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First published online August 31, 2007
Journal of Experimental Biology 210, 3311-3318 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.007914
Jumping behaviour in a Gondwanan relict insect (Hemiptera: Coleorrhyncha: Peloridiidae)
1 Department of Zoology, University of Cambridge, Cambridge CB2 3EJ,
UK
2 Museum für Naturkunde, Humboldt Universität zu Berlin,
Germany
* Author for correspondence (e-mail: mb135{at}hermes.cam.ac.uk)
Accepted 10 July 2007
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Summary |
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Key words: kinematics, jumping, Hemiptera
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). The smaller
size of insects means that the legs can only provide limited leverage even
though bush crickets in particular have exploited this feature
(Burrows and Morris, 2003). To
jump with short legs has led to the evolution of storage mechanisms in which
muscle force is developed slowly and energy stored in deformations of the
skeleton before being released suddenly in a catapult action. This mechanism
has been exploited by the best-known jumping insects such as grasshoppers
(Bennet-Clark, 1975), fleas
(Bennet-Clark and Lucey, 1967;
Rothschild et al., 1972) and
flea beetles (Brackenbury and Wang,
1995; Furth, 1988;
Furth et al., 1983;
Maulik, 1929).
A diverse group of prodigious jumping insects that have been less well
studied are the Hemipteran bugs. Amongst these are the froghoppers, or spittle
bugs (Cercopidae), which outperform all other insects in their jumping
abilities by using a catapult mechanism
(Burrows, 2003;
Burrows, 2006a). The Hemiptera
also contains many other closely related families of bugs, such as tree and
leafhoppers in the probably paraphyletic sub-order `Auchenorrhyncha', and bugs
such as jumping plant lice (Psyllidae) in the sub-order Sternorrhyncha. All
have evolved differing designs for jumping. Some members of an ancient
sub-order, the Coleorrhyncha, also jump. This group split from the
evolutionary lineage of other Hemipterans in the late Permain
(Popov and Shcherbakov, 1996).
The present distribution of this group is restricted to the southern
hemisphere (Australia, New Zealand, New Caledonia and the southern tip of
South America), suggesting that it must have been in existence before the
break up of Gondwana (Evans,
1981). The extant Coleorrhyncha consist of just 25 species
belonging to 13 genera in a single family, the Peloridiidae, all but one of
which are flightless. The close resemblance of living species to those of
Mesozoic fossils, (Popov and Shcherbakov,
1996) and the key position of Coleorrhyncha within the Hemiptera
(Bourgoin and Cambell, 2002;
Schlee, 1969) suggests that
they may give clues about the evolution of jumping and about the jumping
mechanisms that may have been present in the basal ancestral groups.
In this paper, we describe the jumping performance and the associated leg movements as captured with high-speed imaging of a small coleorrhynchan, Hackeriella veitchi, from Australia. We show that it accelerates its body in 1.5 ms by rapid movements of both hind legs to a take-off velocity of 1.5 m s–1, experiencing a force of 100 times gravity. The short hind legs, which provide the main propulsive force for the jump, show few specialisations for jumping compared with those of modern Auchenorrhyncha.
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The anatomy of the hind legs and metathorax was examined in intact insects and in insects preserved in 96% alcohol. Measurements of body and leg length were taken from live insects and from others preserved in 96% ethanol. For scanning electron microscopy, Hackeriella were cleared by boiling in 10% potassium hydroxide and subsequently lightly rubbed with a fine paintbrush and a needle to remove the surface film covering the cuticle. They were dehydrated in increasing concentrations (96–100%) of ethanol, critical-point dried, mounted on aluminium specimen stubs with adhesive pads, and coated with gold–palladium. They were examined in a LEO 1450VP Scanning Electron Microscope at 10 kV. Images were stored digitally and subsequently processed with Adobe Photoshop.
Measurements are given as means ± standard error of the mean (s.e.m.). Temperatures in all experiments ranged from 24–26°C.
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Body form
Hackeriella has a dorso-ventrally flattened body with compound
eyes on lateral protrusions of the head
(Fig. 1B). The adults are
flightless; tegmina are present, but the hind wings are absent. Females are
heavier and larger than the males. They have a mass of 1.39±0.03 mg
(N=10), which is significantly heavier (T-test,
T1,18=9.655, P<0.0019) than the males at
1.05±0.03 mg (N=10). Similarly, they have a body length of
3.04±0.02 mm (N=10) that is significantly longer
(T1,8=3.374, P=0.010) than that of males at
2.83±0.06 mm (N=10).
Jumping movements
Hackeriella jumped infrequently from surfaces of any orientation.
The jumps occurred spontaneously or could be induced by mechanical or
vibrational stimuli from a fine paintbrush. In 17 h of recording, 11 jumps
were captured with high-speed imaging. The descriptions that follow are of
jumps from a flat, horizontal surface.
In preparation for a jump, the angle of the body relative to the ground was first set by movements of the front and middle legs. Once this posture had been adopted, the first movements that initiated jumping were the rapid depression and extension movements of both hind legs (Fig. 2). When viewed from the side, the femora were depressed downwards and backwards and the tibiae extended about the femora. The restricted movement of the coxa about the thorax and the fusion of the trochanter with the femur (see below) implies that the femoral movements resulted from a depression movement of the coxo-trochanteral joint. These movements initially pushed the tarsi of both hind legs firmly against the ground beneath the body but positioned separately from each other. The thrust thereby applied to the ground raised the body and lifted the front and middle legs from the ground (Fig. 2, Fig. 3A,B). In all jumps recorded, the front and middle legs were lifted off the ground before take-off and sometimes the front legs were off the ground before the hind legs even started to move. At take-off, when the weight of the body was no longer supported by the hind legs, each moved medially so that both tarsi were apposed to each other beneath the body. After take-off, the residual thrust of the hind legs moved them further medially so that the tibiae now crossed.
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In the best jumps, the time from the first movements of the hind legs until the insect was launched into the air at take-off took just 1.5 ms (average 2 ms in 10 jumps) (Figs 2, 3). This very short time therefore represents the period during which the body can be accelerated to its take-off velocity.
The trajectory of the jump was variable for different jumps by the same animal and was related to the starting angle of the body relative to the ground. The trajectory was also influenced by the spin of the body after take-off (Fig. 4). In the example shown, the body started to rotate about the long axis of the body at take-off and rotated with an initial periodicity of about 10 ms whilst the insect remained within the view of the camera. In some jumps, the body spun about the transverse axis of the body so that the abdomen periodically reversed positions with the head to be in the lead. In other jumps, the body spun about both axes.
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The asynchrony in the movements of the hind legs could also result in the insect falling from its perching position (Fig. 6). In the example shown, the insect was standing on the edge of the platform with the tarsus of the right hind leg held off the ground. This hind leg then moved suddenly so that its tarsus was pushed against the ground and the subsequent further depression of its femur progressively raised the right side of the body. While this rapid movement was happening the left hind leg did not move. Eventually the tilting of the body was so great that the insect overbalanced and fell from the platform. On different occasions, either the left or the right hind leg could depress by itself so that the fall occurred to one side or the other.
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Structure of the hind legs
In both males and females there is a significant but small difference in
leg length between the hind legs and the front and middle legs (MANOVA –
multivariate analysis of variance – F2,7=1081.715,
P<0.0001). A post-hoc analysis of leg lengths
(Bonferroni-corrected least significant difference) showed that the hind legs
were significantly longer by some 10% than the front and middle legs, which
did not differ significantly from each other. The ratio of lengths for the
front:middle:hind legs is therefore 1:1:1.1. The hind legs are 67% of the
length of the body in the shorter males and 63% in the longer females. The
increased length of the hind legs in both sexes resulted from the longer
lengths of the tibiae with a contribution from the longer tarsi. The hind
tibiae are significantly longer (MANOVA, F2,7=41.959,
P<0.0001) than those of the front and middle legs. Similarly, a
hind tarsus is longer than a middle tarsus (MANOVA,
F2,7=36.003, P<0.0001), which in turn is
longer than a front tarsus (post-hoc test). The other leg segments
are of similar lengths in all three pairs of legs. The coxae and trochantera
of the three pairs of legs are similar in general structure but they differ in
their orientation.
The coxae of the front legs pivot at an angle of about 55° to the longitudinal body axis, the middle coxae at 72° and the hind coxae at right angles (Fig. 7A). The hind coxae are 250 µm wide and are thus larger than the 200 µm wide front and middle coxae. They are not fused to thorax so that they are able to rotate through some 70° about the thorax.
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The rostrum of the sucking mouthparts protrudes between and therefore
separates the left and right coxae of all three pairs of legs
(Fig. 7A). The hind coxae are,
however, set more closely together and may touch each other toward their most
dorsal medial edges where some tubercles (microtrichia) are present. They are
not, however, tightly linked together by attachment devices in contrast to
those of froghoppers (Burrows,
2006b) or leafhoppers
(Burrows, 2007a). The
antero-lateral edge of a coxa has a broad depression, which is surrounded by
tubercles (Fig. 7C). Internally
the depression projects as an apodeme-like structure, which is not present in
a front and middle coxae. A hind coxa has no protrusion on its ventral,
lateral surface (Fig. 7B) as is
present in some Auchenorrhyncha such as froghoppers
(Burrows, 2006b). Similarly
the dorsal surface of the proximal femur has no protrusion
(Fig. 7C). The hind legs
therefore lack an external locking device that could engage the femur with the
coxa and prevent the depression movements of jumping until sufficient muscular
force has been developed to propel take-off. A hair plate consisting of a
group of 4–6 short (6–8 µm) and stout hairs on the anterior
edge of a coxa could monitor the movements of the coxa relative to the thorax
(Fig. 7B).
The angle of rotation of the trochanter about the coxa relative to the
midline is also different for each pair of legs; the angle of the front
coxo-trochanteral joint is 140°, the middle leg joint is 18° and the
hind leg joint is 40° (Fig.
7A). A hind trochanter can be levated and depressed about a coxa
through an angle of some 110°. The articulation of a trochanter with a
coxa consists of a ventral and a dorsal horn of the trochanter, which
articulate in curved indentations of the coxa allowing a movement in one
plane. A small hair plate with up to 5 hairs some 6–8 µm in length on
the lateral edge of a trochanteral horn at its ventral pivot with the coxa
could signal movements of this joint when it is close to full levation
(Fig. 7B). The tendon of the
trochanteral depressor muscle was traced from its insertion on the trochanter,
through the coxa and into the thorax where, in cercopid
(Burrows, 2006b) and fulgorid
(Sander, 1957) bugs, as in
other insects, the main part of this muscle lies.
The demarcation between the hind trochanter and hind femur is marked by a clear suture line that indicates fusion between the two. No movement between them occurred during jumping so that a movement of the trochanter was always accompanied by a movement of the femur. The femur of a hind leg is of similar size to that of the other legs, indicating that the extensor tibiae muscle is not enormously enlarged as in locusts and is thus not a major contributor to the generation of force in jumping. Furthermore, a hind femoro-tibial joint shows no obvious specialisations over those of the middle and front legs.
The hind tibia pivots with the femur in such a way that it can move through an angle of some 155° in the same plane as the movements of the trochanter about the coxa. The movements of this joint are similar to those of the other two pairs of legs. The hind tibia has a semi-circular row of spines on its ventral surface close to its joint with the tarsus (Fig. 7D), which would contact the ground at the same time as the tarsus. They are likely to improve traction with the ground particularly during jumping.
The hind tarsus also moves about the tibia in the same plane as the more proximal joints. It has ungual claws (Fig. 7D) and tarsal pads that should again increase traction with the ground during jumping.
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) suggests that the few extant species in
restricted parts of the Southern Hemisphere have changed little in the
intervening period. Nevertheless, this is the first report that they are
accomplished but reluctant jumpers, although previous reports have indicated
that they do not jump (Popov and
Shcherbakov, 1996), indicating that this form of locomotion is
ancient within the Hemiptera. In its best jumps, Hackeriella
accelerates its small, light body at 1000 m s–2 in 1.5 ms to
a take-off velocity of 1.5 m s–1, expending 1.1 µJ of
energy, developing a power output 0.74 mW and exerting a force of 1.24 mN. It
achieves this with a body design that shows few specialisations for jumping
compared with those of other Hemipterans such as the froghoppers
(Burrows, 2003;
Burrows, 2006a) belonging to
the Auchenorrhyncha.
Design for jumping
The body shape does not immediately suggest a design suited for jumping.
The head is broad with laterally placed eyes with the dorsal surface of the
body is covered by a rigid ribbed tegmina. The hind wings are absent so that
there are no wings that could assist take-off or generate forward propulsion
when airborne.
Propulsion for jumping is delivered by rapid movements of the hind legs,
with the front and middle legs typically losing contact with the ground well
before take-off and showing no movements that could add greatly to the thrust.
The key movements of the hind legs are depression movements of the
coxo-trochanteral joints produced by trochanteral depressor muscles located in
the thorax. Placing the main power-producing muscles in the thorax minimises
the mass of the legs and thus enables them to be accelerated rapidly. The hind
legs are, however, short relative to both the body and to the other legs, so
that the leverage (Alexander,
1995) they can provide in jumping is limited. Structurally, there
is little that sets the hind legs apart from the middle or front legs, or
which obviously proclaims their role in providing the main propulsive force
for jumping. For example, neither the ventral coxae nor dorsal femora have
protrusions, or arrays of microtrichia that could engage with each other and
restrain the movements of the legs in preparation for jumping. This contrasts
with froghoppers, which have an elaboration arrangement of protrusions and
microtrichia that may act as a mechanical lock that allows force to develop
slowly and then be rapidly released in a jump
(Burrows, 2006b).
How are the rapid movements of the hind legs generated?
Evidence from the high speed images of jumping and from the external
anatomy of the legs suggests that the following sequence of events takes
place.
The hind legs are first fully levated at their coxo-trochanteral joints.
This results in a forward and medial movement of the legs so that only the
distal tips of the tarsi touch the ground. The jump movement itself begins
with a depression of the coxo-trochanteral joint that moves the fused
trochanter and femur downwards and backwards and results in the whole ventral
surface of the tarsus being placed firmly on the ground. As a consequence of
the give in the tarsi, there is no upward displacement of the body in this
initial movement. Further depression of the coxo-trochanteral joint then
occurs and this now begins to raise the body. This movement continues and is
accompanied by extension of the femoro-tibial joint until both joints reach
their maxima, at which point the body has been lifted so that the hind tarsi
are the last to leave the ground and the insect becomes airborne. It is
unlikely that a direct contraction of the trochanteral depressor muscles could
power such a rapid movement with such short hind legs as levers. Instead, a
mechanism is implicated in which a slow contraction of the muscle generates
force, which is stored in elastic structures in the hind leg or metathorax and
is then released suddenly to power the movement. Such a catapult mechanism is
proposed for jumping in fleas (Bennet-Clark
and Lucey, 1967) locusts
(Bennet-Clark, 1975) and
froghoppers (Burrows, 2003).
This requires structures in which energy can be stored and which either
restrain movements of the legs until sufficient force has been generated, or
give mechanical advantage to the trochanteral depressor or levator muscle at
different angles of the coxo-trochanteral joint. These structures have been
identified in the insects cited but remain to be discovered in
Hackeriella. Furthermore, this strategy requires a distinct motor
pattern to control all of the muscles involved, particularly in coordinating
the movements of the two hind legs.
Most jumps of Hackeriella result from synchronised movements of the two hind legs with the body set by the front and middle legs to such an angle that a forward trajectory is achieved. Close synchronisation of the hind legs is needed because of the very short time in which the body is accelerated. In some jumps the hind legs move with a time difference of 1 ms and this then results in the body first being tilted to one side and then to another. The result is that the body spins once airborne. The extreme of this asynchrony is reached when only one hind leg extends rapidly while the other does not move. These asynchronous movements of the hind legs may then provide a mechanism by which the insect can fall rapidly from its perch with the potential to avoid a predator approaching from the front and into whose path a forward jump would propel it.
Comparison with other jumping Hemipterans
How does the jumping performance of Hackeriella compare with that
of other Hemipteran insects? Most of our knowledge of Hemipteran jumping comes
from the Auchenorrhyncha and the Cercopidae (froghoppers) in particular
(Burrows, 2003;
Gorb, 2004). Froghoppers
accelerate their body in 0.8 ms to a take-off velocity of 4.7 m
s–1, experiencing 550 g and exerting a force
of 66 mN or more than 400 times their body mass
(Burrows, 2006a).
Hackeriella does not achieve this performance, but it does match the
take-off velocity of the flea (Bennet-Clark
and Lucey, 1967; Rothschild et
al., 1972), some leafhoppers
(Brackenbury, 1996) and some
flea-beetles (Brackenbury and Wang,
1995). Where Hackeriella differs from froghoppers is in
its lack of directional control over a jump, which in turn results from the
lack of a tight synchrony of movements by the two hind legs. Closely
coordinated movements are necessary to produce a directed jump with hind legs
that move in the same plane beneath the body. In Hackeriella the hind
legs can move at the same time, independently, or with different timing
differences between their rapid actions. The inability to generate a directed
jump may, however, be more than offset by the adaptive use each hind leg with
some independence that can result in a rapid escape by falling from their
perch on a plant.
What do the jumping mechanisms in Hackeriella suggest about the evolution of jumping mechanisms and strategies in other Hemipterans? The common and perhaps therefore ancestral features that we report are the following:
Modern auchenorrhynchans have elaborated on this basic plan and have specific mechanisms for the production of faster and more powerful movements:
).
Leafhoppers have a press-stud (popper) arrangement between the hind coxae in
which a protrusion of one engages in a socket of the other
(Burrows, 2007a). In Issids,
the left and right hind coxae are fused at the midline. Larval Fulgoromorpha
[except for the Tettigometridae (Asche,
1988)] also have projections on the medial surface of a trochanter
that can engage with similar projections from the other trochanter rather in
the manner of a cogwheel (Sander,
1957). ),
presumably to improve the probability of synchronous movements of the two hind
legs.
Many other families in the Auchenorryhncha also jump (e.g.
Heilig and Sander, 1986), as
do some Sternorrhyncha (Rietschel,
1952; Weber, 1930)
and some Heteropterans (e.g. Parsons,
1960; Parsons,
1963). Analyses of jumping currently underway of the many
different Hemipteran families should shed light on the evolutionary process
and the functional role of particular mechanisms as related to jumping
performance.
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Acknowledgments |
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Footnotes |
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References |
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