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First published online August 17, 2007
Journal of Experimental Biology 210, 3036-3042 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.008276
Small but powerful: the oribatid mite Archegozetes longisetosus Aoki (Acari, Oribatida) produces disproportionately high forces
University of Tübingen, Zoological Institute, Department of Evolutionary Biology of Invertebrates, Auf der Morgenstelle 28E, 72076 Tübingen, Germany
* Author for correspondence (e-mail: heethoff{at}gmx.de)
Accepted 17 July 2007
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Summary |
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Key words: Acari, Oribatida, force measurement, performance, claw
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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). The morphology of claws can be understood in the context of
adaptation to interact with diverse rough surfaces
(Nachtigall, 1974).
Specialized adhesive attachment devices for smooth surfaces, such as pulvilli
and tarsal hairs, occur in chelicerates and insects, and in the latter they
have been studied experimentally in a variety of taxa
(Beutel and Gorb, 2001).
However, the claw action and its relation to surface structure have been
little investigated. While the performance and functional morphology of insect
tarsi have been studied to some extent
(Stork, 1980;
Lees and Hardie, 1988;
Federle et al., 2000;
Dai et al., 2002;
Betz, 2002), we are unaware of
any studies investigating the performance of chelicerate claws. The only
available study investigated a general correlation of the size of adhesive
devices and body mass in a variety of arthropods, including the spider species
Cupiennius salei and Aphonopelma seemanni, but without
measuring performance or taking claws into account
(Arzt et al., 2003).
Tremendous degrees of apotele modifications appear in different lineages of
chelicerates and have been used together with general leg morphology for
large-scale phylogenetic analyses (Shultz,
1989; Shultz,
2007; Dunlop,
2002). Both the simplest and the most complex apoteles appear in
the Acari (Evans, 1992;
Alberti and Coons, 1999;
Dunlop, 2002). Acari are
grouped into two large taxa, the Anactinotrichida
(=Parasitiformes+Opilioacarida) and the Actinotrichida (=Acariformes). While
the monophyly of Acariformes, Parasitiformes and Opilioacarida is generally
accepted, monophyly of Acari as a whole has been questioned several times, not
least due to the variations in apotele morphology
(Alberti, 2005). Regressive
apoteles comprise two claws, a single claw or no claws at all and are present
in various taxa of Actinotrichida; however, the Anactinotrichida usually
retain their two lateral claws, while the middle claw is modified into a
cushion-like pulvillus (Evans,
1992). Movements of the claws are effected through the action of
depressor and levator muscles, which connect by tendons to the ventral and
dorsal edges of the basilar piece, respectively (Figs
1,
2). The claws are formed of
modified setae and contain birefringent actinopilin in the Actinotrichida
(Grandjean, 1941;
Grandjean, 1943).
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; Betz, 2002).
However, due to the small size of most of the Acari (<1 mm length),
measuring of forces of the claws (
50 µm length) is particularly
difficult.
The size-grain hypothesis (Kaspari and
Weiser, 1999) states that the environmental rugosity significantly
increases with decreasing body size of walking organisms. Hence,
microarthropods experience their environment as a three-dimensional habitat of
interstices and gaps. In this context, small organisms are thought to walk
`through' rather than `over' a landscape. The wide range of ecological niches
inhabited by Acari (predatory, parasitic, herbivorous, fungivorous,
saprophytic) suggests that the landscape microstructure is highly variable for
the different ecological groups. Thus requirements of the locomotory system to
perambulate these landscapes should reflect this variability. We expect the
highest forces of the locomotory appendages in soil-living mites, due to the
necessity of movement within a heterogeneous and unpredictable
environment.
In the present study we provide the first force measurements for such microarthropods. We used two approaches to measure different forces of a single-clawed oribatid mite, Archegozetes longisetosus Aoki: the holding forces perpendicular to the substrate and the pulling forces parallel to the substrate on three test surfaces with different roughness. Our results provide the opportunity for future comparative studies of the performance and adaptive values of different apotele morphologies within the Acari and other microarthropods.
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Materials and methods |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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)]. This
species usually inhabits isolated litter pockets of the soil
(Haq, 1982), so that we expect
high forces of the locomotory system due to digging activity. With exception
of the claw, the primitive chelicerate condition of missing extensor muscles
in the legs is retained, as in all Acari
(Shultz, 1989)
(Fig. 1); the four pairs of
walking legs each have a single claw that is entirely fused with the basilar
piece (Fig. 2) and linked by
condylophores with the sclerotized tarsal cuticle.
The average individual mass of adult specimens, taken from our laboratory
strain A. longisetosus ran
(Heethoff et al., 2007), was
approximately 100 µg, determined by weighing five samples of 30–50
adults. Adult specimens chosen from our laboratory culture were actively
moving, had fed, and contained no eggs. The latter two parameters could be
easily checked due to the translucent cuticle of adults.
Surfaces used in this study
Three different surfaces with defined texture roughness
(Ra) were chosen for force measurements, available as
lapping and polishing films with applied Al2O3 particles
of defined sizes (Ultratec Manufacturing Inc., USA). Roughnesses of 0.05
µm, 1 µm and 30 µm were chosen to span a wide range of
claw–surface interaction strength
(Fig. 3).
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We used X-ray microtomography for the non-invasive analyses of leg and
muscle morphology. This technique is a valuable tool for studying the internal
anatomy of small organisms with sub-µm resolution
(Betz et al., 2007). Fresh
adult specimens were placed in glutaraldehyde for 60 h, dehydrated in an
ethanol series and critically point dried in CO2. Scans of whole
animals were taken at the European Synchrotron Radiation Facility (ESRF,
Grenoble, France) at beamline ID19 (experiment SC2127) with a detector-sample
distance of 20 mm and a pixel resolution of 0.7 µm
(Fig. 1A) and 0.3 µm
(Fig. 2B) at 20.5 keV. Image
analyses of the voxel data and measurements of muscle diameters were performed
using the software VGStudio Max 1.2.1. (Volume Graphics, Heidelberg,
Germany).
Force measurements on living mites
Two kinds of forces were measured on each of the three surfaces: (i)
holding forces and (ii) pulling forces.
) were
calculated as the capability of the mite to remain attached to the surface
when lifted up vertically (Fig.
4). Data were acquired as mass (m) and transformed into
force (F) using the formula F=mg (where
g=981 m s–2). The MP100 WSW data acquisition
system (BIOPAC Systems Inc., USA) with a strain gage force transducer (FORT25,
WPI Inc., USA) was used to calculate the holding forces. The signal was
amplified by a transducer amplifier (BIOPAC Systems Inc.) and calibrated by a
2 g mass. Mites were super-glued with the notogaster (dorsalshield) on an
insect pin (No. 03) and the pin was mounted to the force sensor on a
micromanipulator. The mite was then carefully touched to the horizontal test
surface using the micromanipulator and allowed to attach with all walking legs
(checked with a stereo microscope). The force sensor was then moved upwards
until the mite detached with all legs from the surface, and the masses were
measured. This procedure was repeated 30 times with all specimens and five
specimens were tested on each surface. The difference between the maximum
amplitude of the force sensor and the zero-line (mite hanging free, not
attached to surface, Fig. 6)
was calculated using the software AcqKnowledge 3.8.2 (BIOPAC Systems Inc.,
USA); maximum forces were used for subsequent analyses. ) were
measured as the capability to move upwards on vertical surfaces. Data were
acquired as mass using an analytical scale (GR-202-EC, A&D Instruments
Ltd, UK, Figs 5,
8) and transformed into forces
using the formula F=mg. Mites were glued with the
notogaster on the tip of an insect pin (No. 00); the pin was thoroughly
mounted with the pin head in a 2 g piece of modelling clay, so that it
could not slip. The piece of modelling clay was then placed on the scale and
the scale was set to zero. The test surfaces were mounted vertically on a
micromanipulator and moved towards the mite until all walking legs were
attached to the surface (checked with a stereo microscope). The scale was
connected to a computer and the mass was continuously recorded for 2 min with
the software RsKey 1.34 (A&D Instruments Ltd, UK). A negative mass was
recorded whenever the mites tried to walk upwards, thereby pulling the
modelling clay away from the scale (Fig.
8). On each surface, seven specimens were tested; the maximum
forces from each replicate were used for subsequent analyses.
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Morphological parameters (x,y; Fig. 2) of claws and forces on different surfaces were analysed using one-way ANOVA in SPSS (SPSS Inc., USA); homogeneous groups were identified with a Tukey post-hoc test (P<0.05). Values are means ± standard deviation (s.d.).
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Results |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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Holding forces
Holding forces were calculated in relation to different substrate
roughness; see Fig. 6 for a
typical example of a measurement. Maximum holding forces significantly
increased with the particle diameter of the surfaces for all three values of
Ra (F(2,14)=57.2, P<0.001;
Table 1;
Fig. 7). On surfaces with
Ra=0.05 µm, mites produced forces of 0.0715 mN.
Surfaces with Ra=1 µm provided the mites with enough
irregularities to cling to cavities with forces eight times higher compared to
the 0.05 µm substrate (0.5888 mN). On the roughest surface
(Ra=30 µm), observed holding forces were exceptionally
high, reaching 1.1592 mN. This was 1.8 times higher when compared to
Ra=1 µm and corresponds to 1180 times the body
weight.
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Pulling forces
Pulling forces were also calculated in relation to substrate roughness. A
typical example of a measurement is given in
Fig. 8. Maximum pulling forces
were different among the surfaces (F(2,20)=16.8,
P<0.001; Table 1;
Fig. 9), although differences
between Ra=1 µm and Ra=30 µm
were not significant. The substrate with Ra=0.05 µm led
to pulling forces up to 0.196 mN. On surfaces with Ra=1
µm, pulling forces were 1.5 times higher than on the 0.05 µm substrate
(0.3237 mN). Observed forces on the roughest surface were 2.8 times higher
when compared to Ra=0.05 µm and the pulling capability
reached up to 530 times the body weight.
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Discussion |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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Our results clearly suggest, however, that the interaction strength of
claws and the substrate significantly increases with substrate roughness. This
is a result of the correlation of claw size and surface particle diameter,
together with overall heterogeneity of the surface. A similar correlation of
surface roughness and claw forces has been shown for larger arthropods,
including the beetle Pachnoda marginata
(Dai et al., 2002), beetles of
the genus Stenus (Betz,
2002) and the true bug Pyrrhocoris apterus
(Tölke, 2005).
Holding and pulling forces
To compare holding and pulling forces, it is important to take the
positioning and orientation of the legs on the surfaces into account. The
holding forces were measured at a right angle to the longitudinal axis of the
mite. Thus all legs should make the same contribution to the holding forces
irrespective of the horizontal angle between body and leg, as long as all
claws are the same size. Forces measured along the longitudinal axis, however,
relate only to the first two pairs of walking legs. In oribatid mites, as
generally in Actinotrichida, these are directed anteriorly, while the third
and fourth pairs are directed posteriorly
(Evans, 1992). Pulling forces
are thereby generated only by half of the legs and should therefore be only
about half the magnitude of the holding forces. For rough surfaces
(Ra=1 µm, Ra=30 µm) this is a
suitable approximation; average holding forces divided by pulling forces were
1.86 and 2.43 for Ra=1 µm and Ra=30
µm, respectively. However, on the smooth surface
(Ra=0.05 µm) this relation is completely different and
even inverted (0.42).
The reason for this is apparent from Fig. 3. The particles and the surface heterogeneity on the smooth surface (Fig. 3C) are too small for the claw to fit very far into cavities, thereby losing much of their ability to resist detachment at a right angle to the surface. However, the production of forces along the longitudinal axis is still possible to some extent (approx. twice as high as the holding forces), because here, only the very tip of the claw has to interact with the surface. Correspondingly, mite behaviour is different on the three surfaces. On surfaces of Ra=1 µm and Ra=30 µm, mites almost immediately find a good position with their legs; they keep the legs in one position and start pulling. On the smooth surface, the mite undergoes behaviour more like `ice-skating', where the forces are produced while the claws scratch over the surface to find a position to hook into.
Force/weight ratio and muscle stress
Generally, large animals exert higher forces than small ones. However,
these forces are expected to be proportional to the volume rather than the
body mass because the weight is proportional to the cube of the body length
(Alexander, 1985;
Federle et al., 2000). Thus
force/weight should be proportional to body mass–1/3
(Alexander, 1985); accordingly,
force should be proportional to body weight2/3 as long as animals
have comparable shapes and densities
(Federle et al., 2000).
However, animals of equal mass may exert different maximum forces. Therefore,
an upper and lower threshold for the force/weight ratio of 0.5xbody
mass–1/3 and 20xbody mass–1/3,
respectively, can be given to describe a range of maximum force/weight ratios
(Alexander, 1985). Using this
assumption and the given body mass of 100 µg for A. longisetosus,
we expect a force/weight ratio of 215 (or a value within the force/weight
range between 108 and 4309). The maximum exerted force of 1.1592 mN leads to a
force/weight ratio of 1182. This value is more than five times higher than
expected, but within the theoretical range of maximum possible forces.
Forces, however, depend not only on the weight and size of an organism but
also on the properties of muscles involved in force production. Some muscles
can produce higher stresses (force/cross-sectional area) than others and this
can also be compared among different groups of muscles and animals
(Taylor, 2000). While
vertebrate and insect flight muscles yield stresses of 100–300 kN
m–2, the closer muscle of decapod crustacean claws can
generate stresses up to 1350 kN m–2
(Taylor, 2000).
The functional cross-sectional area of the whole claw depressor muscle of A. longisetosus was calculated by adding the cross-sectional areas of all six bundles. Since the generated holding forces are the result of the action of all eight legs, this value was multiplied by eight and led to a functional cross-sectional area of 1.3568x10–9 m2. Together with the maximum holding force of 1.1592 mN, this results in a muscle stress of 1171 kN m–2, four times higher than the highest values reported for insect flight muscles and vertebrate muscles, but within the exceptionally high range of decapod claw closing muscles.
Comparisons with insect forces
It is difficult to compare forces generated by the claws of insects
directly with those generated by mites because of the different mechanisms
responsible for movements of the claws. The insect unguitractor apparatus is
connected to the tendon of the claw flexor muscle on one side and to the claw
on the other side (Heinzeller et al.,
1989). Gorb (Gorb,
1996) hypothesized that this structure enables an interlocking
mechanism that allows a claw engaged with the surface to be held in a bent
position with a reduced muscular energy investment. Oribatid mites, in
contrast, have no unguitractor apparatus and the claw is actively moved in
both directions by direct muscular action
(Evans, 1992)
(Fig. 1,
Fig. 2B). Other differences are
the number of claws on each leg and the number of legs. While insects have six
legs to generate attachment forces, most mites have eight; and while most
insect species have two claws on each leg, A. longisetosus has only
one. Furthermore, as already described, the contribution of each leg to the
measured pulling forces depends on the positioning of the leg on the surface
(Dai et al., 2002), information
that unfortunately is not available for most of the studies.
Betz (Betz, 2002) reported
that the beetle Stenus cicindeloides generates maximum pulling forces
with claws on rough surfaces (filter paper) with a force/weight ratio of 73
after the tenent setae had been neutralized by covering them with a thin layer
of superglue. Claw forces of the beetle Pachnoda marginata were
investigated by Dai et al. (Dai et al.,
2002); on rough surfaces, the force/weight ratio was 38.
Tölke (Tölke, 2005)
found that the true bug Pyrrhocoris apterus could pull with an
equivalent of 36 times its weight, after pulvilli were removed. On rough
filter paper, Lees and Hardie (Lees and
Hardie, 1988) reported the aphid Megoura viciae having a
claw pulling force/weight ratio of 17, after the pulvilli were
inactivated.
If the pulling forces generated by claws of insects are thus compared with
our results, we can conclude that A. longisetosus has the highest
pulling force/weight ratio, seven times higher than the highest so far
reported in insects (Betz,
2002). Federle et al. (Federle
et al., 2000) reported holding force/weight ratios of 145 for the
ant Crematogaster, which is eight times less than the holding
force/weight ratio reported here. Unfortunately, they used intact tarsi, hence
no holding forces are available for arthropod claws only for comparison with
our results.
However, as described above, the comparison of force/weight ratios can lead
to questionable results due to errors occurring by linear scaling of
parameters that are not linearly proportional. Therefore, we also compared the
force/weight2/3 ratio. Insects have an arithmetic mean for
force/weight2/3 of 3.12±3.01 (mean ± s.d., range:
0.055–8.15). The lowest value was observed in the aphid, M.
viciae (Lees and Hardie,
1988), the highest in the beetle, P. marginata
(Dai et al., 2002). Most
insects studied [S. cicindeloides
(Betz, 2002); P.
apterus (Tölke,
2005); Crematogaster
(Federle et al., 2000)],
however, have a force/weight2/3 ratio between 2.1 and 2.95,
indicating that the factor of proportionality lies within this range. With
2.36, the median is possibly a good approximation to the factor of
proportionality. Taking this into account, we argue that
force/weight2/3 ratios much less than 2 describe disproportional
weakness and ratios much higher than 3 describe disproportional strength. The
aphid M. vicia produces much lower forces than theoretically expected
for an insect; accordingly P. marginata produces disproportionate
high forces. The force/weight2/3 ratio of A. longisetosus
reported here is 11.74, five times higher than theoretically expected.
To conclude, we have shown that the soil-dwelling microarthropod A. longisetosus produces exceptionally high relative claw forces. The high force/weight ratios are not scaling artefacts; on the contrary, A. longisetosus produces disproportionately high forces with its claws. As hypothesized, these high forces are presumably important to move and burrow effectively in soil. The simple organization of its apotele (single claw, direct muscular movement with flexor and depressor), the high muscle stresses, and the high relative forces make A. longisetosus a valuable model system for further investigations of the functioning of arthropod claws.
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Acknowledgments |
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