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First published online November 17, 2006
Journal of Experimental Biology 209, 4622-4637 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02554
Morphology and action of the hind leg joints controlling jumping in froghopper insects
Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK
e-mail: mb135{at}hermes.cam.ac.uk
Accepted 18 September 2006
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The lever arm of the hind trochanteral depressor muscle is smallest at the cocked position, but does not appear to go over the centre of the pivot. It then increases to a maximum after some 80° of depression movement. By contrast, the lever arm of the trochanteral levator tendon is similar over the range of joint movements and is exceeded by that of the depressor only after 40° of depression. Three prominent arrays of hairs on the trochantin, coxa and trochanter are appropriately positioned to act as proprioceptors signalling key movements in jumping.
In the fully levated position, a protrusion on the dorsal, proximal surface of a hind femur engages with a protrusion from the ventral and lateral part of a coxa. These structures are not present on the front and middle legs. Both protrusions are covered with a dense array of small projections (microtrichia) that both increase the surface area and may interlock with each other. To depress rapidly in a jump these protrusions must disengage. If the hind leg of a dead froghopper is forcibly levated, it will lock in its cocked position, from which it can depress rapidly by movement of the coxo-trochanteral joint and disengagement of the femoral and coxal protrusions. A prominent click sound occurs at the start of a jump that results either from the initial movements of the coxo-trochanteral joint, or from the disengagement of the microtrichia on the coxa and femur. Larval Philaenus, which do not jump, lack a femoral protrusion and have no microtrichia in equivalent positions on either the coxa or femur.
Key words: locomotion, tendon, muscle, Auchenorrhyncha
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) that propels
them to a height of about 115 times their body length
(Burrows, 2003). The key
movements of the hind legs in generating these movements are the rapid
depression of the trochantera about the coxae, accompanied by extension of the
tibiae about the femora. The hind legs are, however, only about half the
length of the body, indicating that the mechanical advantages to be gained by
using long levers (Alexander,
1995) are not exploited by froghoppers in jumping. The force
requirement for a jump is much greater than could be provided by direct muscle
contractions in the brief time (less than 1 ms) that the acceleration is
applied. Short hind legs coupled with the intrinsic limitations of muscle
suggest that a catapult mechanism must be used. This, in turn, implies that
there must be anatomical specialisations of the hind legs to allow force to be
generated slowly, whilst they remain stationary, and then to be delivered
suddenly to produce a rapid and powerful movement.
Many insects that jump have accompanying specialisations of their limbs,
joints, muscles and motor patterns that allow rapid movements to be generated.
These specialisations are most marked in the hind legs that generate the
propulsive force. Fleas power their jumps by contracting the enlarged
trochanteral depressor muscles and storing energy in resilin
(Bennet-Clark and Lucey, 1967;
Rothschild and Schlein, 1975;
Rothschild et al., 1972). The
stored force is released apparently by the contraction of a small muscle that
changes the point of action of the depressor muscle so that the short hind
legs can be depressed rapidly. Flea beetles have enlarged hind femora each
containing a spring formed by the curling and chitinisation of the extensor
tibiae tendon (Furth, 1988;
Maulik, 1929). The tibia is
probably locked into the flexed position when a small triangular plate at the
femoro-tibial joint is moved by contraction of the flexor muscle and is
released in a jump by changes in the flexor force and as the spring snaps back
to its original shape.
Grasshoppers have hind legs that are both long and robust with femurs that
are 10 times the mass of those of the other legs. The contractions of the
large extensor tibiae muscle within the femur distort the femoral cuticle and
bend highly sclerotised semi-lunar processes at the femoro-tibial joint, so
storing energy to provide the propulsive force for a jump
(Bennet-Clark, 1975;
Burrows and Morris, 2001). The
tendon of the small flexor tibiae muscle has a pouch that engages with a
femoral lump that changes the line of action of the tendon as the tibia flexes
(Heitler, 1974). These
specialisations restrain movements of the tibiae while the large extensor
muscles contract slowly before a jump. False stick insects (Orthoptera,
Proscopiidae) have long hind legs with curved tibiae, but still require energy
to be stored before a jump. The semi-lunar processes on the femora are much
reduced and are not bent before a jump, but the tibiae themselves do bend
(Burrows and Wolf, 2002).
Again, complex arrangements of the lever arms ensure that the small flexor
tibiae muscle can restrain the contractions of the larger extensor before the
tibia is rapidly extended to power a jump. Both of these insects combine an
ability to store energy in advance of a jump and then release it suddenly,
with the beneficial effects of long legs as levers. Bush crickets, however,
seem to rely more heavily on the leverage from their exceptionally long hind
legs so that some jumps are powered by direct muscle contractions acting on
long levers (Burrows and Morris,
2003).
This paper examines the specialisations of froghopper hind legs that make
possible their extraordinary jumping ability. It focuses on the structure and
actions of the coxo-trochanteral joint of a hind leg, the rotation of which is
the key action in generating rapid jumping movements
(Burrows, 2006). It describes
the structure and articulation of this joint, possible proprioceptors that
monitor its movements and the lever arms of its depressor and levator muscles.
It demonstrates how a novel locking mechanism between the femur and the coxa
might enable the trochanteral depressor muscle to contract slowly without
moving the leg and then suddenly release the rapid jumping movement.
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The anatomy of the hind legs and metathorax was examined in live insects, in insects preserved in 50% glycerol, in 70% alcohol, after fixation in 5% buffered formaldehyde and subsequent storage in 70% alcohol, and when cleared by boiling in 5% potassium hydroxide. Drawings of the legs, joints and muscles were made with the aid of a drawing tube attached to a Leica MZ8 or MZ16 stereomicroscope. Photographs from these microscopes were made with Nikon D1 or Nikon DXM1200 digital cameras. Dried specimens were also mounted on specimen holders, sputter coated with gold and then examined in a Philips XL-30 Scanning Electron Microscope.
Sequential images of the movements of hind legs that underlie jumping, or
imposed movements of the hind legs, were captured at rates of 500, 1000 or
2000 s-1 with a high speed camera (Redlake Imaging, San Diego, CA,
USA) and associated computer. Movements of a hind leg of a restrained
Aphrophora or Cercopis were also recorded by gluing a small
disc of reflective tape to a hind femur close to the femoro-tibial joint. A
modified single lens reflex camera with a concentric light around the lens was
focussed on the disc and the light reflected from it was captured by a
photocell in the film plane of the camera
(Hedwig, 2000).
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). They have a
mass that is only 2% of the total body mass, largely because the main muscles
powering a jump and controlling the movements of the trochantera are in the
thorax (Burrows, 2007). The
tibia of a hind leg is longer than the femur
(Fig. 1). For example, in
Philaenus it is 1.8±0.07 mm (mean ± s.e.m.,
N=34) and thus 64% longer than the femur (1.1±0.03 mm). In
larger froghoppers this difference is accentuated so that in Cercopis
a tibia is 81% longer than a femur and in Aphrophora it is 92%
longer. A hind tibia in all species has two outwardly pointing spines and two
semi-circular rows of smaller spines on its ventral surface close to the
articulation with the tarsus. The two most proximal tarsal joints also have a
ventral, semi-circular row of spines. These spines should increase traction as
thrust is applied through the tibia and tarsus to the ground at take-off in a
jump. The front and middle pairs of legs have no spines and have usually lost
contact with the ground before take-off in a jump.
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)] that engage with those of the other coxa. A hind femur has a protrusion on its dorsal surface that is not present on the front or middle legs (Fig. 2A). As the trochanter levates fully, this protrusion rides over the coxal protrusion and engages with it so that in this cocked position (shown on the right in Fig. 1), the proximal femur is closely apposed to the coxa. The more distal parts of a hind leg, including the femoro-tibial joint, then lie between the thorax and a middle leg.
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Angular movements of the leg joints
The coxae of the two hind legs are closely apposed at their medial
surfaces, in contrast to those of the front and middle legs that are widely
separated, so that the mouthparts are interposed between them. A hind coxa can
move about its articulation with the sternal region of the metathorax by some
25° in a plane that allows the whole hind leg to be rotated forwards and
downwards, or backwards and upwards, in a plane nearly parallel to the long
axis of the body (Fig. 2A).
When viewed ventrally (see drawing in Fig.
1, Fig. 3 and
scanning electron micrographs in Figs
7,
8, and
9), one pivot of this
thoraco-coxal joint is seen to be at the anterior, ventral edge of the lateral
coxa and the second at the anterior, medial edge of the coxa. Movements of the
trochantin, a sclerite embedded in the flexible cuticle linking the coxa
anteriorly with the thorax (Fig.
1) have not been analysed.
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The femur is not fused to the trochanter and can rotate about it through some 30° (Fig. 2B). The dorsal and ventral pivots are both lateral so that the plane of movement is similar to that of the trochanter. The tibia can flex or extend through some 170° about the femur, again in much the same plane, and the tarsus can levate and depress through some 45° about the tibia (Fig. 2D,E). Thus the segments distal to the coxa move in the same plane that is determined by the rotation of the coxa about the thorax. Thrust applied by depression of the trochanter will thus be applied in the same plane through all of the more distal joints of a hind leg.
Movements of the joints during a jump
To resolve the detailed movements of the proximal joints of the hind legs
during jumping, Aphrophora were fixed ventral surface uppermost in
PlasticeneTM in such a way that all the legs were free to move. Very
rapid and simultaneous movements of both hind legs occasionally occurred
spontaneously or could be evoked by gentle tickling hairs on the abdomen with
a fine paintbrush. No differences in the form, sequence, or timing of the
movements of the two hind legs could be discerned in images of these sequences
compared with those in free jumping
(Burrows, 2006;
Burrows, 2007). The sequence of
movements of an individual hind leg could be resolved into the following
sequence of actions (Fig.
3).
First, there was a slow levation of the trochanter about the coxa and a
smaller decrease in the angle of the femur about the trochanter, accompanied
by a flexion of the tibia about the femur. These changes moved the femur
anteriorly so that it passed ventral to a lateral protrusion on the coxa, with
which it then engaged, and medially so that it was pressed against the lateral
edge of the coxa (Fig. 3A,B).
This cocking movement of a hind leg lasted some 110 ms in the example shown,
but in different attempted jumps by Aphrophora, the mean time taken
was 187.3±8.9 ms, range 88-353 ms, median 183 ms, N=45
(Burrows, 2007).
Second, the hind legs remained in this cocked position without moving for a
period of 2 s in this example, though again this period was variable in
different attempted jumps by Aphrophora (mean 2.9±0.16 s,
N=160) (Burrows,
2007). Compression of the ventral thorax during this period meant
that the hind legs moved anteriorly and thus closer to the middle legs,
thereby pressing the trochantin against the flexible membrane of the
metathorax and against the middle legs (see pairs of horizontal lines in
Fig. 3A).
Third, after this long period with no visible movements of the leg joints, both hind legs depressed and extended completely in 1 ms (Fig. 3A,B). The main movement of each hind leg was a rapid depression of the trochanter about the stable coxa, accompanied by a small increase in the trochantero-femoral angle that may have resulted from the centrifugal forces generated by the rapid depression of the coxo-trochanteral joint. The structure of this joint was therefore examined in detail.
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Lever ratios
The lever ratios of the trochanteral depressor and levator muscles were
determined from individual frames captured at rates of 1000 s-1 as
a hind leg moved naturally, or when forcibly moved over its entire angular
range. When viewed ventrally (Figs
1,
4) the line of action of the
depressor tendon always appears to have a positive mechanical advantage, even
when the joint was in its most levated position. It does not appear to go over
the centre of the pivot in this cocked position, and because the pivot is so
lateral there would be little room for it to move into such a position within
the coxa. Images of the joint movement were also captured from a posterior
perspective, enabling both pivots to be seen at the same time
(Fig. 6A-C). In the most
levated (cocked) position, the insertion of the depressor tendon is medial to
the pivot and the line of action of the tendon positive. As the
coxo-trochanteral joint depressed the distance between the insertion of the
tendon and the line of the pivot increased. Plotting the lever arm of the
depressor muscle showed that it was smallest at the most levated position
(Fig. 6D). For the first
10° of depression movement the lever arm remained the same, but then
increased rapidly to a maximum after some 80° of movement and remained at
this value during further depression of the joint. By contrast, the lever arm
of the levator tendon was similar over the full range of joint movement. The
lever arm of the depressor exceeded that of the levator after 40° of
depression.
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First, a hair row on the trochantin consists of some 15 hairs (trichoid sensilla), 15-50 µm long, which are stouter than those distributed more widely over the legs and body (Fig. 7A,B). They would be stimulated when the trochantin either touches the flexible membrane linking it to the mesothorax, or the middle legs. Such a movement occurs during the compression of the thorax when the hind legs are held in their cocked positions in preparation for a jump.
Second, on the anterior, ventral surface of the coxa is an approximately circular hair plate, the coxo-trochantinal hair plate, consisting of some 13 hairs (trichoid sensilla), ranging from 8-15 µm in length (Fig. 7A,C). These hairs could signal contact of the coxa with the trochantin during movements that precede the sudden depression of the trochanter in a jump.
Third, on the lateral edge of the trochanter is a hair plate consisting of two rows of regularly spaced hairs, ranging in length from 5-10 m (Fig. 7D). This hair plate could signal the initial contact of the trochanter with the lateral, distal edge of the coxa when it is levated into its fully cocked position, and the continuing contact while it is held cocked before a jump.
Cocking of the hind legs
In the fully levated position, the femur of a hind leg moves ventral to the
lateral, ventral protrusion of the coxa so that it is closely engaged with it
(Figs 1,
3). The detailed structure of
this region of engagement was therefore examined in the scanning electron
microscope from a ventral perspective for the coxa
(Fig. 8A-D) and from a dorsal
perspective for the femur (Fig.
9A-D).
Coxal protrusion
Micrographs show clearly that the oval, domed-shaped lateral protrusion is
part of the coxa, and that in contrast to the surrounding cuticle of the coxa,
it is covered in a dense array of small protrusions called microtrichia
(Fig. 8B-D). They are similar
in appearance to those on the apposed, medial surfaces of the coxae
(Gorb, 2004). Over most of a
coxal protrusion the microtrichia are 5-8 µm high, 3-6 µm wide and are
separated by intervals of 1-2 µm. The size decreases with distance from the
peak of the protrusion and the spacing between them increases so that they
taper into the smooth cuticle of the rest of the coxa. The microtrichia on the
dome of the protrusion point anteriorly. In many coxae examined in fixed
insects, the dome of the protrusion was dimpled and the microtrichia there
appeared flattened (Fig. 8C).
In live insects, the cuticle on the dome is soft and can easily be dimpled by
slight external pressure, indicating that its appearance in the micrographs
most likely results from the collapse of the underlying, flexible
membrane.
Femoral protrusion
On the dorsal surface of a femur at its proximal end is a prominent
protrusion with its highest point at the anterior edge
(Fig. 5A,B,F;
Fig. 9A,B). It protrudes by 130
µm from the dorsal surface of the femur at a point where the dorso-ventral
depth of the femur is 220 µm (Fig.
5F). It tapers steeply on its distal side but more gradually
toward the proximal end of the femur. This protrusion is again covered in an
array of microtrichia which are somewhat shorter at 4-6 µm high, and
broader at 5-7 µm wide and more widely spaced at 2-3 µm than those on
the coxal protrusion. The size of the microtrichia decreases and their spacing
increases with distance from the slope of the protrusion so that they taper
into the smooth cuticle of the surrounding femur. On the protrusion, the
microtrichia are oriented posteriorly and somewhat medially.
For a hind leg to move into its most levated (cocked) position, the protrusion on the dorsal femur must ride over the coxal protrusion and engage in front of it. This will bring the two surfaces bearing microtrichia into direct contact with each other and the orientation of the microtrichia on these two surfaces should ensure that many will interlock. Furthermore, the flexible dome on the coxal protrusion should allow it to fit snugly into the shape of the femoral protrusion so that the two sets of microtrichia are fully apposed or engaged. The increased surface area provided by the microtrichia, the interlocking between them and the engagement of the femoral protrusion in front of the coxal protrusion together provide an impediment, or an increased area of friction, to the depression of the leg. To allow the femur to be extended in a jump, these two surfaces must disengage and the femoral protrusion must ride over the coxal protrusion. The hind leg could be held in its cocked position by this mechanism and allowed to move suddenly only when sufficient depression force was generated.
Larvae
The larvae, which live in the protected environment afforded by their
secretion of a froth or `cuckoo spit', have a small coxal protrusion that is
smooth and entirely devoid of microtrichia
(Fig. 10A). The dorsal surface
of the proximal part of a hind femur has no protrusion and no microtrichia
(Fig. 10B). The larvae do not
jump and if they do emerge from their froth, they walk to another feeding
site. The femoral and coxal protrusions, and their associated microtrichia,
appear only after the final moult. Within a few minutes of emergence from its
froth, an adult is able to jump.
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Sound association with rapid leg movements
When froghoppers performed an unrestrained jump, a consistent click could
be heard and recorded by a microphone placed at the point of take-off
(Fig. 12A). The sound occurred
during take-off. When a froghopper was restrained on its back in Plasticene
and encouraged by gentle tickling to produce the rapid, simultaneous movements
of its hind legs that underlie jumping, they were again accompanied by a click
sound (Fig. 12B). In this
experimental arrangement, the hind legs made no contact with the substrate or
with any other object, so that the sound must have resulted from the rapid
movements of the hind legs themselves. When images of the movement were
captured at 1000 s-1, the sound could be correlated to the 1 ms
period when the legs moved from their cocked to their fully depressed
position. To provide greater time resolution, a disc of reflective tape was
fixed to the distal femur enabling changes in the angle between the coxa and
femur to be detected (Fig.
12C). Allowing for the conduction of the sound to a microphone 5
mm away, the sound must have occurred just 0.035 ms after the movement started
and full depression of the hind leg was completed in 0.3 ms. The sound must
therefore result from events at the very start of the rapid depression
movement, such as the initial movement of the coxo-trochanteral joint, or the
disengagement of microtrichia on the coxal and femoral protrusions.
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) and the
detailed anatomy presented here suggest that the following sequence of
movements underlie jumping. First, the hind legs are levated slowly into their
cocked positions and the femoral protrusions engage with their respective
coxal protrusions. Second, the hind legs remain stationary for seconds in
their cocked position, but the thoracic cuticle is deformed and may act as an
energy store (Rothschild et al.,
1975; Sander,
1957). Third, both hind legs move rapidly by a depression of their
coxo-trochanteral joints at angular velocities of 75 500 deg. s-1
and an extension of their tibiae about the femora. These movements are
completed in less than 1 ms or 100 times faster than the initial cocking
movements, while accelerating the load of the body. For a hind leg to move
rapidly from its cocked position, the coxal and femoral protrusions must
disengage. A prominent click sound is generated by the early events associated
with the rapid depression and extension movements.
Design of the hind legs for jumping
The differences in the design of the hind legs of froghoppers and their
other two pairs of legs appear to relate to this key role in jumping. The hind
legs, however, are short relative to the body, ranging from only 52-66% of
body length, and relative to the other legs they are only 1.4-1.6 times longer
due entirely to a longer tibia (Burrows,
2006). This contrasts with the hind legs of locusts and bush
crickets, which are much longer relative to body length. The hind legs of
froghoppers are also light, with their mass representing only a small (2%)
proportion of body mass, contrasting again with locusts where the proportion
is 14% (Bennet-Clark, 1975). In
froghoppers, the low mass of the hind legs results from the location of the
main power producing muscles (the trochanteral depressors) in the thorax. This
is the same as in fleas (Bennet-Clark and
Lucey, 1967; Rothschild and
Schlein, 1975), but contrasts with locusts where the
powerproducing extensor tibiae muscles are in the femur.
The specialisations of the hind legs critical for jumping lie in the detailed structure of particular joints, the improved traction provided by the tibio-tarsal and tarsal joints, strategically placed proprioceptors to monitor the large forces, the presence of a mechanism for locking the femur against the coxa when the leg is cocked in readiness for jumping, and in the lever arrangements of the muscles in the thorax that move the trochanter. All of these features are unique to the hind legs and most are not present in the larvae that have restricted lives underground, or above ground in a protective froth.
Mechanisms for increasing traction
The sudden and powerful application of thrust by the hind legs requires
good traction if the force is to be applied effectively to the ground. If both
legs slip then only a very weak jump occurs and if one leg slips the body
skews to one side at take-off. Traction is improved by arrays of small spines
at the ventral surface of the articulation of the tibia with the tarsus and at
the proximal two tarsal joints. The first movement of a hind leg before
take-off results in the tarsus being pushed flat to the ground so that the
spines engage with the substrate (Burrows,
2006). These arrays of spines are not present on the front and
middle legs, and nymphs of Philaenus that do not jump lack prominent
arrays of spines on the hind tarsi.
Proprioceptors
Three external proprioceptors would appear to signal critical aspects in
the preparation for a jump and the consequences of the very large forces that
are generated. It is likely that more internal proprioceptors and campaniform
sensilla exist. First, the prominent hair row on a trochantin is likely to
signal movements of a coxa and trochantin as they move forwards due to the
contractions of the trochanteral depressor muscle. Second, the hair plate on
the medial edge of the coxa could provide additional information about the
contraction of the trochanteral depressor muscle by signalling contact with
the trochantin during preparation for a jump. Third, the hair plate on the
lateral trochanter consists of two rows of stout hairs that are stimulated as
the trochanter is levated about the coxa. They could therefore signal the
levation movement to the fully cocked position, and the maintenance of the
trochanter in this position during preparation for jumping. The movements that
would stimulate the first two proprioceptors only occur when the leg is fully
levated and the trochanteral depressor muscles are contracting without being
able to move the leg from its cocked position. Their likely action is
therefore limited to jumping. A full levation of the trochanter that
stimulates the third proprioceptor is again only likely to occur if the legs
are being cocked in readiness for jumping.
Lever arms
In preparation for a jump, the hind legs are moved into their most levated
(cocked) position and remain in that position for seconds before they are
rapidly depressed and extended in the movement that powers a jump. In this
cocked position, the lever arm of a trochanteral depressor muscle is smallest
and is exceeded by the lever arm of the levator. The tendon of a trochanteral
depressor muscle inserts on the strengthened medial edge of the trochanter
some 500 µm from the joint pivot that is toward the lateral edge of the
coxa. This arrangement means that even in the cocked position the trochanteral
depressor tendon has a positive mechanical advantage and does not go
over-centre. During depression, the lever arm of the depressor muscle
increases rapidly to reach a maximum after some 80° of movement. After
only 40° of movement, it exceeds the lever arm of the levator that changes
little throughout the whole range of joint movement.
Locking mechanisms
To move into the cocked position, a sloping protrusion on the dorsal
surface of a hind femur must ride over and engage with a lateral and ventral
protrusion of a coxa. Once the femoral protrusion is anterior to the coxal
protrusion, the hind leg is effectively cocked and poised to extend by a rapid
depression of the trochanter and extension of the tibia. The effectiveness of
this passive locking device has been demonstrated experimentally in dead
froghoppers. Pushing a hind leg into its cocked position results in it locking
under the passive forces of the engagement between the coxal and femoral
protrusions, and a possible involvement of the coxo-trochanteral joint. If a
force is now applied to a hind leg in the same direction as a depression
movement, then resistance is encountered. When the applied force is sufficient
to overcome this resistance, a hind leg will suddenly and rapidly extend in
about 2 ms, indicating that the cocking movement had stored energy either by
stretching muscles and tendons or distorting cuticular elements. Images
captured at high speed indicate that the rapid movement occurs only when the
femoral and coxal protrusions have disengaged.
The coxal and femoral protrusions are both covered by dense arrays of
microtrichia. These increase the friction between two apposed surfaces, or
those on one surface may interlock with those of the other, much in the manner
of `Velcro' fasteners. Microtrichia are often found in insects wherever two
apposed surfaces must lock together under particular circumstances. For
example, they are found at the articulation of the head with the prothorax in
dragonflies and damselflies where they stabilise the head particularly in
flight; in some beetles they occur in patches on the thorax and on the wings
to lock the wings to the thorax when not flying; in some Lepidoptera on the
front and hind wings to ensure that they are locked together during flight
(Gorb, 2001). In froghoppers,
microtrichia also occur on the medial surfaces of the hind coxa where they
have been interpreted to synchronise the coxae in jumping
(Gorb, 2001;
Gorb, 2004). I have shown that
the critical synchrony in movements of the hind leg during jumping is between
the rapid movements of the trochantera. The microtrichia on the two coxae are
thus more likely to provide a stable base for these rapid movements than to
contribute to the synchrony of the propulsive leg movements.
This elaborate locking mechanism between the femur and coxa is not found in
other insects that jump by a rapid propulsive extension of their hind legs,
but other locks do occur. No similar structure is described in fleas
(Bennet-Clark and Lucey, 1967;
Rothschild and Schlein, 1975),
which jump by depression of the hind trochantera about the coxae, and in
locusts, bush crickets and false stick insects, the propulsion comes from an
extension of the tibiae about the femora. In froghoppers, a key role for these
structures in jumping is emphasised by their occurrence only on the hind legs
of adults. They are not present on the middle and front legs of adults, and in
nymphs that do not jump they are also absent on the hind legs. A meracantha on
the hind coxa engaging with a yellow spot on the proximal femur has been
described on a hind leg of some Homopterans including froghoppers
(Emeljanov, 1987) and a
protrusion from the coxa has also been described on the hind legs of Lantern
Flies (Homoptera, Fulgoridae) (Sander,
1957). Emeljanov suggests that these structures could act as a
possible proprioceptor but provides no evidence. The meracantha may be the
structure to which I have applied the purely descriptive term of coxal
protrusion. I have not seen a yellow spot on the femur but perhaps it
corresponds to the structure I have called the femoral protrusion. My
experiments described here show that these structures provide a locking
mechanism, but their engagement could also stimulate nearby hairs or other as
yet undescribed sensory structures that could provide further proprioceptive
information.
Further understanding of the mechanisms underlying the movements of the
hind legs in jumping, requires that timings and sequences of the muscle
actions be determined. This will be the subject of my next paper
(Burrows, 2007).
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