opening—tethered locust. Records read from right to left. Rising line (small arrow) indicates wind acceleration from rest to 3-5 m./sec. First appearance of uppermost line in each record (large arrow) indicates moment of wing opening. Records show the latency to be about 45 msec.
I arranged a tethered locust in front of the black card as before, the closed forewing tips interrupting the projected 1 mm. light beam. The insect stood on a roughened rubber rod oriented in the plane of the body's long axis. The rod, loosely hinged to a separate ring stand in front of the animal, formed an angle of about 300, front end uppermost, with the body's long axis. The locust usually extended his legs until contact had been made with the rod and then remained quiescent. Because the meta-thoracic legs are longest and the prometa-thoracic shortest, the rod's angle enabled the locust to stand with all legs almost fully extended. Any downward pivoting of the rod on its hinge attachment would therefore release contact of all tarsi almost simultaneously. All tarsi were released well before the rod's tip had moved 1 cm.
The oscilloscope traces were two spots whose vertical positions were independently adjustable. Looking through the lens of the kymograph camera, I positioned the upper spot just below the rod, so that further lowering the rod by as little as 1 mm. would cut off the camera's view of the spot. I then brought the lower oscilloscope spot exactly 1 cm. below the first. The room was kept dark throughout the preparative and experimental procedures.
Locust zoind receptors. Ill 367
tarsal contact by giving a gentle downward tap to the rubber rod. Flight ensued. The photographic record indicated the onset of tarsal release by the rod's interruption of the upper oscilloscope spot. The interruption of the lower spot, 1 cm. below, indicated
= 6
50 msec.
Text-fig. 2. Latency measurements, tarsal release to wing opening—tethered locust. Records read from right to left. Gaps in the two lower beams of each record indicate depression of standing rod platform. Small arrow indicates last possible instant of tarsal contact. Numbers on right indicate number of intact legs. All records show the latency to be at least 30 msec. the instant before which all tarsal release must have been achieved. The first appearance on the film of the projected light beam indicated the first instant of wing opening.
368 JEFFREY M. CAMHI
mechanical shock reached the insect. Hand motions similar to those used in depressing the rod provoked no motion by the locust. I recorded more than 100 flight initiations using six animals^ on three of which I amputated varying numbers of legs successively during the experiment.
Text-fig. 2 shows a series of typical records from one locust. The upper two traces are from the intact animal. The interval between lower beam interruption and wing opening is about 30 msec. In these two cases, then, the latency from the last instant of tarsal release to the first instant of wing opening was at least twice that required for a locust to open his wings following a free jump (PI. 1).
The remaining three traces in Text-fig. 2 show the results of successive leg amputa-tion. Reducing the number of legs to one (any leg) does not alter appreciably the latency from tarsal release to wing opening. This result gives confidence that the latencies reported here are not unnaturally long owing to insufficient tarsal contact with the rod prior to release—reducing over-all tarsal contact to one-sixth (one remaining leg) left the latency unaltered.
In summary, the results of this section suggest that in locusts, neither release from tarsal contact inhibition nor onset of the jump wind detected by the facial setae initiates reflexive wing opening in free flight. Either factor, however, is capable of producing wing opening under the artificial conditions of tethered flight.
(2) Control of forewing twist
To further this study of wind-stimulated symmetrical flight responses, it seemed of interest to determine whether forewing twisting—a critical factor determining the locust's flight path (Weis-Fogh, 1956a; Gettrup & Wilson, 1964)—varies with the speed of a head-on wind directed at the facial wind receptors. Tethered locusts flew with prothoracic wind shields and forewing markers as described in part 2 of the Methods section. I made a total of more than 2400 separate exposures using six different locusts flying in response to winds from o to 4-0 m./sec. For about two-thirds of these photographs the wind tunnel maintained a constant flow rate. For most of the remaining photographs I accelerated the wind and then immediately flashed the strobe light. Finally, I performed several control measurements by moving the wind jet away from the head and directing the air stream directly over the wing. Frequent checks indicated that both forewings acted symmetrically, although only one was consistently photographed.
Plate 2 shows some typical photographic records. Plate za is an example of a multiple exposure at constant flow rate. Seven strobe flashes captured the wing in about the mid-positions of seven different strokes. Of the seven marker images produced, four are almost parallel and show the wing to be supinated. This indicates that in these four cases the wing was captured in the upstroke (Jensen, 1956). The other three parallel marker images show the wing to be pronated, and therefore in the downstroke. The angle between a wing marker and a horizontal oscilloscope grid line provided a con-venient measure of relative twist angle.
Locust wind receptors. Ill 369
labelled along and abscissa, begin arbitrarily with o°, the angle of the most pronated wing image photographed. The left-hand groups of bars represent mid-downstroke and the right-hand groups mid-upstroke twist angles. Stippled bars represent the
•§
E
I
Control
3-4 m./sec.
3-4 msec
2-7 msec.
2 0 msec
1 = L .
1-2 msec.
flr-H
0-9 msec.
0-7 msec.
0-5 msec.
0-3 msec
n II
0 0 msec.
J I 0° 10° 20° 30° 40° 50°
Relative forewing angle
60° 70° 80°
Text-fig. 3. Histogram of relative wing twist angle. Clear bars: constant velocity wind; stippled bars: 80—200 msec, after wind acceleration from rest; bars containing an ' x ': less than 80 msec, after wind acceleration from rest; controls: wind directed toward wings but not toward head. See text for details.
twist angles of wing images produced between 80 and 200 msec, after wind acceleration from rest. Bars marked by an ' x ' show twist angles less than 80 msec, after accelera-tion. Empty bars show twist angles for locusts in wind of maintained velocity.
37° JEFFREY M. CAMHI
1956a; Gettrup & Wilson, 1964), the upstroke twist angle remained essentially un-changed for all wind speeds and accelerations. The downstroke twist angle of the forewing, however, depended upon the velocity of a constant wind. Higher velocities resulted in less than normal downstroke pronation. The effect continued for at least 10 min., the longest interval studied. Text-fig. 4 plots the relationship between maintained wind speed and downstroke twist angle, and shows it to be non-linear; small speed differences have greater effect at low than at high wind speeds.
0 6 1-2 1-8 2-4
Wind velocity (m./gec.)
30 3 6
Text-fig. 4. Wind velocity over facial setae against relative forewing twist angle for downstroke. Plotted from data of Text-fig. 3. O, flight at constant wind velocity; • , flight 80-300 msec, after wind acceleration. Shows that forewings pronate less on the downstroke when facial setae experience higher wind velocity, especially just following wind acceleration.
The histogram of Text-fig. 3 further shows that between 80 and 200 msec, after an acceleration from rest to a given final speed, the decrease of downstroke pronation was exaggerated as compared with values for constant wind at that same speed (see also Text-fig. 4). However, photographs taken less than 80 msec, after such accelera-tion displayed twist angles similar to those of a locust flying in still air. Control measurements, with wind directed at the wing, likewise revealed twist angles like those in still air.
Locust wind receptors. Ill 371
DISCUSSION
(1) Flight initiation
Experiments reported in this paper show that the wind-stimulated flight-initiation response, observed in tethered locusts, proceeds too slowly to be the mechanism in-ducing wing opening following a free jump. The flight initiation resulting from loss of tarsal contact is likewise too slow. It is of interest, then, to inquire how the locust in nature opens his wings and begins to fly.
A fairly simple explanation suggests itself. Locusts, like most other insects, do not possess any flight muscles whose sole function is opening the wings. Rather this task devolves largely upon the wing elevator muscles, which also are crucial to the normal, patterned flapping of the wings (Wilson & Weis-Fogh, 1962). Waldron (1968) has shown that these muscles are the first to contract at the onset of flight. It is reasonable to suggest, then, that to open its wings at the beginning of a flight, a locust need only turn on its central flight motor. One can induce this process in a tethered locust by stimulating the facial wind receptor or by releasing the tarsi. However, when a locust jumps freely into the air, presumably a separate pathway, probably within the central nervous system, turns on the flight motor. Waldron (1968) has recently made a similar suggestion.
(2) Lift regulation
A second finding reported in this paper is that increased wind flow over the facial setae of a tethered locust results in decreased forewing pronation on the downstroke. The extent of the decrease is a function of wind velocity and is exaggerated just following acceleration.
The effects these wing-stroke changes impose on an insect flight are well established (Weis-Fogh, 1956a; Jensen, 1956). Decreased downstroke pronation in both fore-wings increases the ratio lift/drag, resulting in ascending flight.
It was usually not possible to perform the control experiment of covering the facial setae in order to prove definitively the involvement of the aerodynamic sensory structures. Locusts rarely would maintain flight with the setae covered. Nevertheless, with the antennae removed and the shield restricting wind to a course over the head, no other receptors appear capable of subserving this function.
The drastic leg amputation, without which the locusts would break the anemometer wire, was a potential source of error. However, Wilson (1961) showed that apparently normal flight occurs with the legs removed. Moreover, the reproducible flight response to different air speeds could hardly have resulted from amputation shock.
The previous paper (Camhi, 19696) reported neuronal responses of four different types recorded in wind-sensitive interneurones of the cervical connective. Two of these, the 'wind-indicator' and the 'acceleration' cells, gave symmetrical responses to wind from either side of the head, suggesting that any wind-stimulated flight manoeuvres which they mediate would be restricted to movements about the pitch axis. The major difference between these two cell groups was that wind-indicator cells adapted very slowly, while acceleration cells were highly phasic. Also, the wind-indicator cells responded to threshold air speeds as low as o-i m./sec, whereas threshold for acceleration cells was about 1-0-1-5 rn./sec.
372 JEFFREY M. CAMHI
The lift-control reflex reported in this chapter cannot result from the activity of acceleration interneurones. The behavioural response is greatest for wind velocity changes below 10 m./sec, speeds at which acceleration cells remain silent (Camhi, 19696). Moreover, the acceleration cells' phasic response expires after a few hundred milliseconds, whereas the lift-control reflex continues for at least 10 min. (Camhi, 19696). The responses of the other two interneurone types found, the 'wind-direction' and the 'recentre' cells, were also too phasic to subserve the prolonged lift regulation reported here.
It therefore seems reasonable to suggest that wind-indicator interneurones mediate the lift-control reflex. This suggestion implies that wind-indicator cells connect possibly through other interneurones, with mesothoracic motorneurones controlling the forewing twist muscles. Earlier findings (Camhi, 19696) suggested that these same interneurones also connect to the pterothoracic flight motor.
The histogram of Text-fig. 3 indicates that the lift-control reflex requires some 80 msec, or between one and two wingbeats, to affect forewing twisting. That the locust responds thus rapidly to sensory input contrasts with the view of Waldron (1968) that sensory control of flight in this insect generally requires tens of wingbeats to produce any observable effect.
The role that the lift-control reflex might play in natural flight is a matter of specu-lation. However, the physiological data correlate closely with at least one aspect of flight behaviour. As PI. 1 shows, a take-off jump is generally followed by ascending flight, which implies less than normal downstroke pronation of the forewings. The take-off jump produces a wind acceleration of magnitudes which, as this paper shows, result in a temporarily exaggerated decrease in forewing pronation within about 80 msec At this time after the onset of a jump a locust would be in his first or second complete wing cycle. It is possible, then, that one function of this wind-stimulated Hit-control reflex is to promote rapid ascent just following wing opening on the take-off jump. I have also shown (Camhi, in preparation) that the reflex is probably used, in con-junction with changes in the abdomen posture, to prevent stalls at low flight speed
by inducing a dive when air speed becomes critically low.
SUMMARY
1. Tethered locusts begin opening their wings about 45 msec, following wind stimu-lation of the facial setae.
2. Tethered locusts also begin wing opening at least 30 msec, after release of tarsal contact with the substratum.
3. High-speed cine photography reveals that an untethered locust in a take-off jump initiates wing opening as early as 15 msec, after the first detectable jumping motion.
4. Therefore in free jumps neither stimulation of the facial setae by the jump wind nor release of tarsal contact is the factor inducing wing opening. The actual mechanism acts much more rapidly, and probably involves central, rather than peripheral, coding.
Journal of Experimental Biology, Vol. 50, No. 2
Plate 1
Journal of Experimental Biology, Vol. 50, No. 2
Plate 2
Downstroke
(c)
Locust wind receptors. Ill 373
after which the effect is exaggerated for several hundred milliseconds, finally nailing off slightly to the maintained level.
6. The data suggest that the 'wind-indicator' interneurones of the cervical con-nective are active in the lift-control reflex.
7. A suggested behavioural function of this reflex is rapid ascent following the acceleration produced by a take-off jump.
I wish to thank Professors Ian Cooke and Kenneth Roeder for their most helpful critical reading of this manuscript.
REFERENCES
CAMHI, J. (1969a). Locust wind receptors. I. Transducer mechanics and sensory response. J. exp. Biol. 5°» 335-348.
CAMHI, J. (19696). Locust wind receptors. II. Interneurons in the cervical connective. J. exp. Biol. 50, 349-36a.
FRAENKEL, G. (1932). Untersuchungen liber die {Coordination von Reflexen und automatische-nervosen Rhythmen bei Insecten. I. Die Flugreflexe der Insecte und ihre Koordination. Z. vergl. Phyiiol. 16,
371-93-GETTRUP, E. & WILSON, D. M. (1964). The lift-control reaction of flying locusts. J. exp. Biol. 41, 183-90.
JENSEN, M. (1956). Biology and physics of locust flight. III. The aerodynamics of locust flight. Phil. Trans. B 339, 511-52.
WALDRON, I. (1068). Neural mechanisms by which controlling inputs influence motor output in the flying locust. J. exp. Biol. 47, 213-28.
WEIS-FOGH, T. (1949). An aerodynamic sense organ stimulating and regulating flight in locusts. Nature, Lond. 163, 873-4.
WEIS-FOGH, T. (1950). An aerodynamic sense organ in locusts. 8th Int. Congr. Entomol. (Stockholm, 1948), pp. 584-8.
WEIS-FOGH, T. (1956a). Biology and physics of locust flight. II. Flight performance of the desert locust (SMstocerca gregaria). Phil. Trans. B 339, 459-510.
WEIS-FOGH, T. (19566). Biology and physics of locust flight. IV. Notes on sensory mechanisms in locust flight. Phil. Trans. B, 339, 553-84.
WILSON, D. M. (1961). The central nervous control of flight in a locust. J. exp. Biol. 38, 471-90. WILSON, D. M. & WEIS-FOGH, T. (196a). Patterned activity of co-ordinated motor units, studied in
flying locusts. J. exp. Biol. 39, 643-67.
EXPLANATION OF PLATES
PLATE I
Cine sequences of locust jumping into flight; 64 frames per second, (a, 6) Spontaneous jumps, (c-e) Jumps induced by hand movements or gentle prodding. Grid markings: broad lines, 20 cm.; narrow lines, 10 cm. Sequences show that the wings begin to open within one frame (15 msec.) after the onset of the jump. At this instant most legs appear still in contact with the substratum.
PLATE 2
Photographs of wing twist angle and oscilloscope beams. Upper beam marks instant of each strobe flash making the photographic image. Lower beam indicates instantaneous wind velocity, (a) Repetitive strobe flashes, constant wind at 3-5 m./sec.; wing caught in upstroke four times, in downstroke three times. (6) Single strobe flush following wind acceleration from rest to 3-5 m./sec.; wing in downstroke. (c) Same; wing in upstroke. Series shows that wing twist angle changes with wind speed on the down-stroke, but not on the upstroke.
