Ontogenesis of the attachment ability in the bug Coreus marginatus (Heteroptera, Insecta)
1 Evolutionary Biomaterials Group, Max-Planck-Institute of Metals Research,
Heisenbergstrasse 3, 70569 Stuttgart, Germany
2 Department of Insect Ethology and Sociobiology, Schmalhausen Institute of
Zoology, 15 Khmelnitsy Str., 01601 Kyiv, Ukraine
3 Faculty of Biology, Taras Shevchenko University of Kyiv, 64 Volodymyrska
Str., 01033 Kyiv, Ukraine
* Author for correspondence (e-mail: s.gorb{at}mf.mpg.de)
Accepted 3 June 2004
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Summary |
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Key words: friction, adhesion, cuticle, pulvilli, attachment, Insecta, Coreus marginatus
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Introduction |
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Attachment forces on smooth surfaces have been previously measured on the
hairy adhesive pads of reduviid bugs
(Edwards and Tarkanian, 1970),
flies (Walker et al., 1985
;
Gorb et al., 2001a
), beetles
(Stork, 1980
;
Ishii, 1987
), and on the
smooth pads of cockroaches (Roth and
Willis, 1952
), aphids (Lees
and Hardie, 1988
; Dixon et al.,
1990
), grasshoppers (Jiao et
al., 2000
; Gorb and Scherge,
2000
) and ants (Federle et al.,
2000
). Until recently, less attention has been paid to the scale
effects on attachment systems used in locomotion. Relationships between
attachment force and body mass has been studied in the hairy attachment
systems of the beetle Chrysolina polita
(Stork, 1980
), arboreal ant
species (Federle et al., 2000
)
and syrphid flies (Gorb et al.,
2001a
). These results showed that attachment force increases with
an increased body mass, but the ratio between the attachment force and body
weight decreases.
How attachment ability of an insect develops during ontogenesis is the fundamental question for understanding scaling effects on design and performance of biological attachment systems. In hemimetabolic insects, the size of attachment structures gradually increases during their larval growth and, therefore, should be optimised according to their mass in each larval instar. Attachment abilities may be tuned in two ways: (1) by changing the size of attachment pads, and (2) by changing the adhesive properties of the pad material and/or secretion. Which is realised in real systems, however, has remained unknown.
This study was undertaken to understand the relationships between the body
mass, body size, pad area and attachment performance in a heteropteran insect
species during ontogenesis. The approach has two great advantages. It allows
us to discover how an animal's adhesive mechanisms keep up with the increases
in mass that result from growth, and also can shed light on underlying
mechanisms of adhesion. The tarsus of the bug Coreus marginatus L.
(Coreidae) bears a pair of smooth, flexible pulvilli adapted for attachment to
smooth surfaces, such as leaf surfaces of their host plant Rumex
crispus (Polygonaceae). These bugs were ideal for this study because they
can be captured in nature simultaneously at various stages of development and
they possess semitransparent, round pulvilli on their tarsi, whose area can be
measured from the whole-mount preparations. The friction component of the
attachment force of adults and larval insects was measured by using a
computer-controlled centrifugal device equipped with a fibre optical sensor
(Gorb, 2001;
Gorb et al., 2001a
). In a
separate experiment, the effect of angular acceleration on the frictional
force was evaluated. The total area of all pads, body size and weight were
determined for each individual insect used in experiments.
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Materials and methods |
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The position of the insect on the drum was monitored by using a combination of the focused light beam and the fibreoptical sensor. The drum speed was continuously increased until the insect lost its hold on the surface under centrifugal force. The rotational speed at contact loss, position of the insect on the drum (radius of rotation), and the insect mass (determined by weighing on a micro-balance, Mettler Toledo AG 204 Delta Range, Greifensee, Switzerland) were used to calculate the maximum frictional component of the attachment force. Ten repetitions were done with each individual bug. In order to test the effect of acceleration on the attachment force, two measurements were successively carried out in a separate experiment with each individual bug at accelerations of 1.21 and 12.10 rev s-2.
Microscopy
After the force measurements, pads were fixed in 70% ethanol overnight, cut
off, dehydrated in an ascending row of ethanol, and whole mounted in Depex
(Serva, Heidelberg, Germany). Pad areas of the whole-mounted pulvilli were
measured from digital pictures taken with a Sony 3CCD videocamera mounted on a
Zeiss Axioplan light microscope and using AnalySIS (Münster, Germany)
software. Measurements were made individually for all 40 insects (480
pulvilli, 12 pulvilli per insect) and studied separately for the fore-, mid-
and hindlegs.
For scanning electron microscopy, the tarsi were carefully cut off and fixed in a 2.5% solution of glutaraldehyde in phosphate buffer (pH 7.3). After dehydration in the series of ethanol solutions, the samples were critical point dried, coated with gold-palladium, and observed in a Hitachi-S800 (Tokyo, Japan) scanning electron microscope (SEM) at 20 kV.
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Results |
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The material structure of the cuticle on the ventral side of pulvilli is
different from that on the dorsal side. On the ventral side, it resembles the
rod-like branching architecture, as described for other attachment pads of the
smooth type in the grasshopper Tettigonia viridissima
(Gorb, 2001;
Gorb et al., 2000b
), locust
Schistocerca gregaria, plecopterans
(Beutel and Gorb, 2001
),
hymenopterans (Federle et al.,
2001
; Baur and Gorb,
2001
), and cicada Cercopis vulnerata
(Scherge and Gorb, 2001
). A
detailed study of the material ultrastructure of the heteropteran smooth
pulvilli is currently in progress.
Relationships between insect dimensions, body mass and contact surface
The relationship between the linear dimensions of the insect (head width)
and its body mass is given in Fig.
3. Adult bugs have a mass of 79.9±13.8 mg (mean ±
S.D., N=14). Taking into account that an insect
has six legs and 12 pulvilli, we can consider that in an adult insect,
standing on the horizontal drum surface, the average normal force of 66.6
µN acts on a single pulvillus. In a walking insect that keeps three legs in
the stance phase and the other three legs in the swing phase, this force is
doubled (133.2 µN). In the experimental situation, at a certain
rotational speed of the centrifuge (
200 rev min-1), the insect
usually stopped walking and stood on the surface with all six legs.
|
The pulvillus area increases exponentially with the head width (Fig. 4A) and linearly with the body mass (Fig. 4B). However, there was not an exact correlation between the pulvillus area and body mass, because the mass may fluctuate depending on the physiological state of the insect (hungry, additional mass of eggs in females, etc.) and is not always exactly related to the insect size. Data on the pulvillus area vs. body size are less dispersed.
|
Friction force dependence on the body mass
Absolute values of measured friction force increased with an increasing
body mass (Fig. 5A,B). The
friction coefficient (relationship between friction force and body weight) was
always higher than 1. It ranged, for averaged data, from 5 to 40 and for
maximum data from 7 to 70 under different experimental conditions. These
values also show how much of its own weight the insect can hold in the
situation of walking on the wall or resist in the shear direction on the
horizontal surface.
|
The frictional coefficient decreased slightly with increasing body mass
(Fig. 5C,D). This means that
relatively (larger) heavier animals generate relatively lower force. Taking
into account that higher load and pressure
(Fig. 4C) must positively
contribute to measured friction, such an opposite effect can only be explained
by adhesion, which plays a considerably greater role in smaller objects than
in larger ones (Kendall,
2001), and may promote friction to a relatively higher extent in
smaller insects.
Effect of angular acceleration on the friction force
Absolute values of friction force, as well as frictional coefficients, were
higher in experiments in which higher angular acceleration was applied
(Fig. 5). There is a
statistically significant difference between friction measured at different
accelerations (P=0.003; two-way ANOVA for individuals and
accelerations). However, the difference between forces measured at
accelerations differing by one order of magnitude, was in the range of 25-50%.
The characteristics of all relationships mentioned in the previous section did
not depend on the acceleration applied to experimental animals.
The difference between forces measured at different accelerations is slightly increased with an increasing body mass (Fig. 6A). The difference between frictional coefficient obtained at a higher acceleration and that measured at a lower acceleration slightly decreased with an increasing body mass (Fig. 6B).
|
Lateral tenacity
Measured force was higher in animals with higher overall area of pulvilli
(Fig. 7), indicating that
adhesion contributed to the friction force measured. The graphs of the
friction force versus contact area provide information about the
lateral tenacity of the pulvilli material
(Fig. 7). Lateral tenacity is
determined as friction force divided by the overall area of all pulvilli. This
variable was individually measured for experimental insects and pooled for all
animals and accelerations giving 0.097±0.50 N m-2 (mean
± S.D., N=77). Lateral tenacity was not
different for animals with a different pulvilli area [H=0.640,
d.f.=2, P=0.726, Kruskal-Wallis one-way analysis of variance (ANOVA)
on Ranks] (Fig. 8). Lateral
tenacity was lower for low acceleration experiments (0.085±0.39 N
m-2, N=39) than for high acceleration experiments
(0.109±0.57 N m-2, N=39). The values are
significantly different (P=0.005, Mann-Whitney rank sum test). Thus,
the attachment properties of pulvilli are strongly dependent on the
velocity.
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Discussion |
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Ontogenesis of the attachment ability
The simple physical principle that smaller objects adhere relatively
stronger than larger objects (Kendall,
2001) is found in the attachment system of C. marginatus.
Interestingly, in a living system, this has an important biological
significance. Small (juvenile) animals are flightless and stronger attachment
ability might be essential for them to stay attached to the host plant.
The frictional properties of the bug pulvilli do not change during ontogenesis. Thus, pulvilli growth and the consequent increased contact area contribute to the increasing attachment ability in insects at later larval stages. We expected an increase of the lateral tenacity in older instars, since the pressure in the contact area increases in larger insects because of different scaling laws for the mass and surface area (Fig. 4C). However, the lateral tenacity was similar in all instars, which could be explained by (1) pulvilli having possibly stiffer material properties in older (heavier) animals and, therefore, a smaller real contact area of the pulvilli with the substrate at the same pressure and/or (2) different amounts, composition and properties of the pad secretion. These hypotheses, however, need further experimental evidence.
Similar to C. marginatus, lateral tenacity in syrphid flies also
does not depend on the body mass. Frictional and adhesive properties of pads
are precisely adapted to the animal mass. This seems to be an important
feature of locomotory attachment devices. The attachment forces of an animal
should be tuned to an optimum between the ability to hold onto the wall and
ceiling, and simultaneously to walk without high energy expenditure when the
contact breaks. Too high attachment is not desirable, because it could hamper
locomotion. Some animals with smooth attachment devices can actively change
the contact area of pads when carrying loads, as is done by some ants
(Federle and Endlein, 2004).
Hymenopterans and probably other insect groups with a retractable arolium are
also able to passively control the contact area. However, nothing is known
about such an ability in C. marginatus.
Interestingly, the value for lateral tenacity determined for C.
marginatus is within the same range as previously measured for six
species of syrphid flies (Gorb et al.,
2001a), which may reflect the similarity of mechanical properties
of the surface in different types of insect attachment pads. Division of a
large contact area into many single contacts makes the surface less stiff than
the bulk material (Persson,
2003
). Since flies possess a hairy type of pulvilli, the material
of their setae should be stiffer than in that of C. marginatus
pulvillus in order to end up with the same mechanical properties of the
surface. This prediction should be experimentally tested.
The present set of experiments shows stable functioning of the attachment
system during ontogenesis. However, variables such as time after the last
moulting were not determined here. One has to take into account that freshly
moulted animals may have other attachment abilities because of different
material properties of pads in particular
(Ridgel et al., 2003) and the
cuticle in general (Hepburn,
1985
).
Acceleration-dependent attachment
Since the attachment properties of pulvilli are strongly dependent on
velocity, the viscosity of the secretion and viscoelastic properties of
foam-like material of the pulvilli may be important variables contributing to
time-dependent processes in the contact area during attachment and detachment
processes. For smooth systems, such as ant arolium or grasshopper euplantulae,
visco-elastic properties of the pad material have been previously suggested
(Brainerd, 1994;
Gorb et al., 2000b
).
Fluid secretions have previously been reported in hairy adhesive pads
(Edwards and Tarkanian, 1970;
Bauchhenss and Renner, 1977
;
Bauchhenss,
1979a
,b
;
Walker et al., 1985
;
Ishii, 1987
) and in smooth
pads (Roth and Willis, 1952
;
Lees and Hardie, 1988
;
Dixon et al., 1990
). The smooth
pulvilli of various Heteroptera also produce fluid secretions in the contact
area (Hasenfuss,
1977a
,b
,
1978
; Ghasi-Bayat and
Hasenfuss,
1980a
-c
).
It is generally accepted that the pad secretion of diverse insects contains
non-volatile, lipid-like substances
(Hasenfuss, 1977a
;
Bauchhenss, 1979a
;
Ishii, 1987
;
Lees and Hardie, 1988
;
Kosaki and Yamaoka, 1996
).
However, latest studies on secretion in flies
(Gorb, 2001
), locusts
(Vötsch et al., 2002
) and
ants (Federle et al., 2002
)
suggest that the pad secretion is biphasic, probably delivered in the contact
area as a kind of micro-emulsion. The most elaborate chemical study of insect
pad secretion has been published for the locust. It demonstrated that the
water-soluble fraction of the fluid contains amino acids and carbohydrates. It
is obvious that such a biphasic fluid is rather viscous
(Stadler et al., 2001
), the
flow of each phase being constrained by the presence of another fluid that is
not mixed with the first one. The visco-elastic role of the pad material in
attachment has not been previously studied. As a general rule,
visco-elasticity should result in higher adhesion in the case of slow contact
formation and fast contact breaking.
An alternative explanation of the experimental results showing dependence of attachment force on angular acceleration is the presence of active control over attachment ability. Since, in the present experimental situation, high acceleration conditions automatically mean a shorter time under external force for an insect, the results obtained are time dependent: insects can withstand a higher external force for a relatively shorter time. Even if C. marginatus does not posses active control over the pulvilli area, the muscle system is involved in tarsus control and generally in posture control. Since the insect will become tired under external force after a certain time prior to detachment, low acceleration conditions rather than high acceleration conditions would automatically result in lower detachment force. In other words insects can withstand higher external force if it is applied for a relatively short time. In a real situation, it might be important for resisting strong but short wind pulses.
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Acknowledgments |
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