J. Exp. Biol. (1969), 5°. 335-348
235
With 1 plate and 10 text-figures
Printed in Great Britain
LOCUST WIND RECEPTORS
I. TRANSDUCER MECHANICS AND SENSORY RESPONSE*!
BY JEFFREY M. CAMHIf
Biological Laboratories, Harvard
University
{Received 17 June 1968)
INTRODUCTION
Wing movements in locust flight derive chiefly from a central neuronal signal
generator determining the temporal pattern of flight muscle contractions (Wilson,
1961; Wilson & Weis-Fogh, 1962). However, several sensory inputs can modify this
basic pattern in important, though subtle, ways (Weis-Fogh, 1949, 1956a; Goodman,
1959; Gettrup & Wilson, 1964; Wilson, 1963; Gettrup, 1966; Dugard, 1967; Waldron,
1967). One such input derives from the bank of aerodynamic sensory setae on the
dorsal surface of the head in the desert locust (Schistocerca gregaria Forsk.). Guthrie
(1964) found these to be trichoid sensilla possessing hollow shafts 70-250 fi long, and
all curving forward in slightly different directions. He observed a single sensory neurone
under each seta, responding to deflexion of the shaft.
Weis-Fogh (1949, 1950) discovered that directing at the setae of a tethered locust a
' head on' wind stream (analogous to the relative wind which the insect itself creates in
free flight) results in flapping flight, provided that no tarsi make contact with solid
surfaces. Flight continues as long as the wind flows. Weis-Fogh also observed (1949,
1950) that wind directed toward the head from one side, as occurs in free flight during
a yaw, induces the locust to turn into the wind, in an apparent yaw correction manoeuvre. The minimum angle of relative wind needed to produce the turn was about
five degrees. However, since these locusts were severely encumbered by the measuring
apparatus, the actual accuracy may be even greater.
The ability to respond to wind direction with such accuracy poses the question—by
what means does the nervous system accomplish the directional coding implied by the
behavioural response? Haskell (1959) briefly reported that wind flowing from the
front or side of a seta evokes a slowly adapting train of sensory impulses, while flow
from behind produces no response. Sviderskii (1967) extended these observations to
the setae of Locusta migratoria. Guthrie (1964) suggested that a shaft's curvature in
some way limits its freedom of motion, thereby imposing some directionality on the
sensory response.
The experiments reported here were designed to provide a more complete answer
to the question of direction coding, and to lay the groundwork for recordings made
from interneurones, reported in the succeeding papers of this series (Camhi, 1969a, b).
• A preliminary report of this work has been published (Camhi, 1967).
t This work was supported in part by a predoctoral fellowship from National Institutes of Health.
X Present address: Section of Neurobiology & Behavior, Division of Biological Sciences, Cornell
University, Ithaca, New York.
336
J. M. CAMHI
MATERIALS AND METHODS
All experiments were performed on male desert locusts (Schistocercagregaria Forsk.,
phasis gregaria), between 2 and 6 weeks after final moult. Locusts, obtained from the
Anti-Locust Research Centre, London, were maintained at 27 + 3 0 C. and were fed
with fresh grass (fresh lettuce and dried grass during winter), bran middlings and water.
Wind stimulation experiments required an open-throat wind tunnel having the
following specifications: wind stream at least 8 mm. diameter, laminar over a velocity
range of 0^5—4-5 m./sec.; capability to change very rapidly (within about 50 msec.)
wind velocity with the range 0-4-5 m./sec.; capability to make equally rapidly changes
of wind angle up to 150.
A glass tube of inner diameter 13 mm. met these requirements when connected by a
hard rubber hose 3 m. long to the building compressed air supply, equipped with an
accurate reduction valve. By fastening the glass tube to the rim of a wheel the wind
could be directed from any angle towards the insect's head, located exactly at the
wheel's central axis. The rubber hose was supported centrally above the insect in such
a way that turning the wheel did not affect wind velocity. The open end of the jet was
always 1 cm. from the locust's head, which was entirely within the region of laminar
flow. A Flow Corporation (55A1) hot-wire anemometer served to calibrate the wind jet.
For electrophysiological experiments, CO2 or cold anaesthesia immobilized the
insect during dissection. The exposed tissues, if kept in Pringle's (1938) or Weis-Fogh's
(19566) saline at temperatures of 27 ± 30 C , remained responsive for hours. Salinefilled pipette electrodes, slipped over the cut tip of a seta's shaft, recorded a sensory
response when the pipette deflected the shaft. Wind-stimulated sensory responses
could be recorded by teasing out a nerve containing sensory axons and either drawing
up a tiny bundle into a pipette electrode or laying a bundle across fine-tapered
chlorided silver electrodes. Electrical stimulation was by chlorided silver electrodes.
Nerve impulses were amplified by a differential input, capacitance-coupled amplifier
(Grass P4), displayed on a dual beam oscilloscope (Tektronix 502A) and recorded on
moving or stationary film with a Grass C4 camera. Electrical stimulation was by a
Grass SD5 stimulator, producing rectangular pulses of less than 1 msec, duration.
RESULTS
(1) The sensory response to wind
Text-figure 1 presents a schematic drawing of a single seta. To avoid interfering
with the air flow around the setae, it was necessary to record sensory responses as far
away from the cephalic surface as possible. Sensory impulses can be registered from
axons in the circumoesophageal connective. (Experiments reported below (§4) will
show that such impulses are in fact sensory.) Because these axons are very minute
(1 ft diameter, Guthrie, 1964) I have obtained complete sets of data from only three
sensory cells, in three different animals, and partial data on two other sensory cells.
Text-fig. 2 shows examples of sensory responses to a 4-5 m./sec. wind from io° left
of centre, recorded in the left circumoesophageal connective. Wind elicits a train of
impulses at an initial frequency of about 200/sec. Within about \ sec. the cell adapts to
a maintained plateau level of about 125 impulses/sec. The plateau frequency decreases
further by some 10% within 5 min.
Locust wind receptors. I
337
Text-figure 3 is a polar plot of this cell's responses, showing the plateau frequency as
a function of wind direction at three different flow velocities. As these curves indicate,
for anyflowvelocity, the cell responds maximally to wind flowing in the plane of the
shaft's curvature. In each of the five cells studied the optimal direction differed by
less than 3° from the observed angle of shaft curvature. This was within the error
of observation, about + 5°.
Shaft
Socket
Dendrite
Cuticle
Cell body
Axon
SO/'
Text-fig. 1. Aerodynamic sensory seta, schematic drawing. All cuticular elements are stippled.
Note the alignment of the two asymmetries—shaft curvature and dendritic attachment. Similar
to diagram by Guthrie (1964).
^
200 ji V.
0 2 sec.
Text-fig. 2. Sensory responses to wind. Recorded in the left circumoesophageal connective.
Wind at 4-5 m./sec. from io° left of centre. Upward arrow indicates the approximate moment
of wind onset; downward arrow, wind cessation. Record reads left to right. The initial burst of
spikes levels off within J sec. to a plateau frequency.
From a direction plot such as that of Text-fig. 3, one can describe the accuracy with
which a single sensory cell discriminates wind direction. This accuracy can be expressed
338
J. M. CAMHI
quantitatively as the angle between the two half-peak values of the curve. The morei
acute the half-peak angle, the more accurate is the direction response.
The sensory response plotted in Text-fig. 3 is fairly directional, showing a half-peak
angle of 540 ± 40 for the three wind speeds. Two other axons recorded gave values of
48° + 3 0 and 57° ±6°.
Text-figure 3 also shows that impulse frequency is approximately linearly related to
wind velocity, a point which I shall consider in the Discussion section.
180°
Text-fig. 3. Wind direction against plateau spikefrequency.Maximal response to wind from
15° left. Arrow indicates angle of seta curvature plane, 130 left. O, 4-5 m./sec.; x , 3-0 m./sec.;
• , 1-5 m./sec. The graph shows that the sensory frequency depends sharply on wind direction,
and is about linearly related to wind speed.
(2) Transducer mechanics
The aerodynamic setae, then, can register wind direction because each seta responds
maximally only to wind flowing in the plane of its shaft curvature, and different shafts
have different curvature planes. As Text-fig. 1 shows, two clearly defined structural
asymmetries are detectable in each seta (Guthrie (1964), and my histological sections):
the shaft curvature and the anterior dendritic attachment to the shaft base. (In cross-
Locust wind receptors. I
339
section the shaft is circular with minute, regular flutings around the entire circumference.) There also existed the possibility of some structural asymmetry within the
socket itself. I investigated each of these three potential sources of directional
information—dendrite, socket, and shaft—under conditions precluding interference
from the other two.
To determine the contribution of the eccentric dendrite attachment to direction
coding, I recorded sensory responses using a pipette electrode fitted over the shaft tip.
180°
Tert-fig. 4. Deflecting direction against spike frequency. Recorded with pipette electrode over
shaft. 50 /* deflexions. Maximal response to bending is from 20° left. Arrow indicates angle of
seta curvature plane, 20° left. Note that this response is less directional than the sensory
response to wind (Text-fig. 3).
With the pipette very firmly attached to a calibrated micromanipulator, advancing the
manipulator could deflect the shaft a known distance in any desired direction. Bending
always occurred at the socket, the rigid shaft retaining its shape during all movements.
The use of tactile, rather than wind, deflexion precluded any possible aerodynamic
effect of the shaft curvature. This method of stimulation probably also negated any
influence of the socket's elastic resisting force because, by comparison, the force used
here to deflect the seta was enormous.
Sixteen experiments gave complete sets of data. Text-figure 4 shows the polar
curve for a typical seta deflected 50 /i in different directions. (Wind deflects a seta
22
Exp. BioL jo, 2
34°
J- M. CAMHI
about the same distance.) The optimal direction for each seta was within io° of its
curvature plane. The frequency at this optimal direction was about ioo spikes/sec.
Half-peak angles for the i6setaewere85° ± 130. This, then, is a less directional response
than that of the sensory axon in wind, and therefore the eccentric dendrite attachment
cannot by itself account for the directional properties of the receptors.
Determining any preferred direction prescribed by the socket's elastic force called
for a method of comparing the magnitudes of the forces required to deflect the shaft in
180°
Text-fig. 5. Deflecting direction against relative deflexion force. Bending seta 50 /* required
least force from about 200 right. Arrow indicates seta curvature angle, 27° right. This socket
behaviour is much less directional than the wind-stimulated sensory response (Text-fig. 3).
different directions. Carefully excising a small strip of cephalic cuticle and gently
peeling away its loose hypodermis did not visibly deform the cuticle or setae. Such
strips could be mounted on a transparent glass dial which was rotatable through 3600,
and individual setae could then be observed with transmitted light through a compound
microscope. It was crucial, for the avoidance of optical errors, that the tip and base of
any seta measured be in perfect vertical alignment.
Using the fine control knob of a calibrated micromanipulator, I carefully rested a
tiny loop of elastic silk, taken from the cocoon of a silk moth (Antheraea polyphemus),
tangentially against the shaft near its tip. Further advancing the silk fibre deflected
the shaft upon its socket articulation. Because a silk loop is highly elastic, the distance
Locust wind receptors. I
341
that the loop must advance to produce a given shaft deflexion is proportional to the
socket's elastic resisting force.* I determined the shaft deflexion using an ocular
micrometer, and the loop movement by reading the manipulator's calibration. At
different angles (relative to the direction of shaft curvature) I imposed on the shaft a
50 [i tip deflexion and recorded the loop movement. Repeating the measurement at
different angles (different settings of the transparent dial) allowed determination of the
relative force as a function of angle. Complete sets of data were taken for eight setae.
Text-figure 5 shows a typical polar plot of relative force for a 50 ji deflexion of a
freshly prepared seta. As the graph indicates, the socket does impose upon the shaft a
preferred direction. However, the half-peak angles for the eight sockets studied were
very broad, 1830 ± 250. The optimal directions again were closely similar to the directions of shaft curvature.
Studies of the socket's morphology indicate a possible structural basis for this
mechanical behaviour. Plate 1 show photomicrographs of the outer and inner surfaces
of a strip of cuticle just after its excision from the head. Just behind each seta is a clear
cuticular specialization, visible from either surface. Because sectioning this material
proved difficult, I did not attempt a detailed structural investigation. However, it
seems reasonable that the clear area is related to the socket's force asymmetry, since
its outline is almost identical in shape, and just opposite in orientation to the polar
curve of the deflexion force.
In order to study any possible aerodynamic influence of the shaft curvature upon the
seta's direction discrimination, it was necessary to replace the direction-giving socket
with an ' ideal', non-directional socket. An individual shaft could be plucked or excised
delicately from its socket and remounted in a tiny drop of rubber cement applied to the
surface of the transparent dial. Holding the shaft in place with a tiny probe on a
micromanipulator allowed the glue to dry with the base and tip in vertical alignment.
Perfect alignment was crucial for avoiding optical errors of measurement.
A preliminary test, using the silk fibre deflecting method, indicated whether such a
newly synthesized socket was in fact non-directional (polar curve a circle about the
zero point). I selected 12 such non-directional mountings for further study.
A Pasteur pipette replaced the wider tube of the wind jet and directed towards the
shaft a horizontal stream of wind. Flow was laminar at all velocities used. When the
wind flowed, the shaft deflexion, measured by an ocular micrometer, was proportional
to the shaft's aerodynamic drag for that particular angle between shaft curvature plane
and wind direction. Repeating the measurement at different rotational settings of the
dial allowed a determination of drag as a function of angle, f
Text-figure 6 shows the polar curve of a typical shaft's relative aerodynamic drag for
different wind velocities. The curve is bimodal, the two maxima corresponding closely
to the direction of shaft curvature and the exact opposite direction. The half-peak
angles for the front mode (angle of curvature) of the 12 shafts measured were 1030 ± 120.
The opposite mode gave more variable readings. However, this latter mode has no
• The elastic characteristics of the silk fibre are unknown. If the elastic modulus were not a constant,
the measurements reported here would underrate, rather than exaggerate, any asymmetrical forces of the
socket.
t The elastic properties of the rubber cement socket are unknown. As was true in the previous series
of measurements, a non-constant elastic modulus would cause these determinations to underrate rather
than exaggerate any effect of shaft curvature upon drag at different angles.
342
J. M. CAMHI
obvious biological significance, since deflecting a seta forward elicits no sensory
response (Text-fig. 3).
In summary, then, each shaft's asymmetric drag, each socket's asymmetric elastic
force, and each dendrite's eccentric attachment contributes in registering that sensory
cell's preferred direction. No one of these three factors appears sufficiently sharp
directionally to produce by itself the high degree of angular sensitivity observed in
wind-stimulated sensory recordings. Presumably the sensory cell sums the three
directional effects to produce its more accurate angular discrimination.
180°
Text-fig. 6. Wind direction against shaft relative drag. Drag maximal for wind from 8° right.
Additional peak for wind from 175° left Wind velocity: 0-3 m./sec., 0-1-5 m./gec. Arrow indicates seta curvature direction, 9° right. This response is less directional than the wind-stimulated
sensory response (Text-fig. 3).
(3) Direction optima for the entire sensory organ
Since for each seta and its sensory cell the direction of maximum sensitivity is
identical to the shaft curvature plane, one can determine this optimal direction for
any sense cell simply by visual inspection. Text-figure 7 maps the optimal wind directions for the largest of the setae. Smaller setae, not shown here, curve as their larger
neighbours. This map is bilaterally symmetrical and nearly identical from one locust
to the next.
Locust wind receptors. I
343
(4) Functional neuroanatomy
Guthrie's (1964) light-microscopic observations suggested that at least some of the
sensory cells have processes which traverse the entire brain, synapsing in either the
suboesophageal, or one of the thoracic ganglia. To investigate this question more fully
I first recorded the wind responses of single axons in the circumoesophageal connective.
Then, after locating any individual seta whose deflexion would evoke a response in this
Antenna
Ocellus
^
Compound
eye
Text-fig. 7. Map of shaft curvature plane angle, for largest seta shafts. Smaller shafts, not shown
here, curve as their larger neighbours. The shaft angle is almost identical to the seta's optimum
wind direction.
axon, I recorded simultaneously from the axon and the cut tip of the seta. Owing to
the small axon diameters, only three successful experiments were performed.
As Text-fig. 8 shows, every impulse was recorded by both electrodes, demonstrating
that either no synapses or only one-to-one conducting synapses intervene in the brain.
The latency separating the two records of any action potential was about 10 msec.
Deflecting any other seta neither elicited an axon response nor altered the axon spike
frequency evoked by deflecting the first seta. In each case, the circumoesophageal
connective was ipsilateral to the seta inducing the response.
Next, when recording over an undeflected seta, electrical stimulation of the entire
ipsilateral circumoesophageal connective evoked one antidromic spike per stimulus
344
J- M. CAMHI
(Text-fig. 9, upper trace). In each of the more than 40 setae studied, the latency was
7-14 msec, giving a conduction velocity of about 0-5 m./sec, a value consistent with
the small axon diameter. Contralateral setae gave no response. This antidromic throughconduction requires that if any synapse exists on the pathway from set a to suboesophageal ganglion it must be non-polarized. Since Guthrie's (1964) observation sug-
1OO/(V.
0 5 sec.
Text-fig. 8. Simultaneous recordings from a sensory seta and an axon of the circumoesophageal
connective, as illustrated in the diagram. Each action potential is recorded by both electrodes.
sens, sensory setae; br, brain; to, suboegophageal ganglion; tu t,, t3 resp., 1st, 2nd and 3rd
thoracic ganglia; Rlt Rlt two recording positions.
,100
_ J /.V.
20 msec.
r~ "
100
1 «V.
40 msec.
it*-*—•—w
,100
20 msec.
Text-fig. 9. Pipette recordings over seta. Upper trace, 0 6 V., 1-5 msec, shocks to circumoesophageal connective. One sensory impulse evoked per stimulus. Middle trace, increasing voltage
gradually to 1-5 V.; brings in a second active unit. Lower trace; 1-5 V.,i msec, shocks to anterior
prothoracic connective; only the second unit fires. Last three records of lower trace; same, with
pipette deflecting the shaft to produce sensory impulses (large arrows). Small arrows indicate
shock artifact, tu st, two stimulating positions. Other labels as in Text-fig. 9.
Locust wind receptors. 1
345
gested no brain synapses for at least some sensory cells, and since non-polarized
synapses are relatively rare, the most likely condition is that all aerodynamic sensory
cells have axons which traverse the brain without synapsing.
When recording from a cut seta tip, stimulation of the cervical connective (just
posterior to the suboesophageal ganglion) evoked no response in more than 30 setae
tested. This finding suggests that, contrary to Guthrie's (1964) observations, the
sensory axons synapse in the suboesophageal ganglion, probably none of them
continuing through to the thorax.
(5) Accessory neurone
If, when stimulating the circumoesophageal connective, the stimulus voltage was
increased to two or three times threshold, a second unit could be recorded by the
pipette over any ipsilateral seta. For each of 22 setae studied, this 'accessory neurone'
gave a lower amplitude response and was always slower conducting (latency about
25 msec.) than the sensory cell (Text-fig. 9, middle trace).
MOO^V.
20 msec.
100AV.
0-2 msec.
Text-fig. 10. Pipette recordings over a seta, (a) Sensory impulses (large spikes) produced by
deflecting shaft. Accessory cell impulses (arrows) evoked by 1-8 V., i msec, stimulation of
cervical connective. Accessory cell spike causes no change in sensory inter spike interval.
(6) Continuous sensory discharge produced by deflecting seta. Stimulation of anterior
prothoracic connective, 2-2 V., i*a msec., repeated at io/sec. Accessory cell responses (arrow)
produced no consistent change in sensory interspike interval.
Stimulation of the cervical connective, while not evoking a sensory response, did
induce, in 12 setae studied, one accessory cell impulse per shock (Text-fig. 9, lower
trace). The latency was some 30 msec. The accessory cell impulses were again of
lower amplitude than sensory spikes produced by simultaneously deflecting the seta.
Stimulation of the connective between the prothoracic and mesothoracic ganglia
never produced a response in the 12 setae studied. Thus the accessory neurons link the
sensory setae with the prothoracic ganglion. This pathway presumably either lacks
synaptic interruption, or includes one-to-one transmitting synapses.
Histological and electrophysiological findings indicate the presence of only one
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J. M. CAMHI
sensory cell under each seta (Guthrie, 1964, and this paper). Nevertheless, the possibility remained that the accessory cells were sensory in function. Careful scrutiny of
the oscilloscope records following bending, twisting or otherwise manipulating the
shaft with a recording pipette always revealed only the large sensory spikes. Adding
grass extracts or various sugars to the saline of the recording pipette also produced no
accessory cell response, though deflexion by such a pipette evoked sensory spikes.
Another possibility was that the accessory neurones function as efferent controls of
the sensory cells. Histological studies reveal no muscle cells in the area of the setae, but
synaptic control upon the sensory cell itself remained to be tested. Guthrie (1964)
cauterized the cephalic cuticle of locusts, destroying most or all of the sensory cell
bodies. After allowing 3 weeks for nerve degeneration, he found histologically that
some axons persisted in the sensory nerve. One possible interpretation is that the
remaining axons belong to efferent neurons.
To see whether the accessory neurones exert efferent control on the sensory response,
I first deflected a seta with the pipette electrode, evoking a train of impulses. After
a few seconds I stimulated, with repeated shocks, the cervical connective. Each
stimulus evoked one spike in the accessory cell, but no sensory impulses. The appearance of an accessory cell spike does not correlate with any consistent changes in
sensory cell inter-spike interval (Text-fig. 10). The accessory cells therefore appear not
to function as efferent controls of the sensory response.
All attempts to evoke an accessory cell response by stimulating more posterior or
peripheral parts of the nervous system, including the peripheral nerves to the prothoracic ganglion, were without result. Thus both the function and the central connexions of these cells remain unknown.
DISCUSSION
The experiments reported in this chapter show that each aerodynamic sensory cell
responds to wind with a train of impulses whose initial high frequency adapts to a
maintained plateau level. Each sensory cell responds maximally to wind flowing in a
specific direction. It is important to recall, however, that the frequency of a windstimulated sensory response is a function of both wind direction and wind velocity.
This means that without some independent velocity measurement the locust could not
employ individual sensory cells to achieve an accurate direction reading such as that
implied by Weis-Fogh's (1950) behavioural experiments. As the next paper in this
series will show (Camhi, 1969 a), this requirement is met in an interesting way by
certain intemeurones.
One factor determining the directional nature of the sensory response was the variation with wind angle of a shaft's aerodynamic drag. This was in one sense a surprising
result. Aerodynamic theory prescribes that in conditions of low Reynolds number, drag
is determined primarily by overall surface area, and not by the physical conformation
of the object. Since the Reynolds number of a single shaft at physiological air speeds
is very low, one should expect drag to be relatively independent of wind angle. The
empirical result is therefore difficult to reconcile with theory.
All three observed structural asymmetries of the seta—shaft curvature, socket force
asymmetry, and eccentric dendrite attachment—probably contribute to the directional
Locust wind receptors. I
347
sensitivity of the sensory response. However, since each of these three factors is
measured in different units (drag, elastic force, and spikes per second) it is impossible
to sum these quantitatively in any meaningful way. It is therefore also impossible to
determine which, if any, of the three factors exerts the greatest effect in determining
the directional properties of the sensory response.
Finally, Nicklaus (1965) has shown that responses from wind receptor hairs on the
cockroach cercus are directionally sensitive. Those hairs resemble in many ways the
ones presently investigated, suggesting that organs of this type may be fairly general
features of insect sensory equipment.
SUMMARY
1. The sensory cell innervating each wind-receptor hair on the face of the desert
locust responds to wind with a slowly adapting train of impulses.
2. Each sensory cell responds maximally to wind flowing in a specific direction. The
optimal direction for any sense cell is the same as the angle of curvature of its hair
shaft.
3. The optimal wind direction has been determined for each sensory cell of the
organ.
4. Three independently measured factors determine a sense cell's direction response : drag asymmetry of the curved shaft, elastic force asymmetry of the socket, and
the eccentric attachment of the dendrite.
5. The sensory cells probably continue uninterrupted through the brain, synapsing
first in the suboesophageal ganglion.
6. An accessory neurone of unknown function and unidentified central connexions
links each seta with the prothoracic ganglion.
My thanks to Professors Ian Cooke and Kenneth Roeder whose critical reading of this
manuscript was most helpful.
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WILSON, D. M. (1961). The central nervous control of flight in a locust. J. exp. Biol. 38, 471-90.
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EXPLANATION OF PLATE
Plate. 1. Photomicrograph of cephalic cuticle surface, showing asymmetrical socket morphology. Upper
micrograph: outer cuticle surface; lower micrograph: inner cuticle surface. S, socket. Arrow heads
point to asymmetrical socket structures. Each shaft (out of focus) curves in the direction indicated by the
arrow head on that socket. There is a close correspondence between the shape and orientation of the
socket and of the polar curve in Text-fig. 5.
Journal of Experimental Biology, Vol. 50, No.
JEFFREY M. CAMHI (I)
Plate 1
(Facing p. 348)