Friction Forces In Syrphidae

The Journal of Experimental Biology 204, 1421–1431 (2001)
1421
Printed in Great Britain © The Company of Biologists Limited 2001
JEB3222
SCALE EFFECTS ON THE ATTACHMENT PADS AND FRICTION FORCES IN
SYRPHID FLIES (DIPTERA, SYRPHIDAE)
STANISLAV GORB*, ELENA GORB AND VICTORIA KASTNER
Biological Microtribology Group, Biochemistry Department, Max-Planck-Institute of Developmental Biology,
Spemannstrasse 35, D-72076 Tübingen, Germany
*e-mail: Stas.Gorb@tuebingen.mpg.de
Accepted 24 January; published on WWW 28 March 2001
Summary
To test the role of constructional and dimensional factors
The frictional properties of the material of the setal tips
in the generation of friction force by systems of setose
were not dependent on the dimensions of the ﬂy species.
attachment pads, six species of syrphid ﬂy (Platycheirus
Similar results were obtained for the frictional properties
angustatus, Sphaerophoria scripta, Episyrphus balteatus,
of the pulvillus as a whole. Thus, the properties of the
Eristalis tenax, Myathropa ﬂorea and Volucella pellucens)
secretion and the mechanical properties of the material of
were studied using light and scanning electron microscopy.
the setal tips are approximately constant among the species
Flies were selected according to their various body mass and
studied. It is concluded that differences in friction force
attachment pad dimensions. Such variables as pad area,
must be related mainly to variations in the real contact area
setal density, the area of a single setal tip and body mass were
generated by the pad on the smooth surface. The real
individually measured. A centrifugal force tester, equipped
contact area can be estimated as the summed area of the
with a ﬁbre-optic sensor, was used to measure the friction
broadened setal tips of the pad in contact with the surface.
forces of the pads on a smooth horizontal surface made of
The real contact area depends on such morphological
polyvinylchloride. Friction force, which is the resistance
variables as setal density and the area of a single setal tip.
force of the insect mass against the sum of centrifugal and
Although individual variables vary among ﬂies with
tangential forces, was greater in heavier insects such as Er.
different dimensions, they usually compensate such that
tenax, M. ﬂorea and V. pellucens. Although lighter species
smaller setal tip area is partially compensated for by higher
generated lower frictional forces, the acceleration required
setal density.
to detach an insect was greater in smaller species. The area
of attachment pads, setal tip area and setal density differed
signiﬁcantly in the species studied, and the dependence of
Key words: morphology, cuticle, material properties, scale effect,
these variables on body mass was signiﬁcant.
friction, attachment, Insecta, Diptera, syrphid ﬂy.
Introduction
During their evolution, insects have evolved two distinctly
Data on the setose pad system of an adult reduviid bug
different mechanisms to attach themselves to a variety of
(Rhodnius prolixus) led previous authors to suggest that
substrata: smooth ﬂexible pads and setose surfaces. Attachment
mechanical interlocking between adhesive setae and
forces mediated by friction or adhesion are usually proportional
irregularities in the substratum is responsible for attachment to
to the area of real contact between two surfaces (Persson, 1998).
the substratum (Gillett and Wigglesworth, 1932). Most authors
Because of the ﬂexible material of the pads, both mechanisms
agree that at least two factors related to the pad material can
can maximise the possible real contact area with the substratum,
contribute to the attachment force: (i) material ﬂexibility and
regardless of its microsculpture. Setose systems always contain
(ii) the presence of an epidermal secretion in the contact area.
cuticle protuberances in their surfaces. The protuberances
The deformability and visco-elastic properties of smooth pads
occurring on the setose pads of Coleoptera (Stork, 1980a; Stork,
have been suggested to be important (Brainerd, 1994), and this
1980b), Dermaptera (Beutel and Gorb, 2000) and Diptera
was recently conﬁrmed experimentally (Jiao et al., 2000; Gorb
(Bauchhenss, 1979; Bauchhenss and Renner, 1977; Gorb,
et al., 2000). It has been shown that the adhesive secretion is
1998b) belong to different types. Representatives of the ﬁrst
an essential component of attachment in both setose and
two lineages have setae, 5–50 µm long, with sockets on the
smooth systems. Pad ﬂuids have been found on the smooth
ventral surface of their tarsal segments. Dipteran protuberances
pads of cockroaches (Roth and Willis, 1952), aphids (Lees and
are acanthae, protuberances originating from a single cell and
Hardie, 1988; Dixon et al., 1990) and bugs (Hasenfuss, 1977;
lacking sockets (Richards and Richards, 1969).
Hasenfuss, 1978; Ghasi-Bayat and Hasenfuss, 1980) and on the

1422 S. GORB, E. GORB AND V. KASTNER
setose adhesive pads of reduviid bugs (Edwards and Tarkanian,
without anaesthesia. Anaesthesia was not used because,
1970), ﬂies (Bauchhenss and Renner, 1977; Bauchhenss, 1979;
without a recovery period, it may disturb the normal posture
Walker et al., 1985) and coccinellid beetles (Ishii, 1987).
of animals and, thus, inﬂuence attachment ability. To minimise
Although the morphology and ultrastructure of the setose
water loss through the cut wing bases, experiments were
attachment devices have been described in numerous studies,
carried out 5–15 min after wing excision. After the
various aspects of the functioning of these systems still remain
experiments, the insects were labelled and placed in 70 %
unclear. Among lizards, which are also able to walk on a
ethanol for processing for microscopy.
smooth surface using ‘setose pads’, the pad area has been
shown to be the primary factor inﬂuencing clinging ability in
Force measurements
geckos, skinks and iguanids (Irschick et al., 1996). However,
A centrifugal technique was used. The main advantage of this
despite the close correlation between pad area and attachment
method, especially in the case of small organisms, is that no
ability, pad area depends on body mass less than does
prior treatment of the insects is required. This method is
attachment ability. When the effect of body size is removed,
commonly used for the measurement of friction and adhesive
approximately 50 % of the variation in clinging ability
forces for a variety of objects. It has been applied to measure
remains unexplained, which suggests that microsculptural and
the adhesion strength between starch microspheres and
ultrastructural differences may affect clinging ability. Geckos,
microcrystalline cellulose (Podczeck and Newton, 1995), the
skinks and iguanids differ in the structure of the setae covering
frictional properties of skin (Highley et al., 1977), the strength
the attachment pads (Ruibal and Ernst, 1965; Ernst and Ruibal,
of barnacle cement (Dougherty, 1990) and insect attachment
1967). However, the effect of ultrastructural properties has not
forces (Dixon et al., 1990; Brainerd, 1994; Federle et al., 2000).
been investigated systematically.
Our equipment was improved to enable such variables as
In the beetle Chrysolina polita, attachment force increases
initial motor speed and motor acceleration to be varied and to
with the total number of adhesive setae (Stork, 1980b). The
acquire, automatically, data on motor speed and insect position
number of adhesive setae can contribute to the attachment
relative to the rotor centre (Fig. 1). The motor rotating the
force by increasing the number of single contact points and/or
drum was controlled by a computer. Just above the drum, a
by increasing the overall contact area with the substratum. To
laser beam, serving as a light source, and a ﬁbre-optic sensor
test the role of constructional and dimensional factors in
were mounted. The sensitivity of the ﬁbre-optic sensor could
attachment, a larger number of species must be tested. Such
be tuned according to the subject’s dimensions. Its sensitivity
variables as pad area, setal density, the area of a single setal
was high enough to monitor subjects of approximately 1 mm
tip and body mass must be individually measured to investigate
in diameter rotating at 2000–2500 revs min−1. The light source
their effects on the resulting attachment force. Previously,
and the sensor were displaced by a distance (d) from the rotor
measurements of attachment forces in living insects have been
centre. Given the angular speed (ω) of the motor and the time
confounded by difficulties in experimental design or by the
between the two interruptions of the light sensor signal (∆t1)
time-consuming processing of video recordings. In this study,
as an insect rotates on the disc, the radius of the position of
we improved the previously used centrifugal method of
the subject could be calculated after each rotation (Fig. 1B–D).
measuring attachment force by incorporating a laser beam
All calculations were carried out automatically by the
system and ﬁbre-optic sensor to monitor the position of the
controlling software. The radius and other variables were
insect on a drum. This method was used to test individual
displayed on the computer screen, allowing the subject’s
attachment performance in six species of syrphid ﬂy, chosen
position on the drum during the experiment to be monitored
according to their body mass and attachment pad dimensions.
directly. An insect standing on the horizontal surface of the
The variety of attachment pad design in a number of taxa of
drum covered by a smooth polyvinylchloride (PVC) plate (the
the family Syrphidae has been described previously (Röder,
contact angles of water droplet on the plate were 81–86 °) was
1984). In our study, most morphological data were collected
accelerated until it lost contact with the drum. When the insect
individually, allowing us to compare directly the effects of
left the drum surface, the sensor no longer received a signal
structural properties on attachment ability. Lateral attachment
from the subject, and the motor acceleration was automatically
force, to which friction force is the main contributor, was
interrupted, the experiment stopped and data saved. The
measured.
following variables were measured: sensor displacement from
the rotor centre d (cm), motor speed v (revs min−1), mass m
Materials and methods
(mg), time from the beginning of rotation ∆t (s) and the time
between two sensor signals ∆t
Animals
1 (s). In addition, the following
variables were calculated. The angular speed ω (rad s−1) was
Male ﬂies of six common species from the family Syrphidae
calculated as:
were captured in July 1999 in the Schönbuch forest (near
1
Tübingen, southwest Germany): Platycheirus angustatus (Z.),
ω =
πν .
(1)
Sphaerophoria scripta (L.), Episyrphus balteatus (De Geer),
30
Eristalis tenax (L.), Myathropa ﬂorea (L.) and Volucella
The angular speed was used to calculate the radius of rotation
pellucens (L.). After capture, the wings were carefully cut off
r (cm):

Friction forces in Syrphidae 1423
1
A
Fc =
mac .
(5)
ls
fos
1000
Knowing at, the tangential component of the friction force Ft
pt
cp
(mN) was calculated:
1
Ft =
mat .
(6)
dr
1000
cs
The total friction force F (mN) was obtained from the
centrifugal Fc and tangential Ft components of the friction
cm
force:
rt
F 2
2
F =
c + Ft
,
(7)
B
C
and the total acceleration a (m s−2) was obtained from the
ls
fos
centrifugal ac and tangential at components of the acceleration:
a 2
2
a =
c + at .
(8)
Fly
Prior to force measurements, an individual insect was
weighed using a Mettler Toledo AG204 balance with a
pt
precision of 0.1 mg. The number of individual ﬂies (N) and
D
number of tests (n) for each species was as follows:
Platycheirus angustatus (N=6, n=60), Sphaerophoria scripta
(N=4, n=40), Episyrphus balteatus (N=11, n=110), Eristalis
tenax (N=9, n=90), Myathropa ﬂorea (N=3, n=30) and
d α
Volucella pellucens (N=5, n=50). Ten repetitions of force
r
measurements were made for each individual ﬂy. No
statistically signiﬁcant differences were revealed in ﬂy
Fig. 1. Centrifugal device for measuring frictional force. (A) Layout
performance depending on the number of experiments. For
of the centrifuge. The metal drum (dr), covered by a polyvinyl-
example, the relationship between friction force and experiment
chloride disc (pt), is driven by the computer-controlled motor. The
number was estimated for Er. tenax (N=8, n=10 tests for each
ﬁbre-optic sensor (fos) is adjusted to be just above the disc. The
insect, Kruskal–Wallis one-way analysis of variance, ANOVA,
sensor signal is monitored by a computer (cp). (B–D) Diagrams
on ranks, H=7.634, d.f.=9, P=0.571). Similar results have been
showing the technique used to monitor insect position (view onto the
disc surface from above). The sensor is shifted to one side of the disc
reported for ants (Federle et al., 2000).
by a distance d from its centre. The ﬂy, rotating clockwise, passes the
We made corresponding calculations to evaluate possible
laser beam twice per rotation, thus interrupting the sensor signal
drag Fdrag for each experiment (n=371):
twice. Given the speed of the motor and the time between signal
1
interruptions, the position of the ﬂy on the disc can be calculated (D).
Fdrag =
CArv2 ,
(9)
α, angle between detected ﬂy positions and the drum centre; cs,
2
sensor control electronics; cm, motor control electronics; d,
where C
is the empirical drag coefficient (usually
displacement of the sensor from the drum centre; ls, light source; r,
approximately 1, dimensionless), A is the area of the object in
radius of the position of the ﬂy from the rotor centre; rt, rotor of the
a plane perpendicular to the object’s motion, r is the density
motor.
of the medium (rair=1.29 kg m−3) and v is the linear speed.
Since we did not know the orientation of the insect on the
 ω 
drum, we assumed that the insect always stood parallel to the
r = d cos 
 ,
(2)
 2∆t1 
drum radius and that the insect’s area, in a plane perpendicular
to the object’s motion, was the maximum possible. These data
the centrifugal component of the acceleration ac (m s−2):
were obtained from digitised video frames for all six species.
The maximum linear speed of the insect was calculated from
1
ac =
rω2 ,
(3)
the maximum rotational speed, which was recorded during
100
each experiment. Calculated average drag was 0.056 mN (P.
and the tangential component of the acceleration at (m s2):
angustatus, 3.5 % of the average friction force), 0.110 mN (S.
scripta, 1.28 %), 0.200 mN (Ep. balteatus, 0.88 %), 0.404 mN
∆ωr
at =
.
(4)
(Er. tenax, 0.26 %), 0.504 mN (M. ﬂorea, 0.24 %) and
100∆t
0.740 mN (V. pellucens, 0.13 %). According to these
Knowing ac, the centrifugal component of the friction force Fc
calculations, the inﬂuence of ﬂuid ﬂow must be minimal for
(mN) was calculated:
such a body size and velocity.

1424 S. GORB, E. GORB AND V. KASTNER
Light microscopy
200
The pretarsi of ﬂies used in force measurements were cut off
A
160
the legs, dehydrated in ethanol and whole-mounted in DePeX
(Serva). Each individual leg was processed separately. Digital
120
images of pulvilli were obtained using a Sony 3CCD video
camera DXC-950P mounted on a Zeiss-Axioscope light
80
microscope. Pulvillus areas were measured from digital images
Body mass (mg)
40
using Sigma-Scan 5.0 (SPSS) software. Data were averaged
separately for each individual, for each species and for each
0
s
a
leg pair (fore-, mid- and hindlegs) within each species.
horia
phus
op
op
Eristali yathr
M
Volucella
Scanning electron microscopy
Platycheirus
haer
Episyr
Sp
Pretarsi of ﬂies, ﬁxed in 70 % ethanol, were dehydrated in
ethanol, critical-point-dried, mounted on holders, sputter-
coated with gold–palladium (10 nm) and examined in a Hitachi
90×103
S-800 scanning electron microscope at 20 kV. Measurements
)
B
2
of setal tip area and setal density per 1000 µm2 were made from
µ
m
60×103
digital pictures using AnalySIS 2.1 image-analysis software
a (
(Soft-Imaging Software GmbH, Münster, Germany). The setal
re
s a
tip areas were measured in a total of 120–150 tenent setae from
30×103
two insects for each species. The density of tenent setae was
averaged for 20–60 selected pulvillar areas in each species.
Pulvillu
0
Data processing
s
a
horia
phus
op
ANOVA was used to estimate differences between species
op
Eristali yathr
M
Volucella
according to particular variables. If the raw data were not
Platycheirus
haer
Episyr
Sp
normally distributed, Kruskal–Wallis one-way ANOVA on
ranks was used. To calculate dependencies between different
Fig. 2. Body mass (A) and area of a single pulvillus (B) for each
variables measured, linear models were applied. The ANOVA
species. Values are means ± S.D. (n=10 per species, N=3–11
statistics (F) were also estimated for the regressions. The F-
depending on species, see Materials and methods).
test statistic assesses the contribution of the independent
variable in predicting the dependent variable. It is the ratio
between the regression variation from the dependent variable
Friction force
mean and the residual variation about the regression line. The
The acceleration at which an insect loses contact with the
P-value is the probability of being wrong in concluding that
surface is hereafter termed ‘acceleration’. This value was
there is an association between the dependent and independent
signiﬁcantly different among species studied (one-way ANOVA
variables. The smaller the P-value, the greater is the probability
on species: F=91.31, P<0.001). The acceleration a (m s−2) was
that there is an association. We have concluded that the
greater in representatives of lighter species. It was related to the
independent variable could be used to predict the dependent
body mass m as a=217.9−0.9m (linear regression: F=7.50,
variable when P<0.05.
P=0.010, one-way ANOVA) (Figs 4A, 5A). Friction force F
(mN), which is the resistance force of the insect mass to the sum
of centrifugal and tangential forces, was also signiﬁcantly
Results
different among species studied (one-way ANOVA on species:
Body mass and pulvillus area
F=19.32, P<0.001). Friction was greater in heavier insects, such
In the series of species P. angustatus, S. scripta, Ep.
as Er. tenax, M. ﬂorea and V. pellucens (Figs 4B, 5B). It was
balteatus, Er. tenax, M. ﬂorea and V. pellucens, body mass m
related to the body mass m as F=0.601+0.019m (linear
(mg) increased from 4.8 to 164.0 mg (Fig. 2A). Only
regression: F=39.34, P<0.001, one-way ANOVA). Acceleration
representatives of two species (Er. tenax and M. ﬂorea) were
data show that, although lighter species generated lower friction
similar in body mass. All other species differed signiﬁcantly
forces, the ratio of friction force to the body mass is greater in
from each other (one-way ANOVA on species: F=89.14,
smaller species. In other words, lighter species demonstrated
P<0.001). Although the area of a single pulvillus S1 (µm2)
relatively higher attachment ability.
correlates with an increased body mass (linear regression:
The acceleration averaged for all trials with one species was
S1=10 350−286m, F=40.00, P<0.001, one-way ANOVA), the
2–6 times lower than its maximal value for each species
heaviest species (V. pellucens) has signiﬁcantly smaller pulvilli
(Fig. 4A). A similar relationship was also obtained for friction
than Er. tenax and M. ﬂorea (one-way ANOVA: F=14.29,
force (Fig. 4B). The relative difference between the average
P=0.001; F=24.48, P=0.001, respectively) (Figs 2, 3).
measured friction force Fave (mN) and the maximum measured

Friction forces in Syrphidae 1425
Fig. 3. Wholemounts of pretarsi of the species studied. Such preparations were used to quantify the area of a single pulvillus. Red lines indicate
measured areas (ma). Insets show the species used. cl, claw; pu, pulvillus; ta, terminal tarsomere. The 200 µm scale bar applies to all
micrographs, whereas the 5 mm scale bar applies to all ﬂy insets. (A) Platycheirus angustatus. (B) Sphaerophoria scripta. (C) Episyrphus
balteatus. (D) Volucella pellucens. (E) Eristalis tenax. (F) Myathropa ﬂorea.
friction force Fm (mN) was smaller in heavier insects
surface characteristics of the attachment devices may
[∆F=64.740−0.129m, where m is mass and ∆F=100(Fave/Fm);
contribute to differences in friction in the species studied.
F=15.580, P<0.001, one-way ANOVA]. A similar relationship
Two variables of the pulvillus surface were also quantiﬁed: (i)
was found for the acceleration data.
setal density and (ii) the area of the setal tip. Data obtained
from scanning electron microscopy (Fig. 8) revealed
Relationship between pulvillus area and friction force
signiﬁcant differences in setal size and density among species
There was signiﬁcant relationship between body mass m
(one-way ANOVA on species: setal tip area: F=489.50,
and the area of a single pulvillus S1 (linear regression:
P<0.001; density: F=67.55, P<0.001). Setal tip area increased
S1=10 350−286m, F=40.00, P<0.001, one-way ANOVA) (see
and setal density slightly decreased with increased body mass.
Fig. 7A). Since, in general, pulvillus area is larger and friction
There was a signiﬁcant relationship between setal tip area St
force is higher in heavier animals, it has been suggested that
(µm2) or setal density D (the number of setae in 1000 µm−2)
heavier animals generate higher friction force as a result of a
and increased body mass (St=2.608+0.018m, F=543.30,
larger area in contact with the substratum. The measured
P<0.001; D=109.1−0.331m, F=64.73, P<0.001, one-way
acceleration does not depend clearly on the pulvillus area
ANOVA) (Fig. 9A,B). At the same time, setal density was
(Fig. 6A,C), whereas the friction force increased linearly with
correlated with setal tip area. With an increase in setal tip area,
an increase in attachment area (Figs 6B,D, 7B).
setal density decreased signiﬁcantly (D=151.371−18.549St,
F=25.51, P<0.01, one-way ANOVA) (Fig. 9C).
Surface characteristics of pulvilli
Given that the friction force is dependent on the area of real
Lateral tenacity
contact between two surfaces, it is possible that different
Tenacity is the adhesive force per unit apparent contact area

1426 S. GORB, E. GORB AND V. KASTNER
1000
700
A
A
)
600
Platycheirus
-
2
800
s
)-2 500
600
Sphaerophoria
400
a
tion (m
400
Episyrphus
Eristalis
l
er
300
Myathropa
cce
A
200
200
Volucella
Acceleration (m s
0
100
a
op
horia
phus
0
op
Eristalis yathr
0
50
100
150
200
M
Volucella
Platycheirushaer
Episyr
Sp
25
B
70
Myathropa
B
20
60
)
Platycheirus
Volucella
N
50
Eristalis
15
Sphaerophoria
40
f
orce (m
30
10
20
riction
F
Friction force (mN)
5
10
Episyrphus
0
s
0
a
op
0
50
100
150
200
horia
phus
op
Eristali
yathr
M
Volucella
Body mass (mg)
Platycheirushaer
Episyr
Sp
Fig. 5. Acceleration (A) and friction force (B) versus body mass for
Fig. 4. Acceleration (A) and friction force (B) of the ﬂy species.
each specimen studied.
Filled columns give maximum values; open columns give means +
S.D. (n=10 per species, N=3–11 depending on species, see Materials
and methods).
Discussion
In a previous study, the vertical attachment forces (adhesive
between two surfaces. Since, for syrphid pulvilli, friction force
components) of ants measured with a centrifugal apparatus
is mediated by adhesion, the lateral tenacity of pulvilli, τp
were signiﬁcantly greater than those measured with a strain-
(µN µm−2), was calculated. This represents the friction force F
gauge force transducer (Federle et al., 2000). This result has
per unit apparent contact area τp=F/12S1, where S1 is area of a
been explained by the fact that tethered insects generally
single pulvillus. The value of τp ranged from 0.015 to 0.035 µN
continued to move during the strain-gauge measurements and
µm−2 in the species studied and was dependent on the body
only rarely were all six legs simultaneously in contact with the
mass (Fig. 10A). However, τp was not dependent on pulvillus
surface. Therefore, the centrifugal method was chosen in the
area (Fig. 10B).
present study to compare attachment abilities of syrphid ﬂies.
However, pulvillus area is not the same as the real area of
contact. The real area of contact directly inﬂuences the adhesive
Adhesion-mediated friction
and frictional forces. The real contact area Sr (µm2) was
In our experimental design, the horizontal force resisted by
evaluated as the aggregate area of all setae on all pulvilli
the insect during drum rotation was measured. This situation
Sr=12DS1St, where D is the density of tenent setae per 1 µm2, S1
is more closely related to the situation when an insect walks
is area of a single pulvillus in µm2, and St is the area of the single
on a vertical wall, and is not comparable with the situation
setal tip in µm2. There was no signiﬁcant relationship between
when an insect walks under a horizontal surface because of the
Sr and body mass (Sr=38.02+0.93m, F=2.36, P=0.20, one-way
different directions of the forces acting on the insect. In the
ANOVA) (Fig. 11A). The lateral tenacity of pulvillus material,
latter situation, the insect’s weight acts in a direction more-or-
τm (µNµm−2), was also calculated. Since the calculated real
less perpendicular to the surface, and adhesion is the main
contact area was 3–5 times smaller than the area of apparent
contributor to insect attachment. In our experimental situation,
contact (pulvillus area), τm was greater than τp. τm ranged from
an insect resisted the force acting to move it in a direction
0.06–0.13 µN µm−2 in the species studied and was not dependent
parallel to the surface. The insect is able to resist this external
on body mass (τm=0.10−0.0002m, F=0.48, P=0.53, one-way
force because of the friction between its attachment pads and
ANOVA) (Fig. 11B).
the substratum.

Friction forces in Syrphidae 1427
1000
80
A
B
)
800
)
-
2
N 60
s
m
m
600
c
e (
t
ion (
40
ion for
elera
400
ct
cc
20
ean a
200
ean fri
M
M
0
0
1000
80
C
D
)
)
-
2
N
s
800
m
m
60
c
e (
t
ion (
600
ion for
elera
40
ct
cc
a
400
fri
m
m
u
u
i
m
i
m 20
200
a
x
a
x
M
M
0
0
0
20×103
40×103
60×103
0
20×103
40×103
60×103
Area of a single pulvillus (µm2)
Fig. 6. Mean aave (A) and maximum am (C) acceleration and mean Fave (B) and maximum Fm (D) friction force versus the area of a single
pulvillus S1 for all specimens studied. (A) Linear regression: aave=195.7−1.21×10−3S1, F=1.788, P=0.190, one-way ANOVA. (B) Linear
regression: Fave=1.257+2.345×10−4S1, F=84.618, P<0.001, one-way ANOVA. (C) Linear regression: am=291.5−8.826×10−6S1, F=3.987×10−5,
P=0.995, one-way ANOVA. (D) Linear regression: Fm=1.892+5.241×10−4S1, F=38.248, P<0.001, one-way ANOVA.
)
25
2
A
80×103
B
µ
m
s (
5
)
20
N
60×103
15
4
5
4

pulvillu
r
c
e
(m
le
6
g
40×103

f
o
n
in
10
6
i
o
ct
f
a s
ri
20×103
F
2
5
3
a o
1
3
1
Are
2
0
0
0
50
100
150
200
0
20×103
40×103
60×103
80×103
100×103
Body mass (mg)
Area of a single pulvillus (µm2)
Fig. 7. Dependence of the area of a single pulvillus S1 on body mass m (A) and of friction force F on the area of a single pulvillus (B) for the
six species studied. (A) Linear regression: S1=10350−286m, F=40.00, P<0.001, one-way ANOVA. (B) Linear regression:
F=1.379+2.205×10−4S1, F=32.87, P=0.005, one-way ANOVA. Values are means ± S.D. (n=10 per species, N=3–11 depending on species, see
Materials and methods). 1, Platycheirus angustatus; 2, Sphaerophoria scripta; 3, Episyrphus balteatus; 4, Eristalis tenax; 5, Myathropa ﬂorea;
6, Volucella pellucens.

1428 S. GORB, E. GORB AND V. KASTNER
Fig. 8. Tenent setae of the species studied. Such images were used to quantify setal density and the area of the setal tip. The 10 µm scale bar
applies to all micrographs, whereas the 5 mm scale bar applies to all ﬂy insets. (A) Platycheirus angustatus. (B) Sphaerophoria scripta. (C)
Episyrphus balteatus. (D) Volucella pellucens. (E) Eristalis tenax. (F) Myathropa ﬂorea.
Factors such as the proximity of two surfaces, the thickness
to the surface (Bauchhenss, 1979). Attachment under a
of the ﬂuid layer between the surfaces, surface chemistry and
horizontal surface is mediated mainly by adhesive forces;
ﬂuid viscosity will contribute to the adhesion force. The ﬁne
these have been measured only for two insect species
structure of the adhesive setae in syrphid ﬂies has been
possessing setose attachment pads: the calliphorid ﬂy
reported previously for the ﬂy Episyrphus balteatus (Gorb,
Calliphora vomitoria (2.4 mN) (Walker et al., 1985) and the
1998b), whose setae (acanthae) are hollow, with some
coccinellid beetle Epilachna vigintioctomaculata (2.9 mN)
containing pores under the end plate. These pores presumably
(Ishii, 1987). Given that the pad material is designed to
allow the adhesive secretion to pass directly to the contact
deliver secretion continuously to the contact area and that
area. Porous canals, located at the base of the shaft, reported
adhesive forces are involved in holding an insect under a
in other ﬂies, are also involved in the transport of secretions
surface, we might expect that adhesive forces will contribute

Friction forces in Syrphidae 1429
to the friction force (Rabinowicz, 1995) when an insect walks
180
on a vertical surface. Such adhesion-mediated friction can be
A
2
160
relatively high, so that surfaces with ﬂuid between them

µ
m
140
demonstrate a friction coefficient greater than 1. In the
2
000
120
present study, in different species of syrphid ﬂy, we measured

1
friction coefficients ranging from 7 to 35, supporting a role
100
per
3
4
for adhesion-mediated friction. Friction between the ﬂy
y
80
5
s
it
attachment system and a smooth surface is eight times greater
en
60
than adhesion (Walker et al., 1985). If we assume that this
1
6
a
l
d
40
relationship is similar among various ﬂy species, the expected
Set
20
adhesive properties of the pad material can be calculated for
0
species in the present study.
0
50
100
150
200
In the case of friction, the mechanical properties of a
Body mass (mg)
material may also contribute to the overall attachment force.
The material of the ﬂy pulvillus is soft; the membranous cuticle
8
of setose pads is a ﬁbrous composite material in which the
B
7
ﬁbres are not densely distributed. In Coleoptera, the setal bases
)2
6
are embedded in this material, providing high mobility of setae
6
µ
m
(
and thus adaptability to a variety of surface proﬁles (Beutel and
5
Gorb, 2000). The setae or setal ends are also composed of an
a
l

tip
1
4
extremely ﬂexible material. Among setose attachment systems,
et
4
5
f
s
3
the ﬂexible nature of setae has been demonstrated only for
3
a o
beetles, using Mallory’s single stain (Stork, 1983), which
re
2
A
stains tanned and untanned cuticle differently.
2
1
Scale effects on the surface microsculpture
0 0
50
100
150
200
Scale effects on the surface microsculpture have been
Body mass (mg)
reported previously in other insect attachment systems. The
size and density of the seta-like hooks that couple the fore-
180
and hindwing in Hymenoptera are dependent on animal size
C
2
160
(Schneider and Schill, 1978). Scale effects have been
2
reported for the microtrichia in different frictional devices of

µ
m
140
insects, such as the beetle elytra-to-body locking device
000
120

1
(Gorb, 1998a) and ﬂy armoured membranes (Gorb, 1997). In
100
per
3
4
the beetle wing-locking device, a ﬁvefold increase in body
y
80
size results in an increase in both the length (up to fourfold),
s
it
5
en
60
1
6
and the width (up to 2.3-fold) of microtrichia and a decrease
a
l
d
40
in their density (up to ﬁvefold). The dependence of the length
Set
20
and width of microtrichia on beetle size was linear, whereas
that of microtrichial density was logarithmic. Among the ﬂy
0 0
2
4
6
8
species studied, the surface microsculpture (the area of the
Area of setal tip (µm2)
setal tip and setal density) varied signiﬁcantly: larger ﬂies
have a lower setal density and larger setal tip areas.
Fig. 9. (A) Setal density D versus body mass m. (B) Setal tip area S1
Therefore, the area of the setal tip correlated negatively with
versus body mass m. (C) Dependence of setal density D on the area
setal density.
of the setal tip S1. Values are means ± S.D. (n=10 per species,
N=3–11 depending on species, see Materials and methods).
Frictional properties of the pad material
1, Platycheirus angustatus; 2, Sphaerophoria scripta; 3, Episyrphus
The most interesting ﬁnding of this study is that the
balteatus; 4, Eristalis tenax; 5, Myathropa ﬂorea; 6, Volucella
pellucens.
frictional properties of the material of the setal tips are not
dependent on the dimensions of the ﬂy species. In other words,
frictional forces generated by the surface unit of the pulvillus
variations in real contact area generated by the pad. Real
are similar among the species studied (Fig. 10). The same
contact area is the sum of the areas of the broadened setal tips
result was obtained for the material of the setal tip (Fig. 11).
of the pad in contact with the surface. Real contact area
Thus, the adhesive properties of the secretion and the
depends on the overall pad area, setal density and the area of
mechanical properties of the setal tip material vary little, and
a single setal tip. Although these variables vary among animals
differences in friction force must therefore mainly relate to
with different dimensions, the smaller setal tip area is

1430 S. GORB, E. GORB AND V. KASTNER
0.05
0.020
A
)
A
2
5
s
0.04
p
0.016
3
1
6
a
l

ti
s (mm
u
4
0.03
et
ill
0.012
4
f
s
l
v
a o
0.02

pu
5
re
0.008
6
)
2
le
g
-
2
t
a
l
a
in
o
0.01
0.004

µ
m
T
a s
2
3
µ
N
in
1
(
0
0 0
50
100
150
200
0
20 40 60 80 100 120 140 160 180
Body mass (mg)
f

pulvilli
)
0.14
-
2
1
B
3
0.05
0.12
a
c
i
t
y o

µ
m
B
y
µ
N
0.10
6
a
l

t
en
0.04
a
cit
3
a
l
(
0.08
a
t
er
teri
2
L
6
4
0.03
a
l

ten
0.06
5
1
4
a
ter
a
l
ma
L
0.04
et
0.02
s
2
5
0.02
f

the
o
0.01
0
0
20 40 60 80 100 120 140 160 180
0
Body mass (mg)
0
30×103
60×103
90×103
Fig. 11. (A) Real contact area Sr of all setae in a single pulvillus
Area of single pulvillus (µm2)
versus body mass m. Linear regression: Sr=3.186+0.077m, F=2.36,
Fig. 10. Lateral tenacity of the ﬂy pulvillus. (A) Lateral tenacity
P=0.199, one-way ANOVA. (B) Lateral tenacity of the ﬂy setae τm
of the pulvillus τ
versus body mass m. Linear regression: τ
p
versus body mass m. Linear regression:
m=0.100−1.550×10−4m,
τ
F=47.65, P=0.528, one-way ANOVA. (n=10 per species, N=3–11
p=0.025+6.351×10−6m,
F=0.01, P=0.923, one-way ANOVA.
(B) Lateral tenacity τ
depending on species, see Materials and methods). 1, Platycheirus
p versus pulvillus area S1. Linear regression:
τ
angustatus; 2, Sphaerophoria scripta; 3, Episyrphus balteatus;
p=0.029−1.380×10−6S1, F=1.03, P=0.367, one-way ANOVA.
Values are means ± S.D. (n=10 per species, N=3–11 depending on
4, Eristalis tenax; 5, Myathropa ﬂorea; 6, Volucella pellucens.
species, see Materials and methods). 1, Platycheirus angustatus;
2, Sphaerophoria scripta; 3, Episyrphus balteatus; 4, Eristalis tenax;
5, Myathropa ﬂorea; 6, Volucella pellucens.
a further increase in pulvillus area could cause problems with
the operation of a large attachment area so that a smaller
number of setae would be able to make contact with the
compensated for by a higher setal density in the smaller species
substratum. Data obtained from force measurements on a
(Fig. 9C).
single hair of the gecko attachment pad support this suggestion:
the attachment force of a single hair is greater than the force
Scale effects on friction force
recalculated from measurements on a complete gecko foot
Although heavier species demonstrated higher friction
(Autumn et al., 2000).
forces (Fig. 4B), the ratio of friction force to body mass was
signiﬁcantly higher in the smallest species (P. angustatus:
M. Mondon (Institute of Physics, University of
36.16±18.12 mN mg−1, N=6) compared with the largest species
Kaiserslautern, Germany) helped with contact angle
(V. pellucens: 6.29±1.56 mN mg−1, N=5) (means ± S.D.;
measurements. Valuable discussions with Dr Y. Jiao, S.
F=13.27, P=0.005, one-way ANOVA). The other four species
Niederegger (MPI of Developmental Biology, Tübingen,
had ratios intermediate between these two values. Friction
Germany), Dr M. Scherge (Ilmenau Technical University,
force increases with an increase in pulvillus area (Figs 6B,D,
Germany) and Dr W. Federle (Würzburg University,
7B). However, the largest species does not have the largest
Germany) are greatly acknowledged. Two anonymous
pulvilli. Possibly, there are some design constraints on a further
reviewers helped to improve an early version of the
increase in the pad area. Since a twofold increase in pulvillus
manuscript. TETRA GmbH (Ilmenau, Germany) contributed
area results in a 1.5-fold increase in friction force (from 10 to
to the design of the centrifuge tester. This work is supported
15 mN), it seems that a further increase in pulvillus area does
by the Federal Ministry of Education, Science and
not improve attachment ability. A possible explanation is that
Technology, Germany, to S.G. (project BioFuture 0311851).

Friction forces in Syrphidae 1431
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