First published online October 7, 2008
Journal of Experimental Biology 211, 3333-3343 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.020941
Comparison of smooth and hairy attachment pads in insects: friction, adhesion and mechanisms for direction-dependence
James M. R. Bullock,
Patrick Drechsler and
Walter Federle*
Department of Zoology, University of Cambridge, Downing Street, Cambridge
CB2 3EJ, UK
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Fig. 1. Adhesive pad morphology in G. viridula (male, A–C) and
C. morosus (D–F). Hind (A) and front tarsus (D), the
distal-most adhesive pad (B,E) and the contact area of each of the pads in
contact with glass, as viewed via epi-illumination (C,F). Arrows
indicate distal direction. Ti, tibia; Cl, claws; Ta, tarsal segments; Eu,
euplantulae; Ar, arolium.
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Fig. 2. Diagram showing the two restraining conditions for the stick insect tarsus.
(A) `Immobilised', the pretarsus is fully restrained using dental cement. (B)
`Footloose', the leg is fixed at the tibia, leaving the tarsus free to
move.
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Fig. 3. (A)Experimental setup for recording friction, adhesion and contact area of
insect adhesive pads. The pad is brought into contact with a glass cover slip
attached to a 2-D strain gauge force transducer. The transducer is moved by a
3-D motorised positioning stage. Force signals are amplified and recorded on a
computer. A feedback mechanism allows a constant normal force to be
maintained. Contact area is recorded using a stereo microscope with coaxial
illumination. (B) G. viridula pad contact area. Arrow shows distal
direction. (C)Threshold image providing a measure of the real contact area.
The red frame shows the estimate of projected pad area. Scale bar=100
µm.
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Fig. 4. Example force curves for the stick insect (A,B) and the dock beetle (C,D).
A and C show proximal slides (resulting in a positive friction force); B and D
show distal slides (resulting in a negative friction force), preceded by a
short proximal movement. Dark grey background denotes proximal, light grey
background denotes distal movements. Sliding velocity 500 µ
ms–1, each slide followed by a 5 s pause before pull-off.
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Fig. 5. Effect of build up or depletion of secretion on pad adhesive stress (A,B)
and shear stress (C,D) for G. viridula. A and C show the result of
multiple slides on the same area of glass allowing the fluid to build up; B
and D show repeated slides, each time on a fresh area, depleting the fluid.
Plot shows medians (centre lines), interquartile ranges (boxes), and the
largest and smallest values (whiskers) that are not outliers (circles);
N=5.
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Fig. 6. Build up of pad secretion on glass after one slide (A, `little' secretion)
or five consecutive slides (B, `accumulated' secretion) visualised by
Interference Reflexion Microscopy. The dark marks indicate droplets of fluid
secretion, and fluid build up is evidenced by the increased number of `spots'
visible in B, `accumulated' secretion. The sliding direction of the pad is
indicated by arrows.
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Fig. 7. Effect of applied normal force on friction force (A), contact area (B) and
shear stress (C) in G. viridula. Plot shows medians (centre lines),
interquartile ranges (boxes), and the largest and smallest values (whiskers)
that are not outliers (circles); N=6.
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Fig. 8. Direction-dependence of friction (A,D), contact area (B,E) and shear stress
(C,F) for G. viridula (hairy system, A–C) and C.
morosus (smooth system, D–F) for immobilised pads (N=7) in
both `little' and `accumulated' secretion regimes. Significance levels;
*P<0.05, **P<0.01,
***P<0.001.
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Fig. 9. G. viridula, changes in adhesive contact area during proximal and
distal slides. (A) Full contact during a proximal slide. (B–E) The
following distal slide. Note that many individual tips have significantly
decreased in contact during the movement. Contact visualised by
epi-illumination. Scale bar=100 µm; arrow shows distal direction.
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Fig. 10. Tarsal movements during proximal and distal slides in C. morosus
and G. viridula in the `footloose' condition. In the stick insect (A
proximal, B distal), a distal slide resulted in buckling, peeling off the
contact area from its proximal side. In the beetle (C proximal, D distal), a
distal push caused the whole pad to rotate due to its lateral instability.
When fixed in all but the vertical direction (E proximal, F distal), proximal
to distal peeling of the entire pad was observed. (A D) side views, (E,F)
contact recorded via epi-illumination. Arrow shows distal
direction.
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Fig. 11. Diagram of setal movement during a proximal pull (A) and a distal push (B).
During distal movements, hairs appeared to contact only with their tips. The
thicker fluid film may explain the observed distal decrease in shear
stress.
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© The Company of Biologists Ltd 2008
