Arachnids secrete a fluid over their adhesive pads.
ABSTRACT Many arachnids possess adhesive pads on their feet that help them climb smooth surfaces and capture prey. Spider and gecko adhesives have converged on a branched, hairy structure, which theoretically allows them to adhere solely by dry (solid-solid) intermolecular interactions. Indeed, the consensus in the literature is that spiders and their smooth-padded relatives, the solifugids, adhere without the aid of a secretion.
We investigated the adhesive contact zone of living spiders, solifugids and mites using interference reflection microscopy, which allows the detection of thin liquid films. Like insects, all the arachnids we studied left behind hydrophobic fluid footprints on glass (mean refractive index: 1.48-1.50; contact angle: 3.7-11.2°). Fluid was not always secreted continuously, suggesting that pads can function in both wet and dry modes. We measured the attachment forces of single adhesive setae from tarantulas (Grammostola rosea) by attaching them to a bending beam with a known spring constant and filming the resulting deflection. Individual spider setae showed a lower static friction at rest (26%±2.8 SE of the peak friction) than single gecko setae (Thecadactylus rapicauda; 96%±1.7 SE). This may be explained by the fact that spider setae continued to release fluid after isolation from the animal, lubricating the contact zone.
This finding implies that tarsal secretions occur within all major groups of terrestrial arthropods with adhesive pads. The presence of liquid in an adhesive contact zone has important consequences for attachment performance, improving adhesion to rough surfaces and introducing rate-dependent effects. Our results leave geckos and anoles as the only known representatives of truly dry adhesive pads in nature. Engineers seeking biological inspiration for synthetic adhesives should consider whether model species with fluid secretions are appropriate to their design goals.
- SourceAvailable from: cmu.edu[show abstract] [hide abstract]
ABSTRACT: Geckos are exceptional in their ability to climb rapidly up smooth vertical surfaces. Microscopy has shown that a gecko's foot has nearly five hundred thousand keratinous hairs or setae. Each 30-130 microm long seta is only one-tenth the diameter of a human hair and contains hundreds of projections terminating in 0.2-0.5 microm spatula-shaped structures. After nearly a century of anatomical description, here we report the first direct measurements of single setal force by using a two-dimensional micro-electromechanical systems force sensor and a wire as a force gauge. Measurements revealed that a seta is ten times more effective at adhesion than predicted from maximal estimates on whole animals. Adhesive force values support the hypothesis that individual seta operate by van der Waals forces. The gecko's peculiar behaviour of toe uncurling and peeling led us to discover two aspects of setal function which increase their effectiveness. A unique macroscopic orientation and preloading of the seta increased attachment force 600-fold above that of frictional measurements of the material. Suitably orientated setae reduced the forces necessary to peel the toe by simply detaching above a critical angle with the substratum.Nature 07/2000; 405(6787):681-5. · 38.60 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: The pretarsus of the female miteVarroa jacobsoni Oudemans (1904) consists of two main parts, a cuticular basal stalk and an extrudable, membranous ambulacral pad, the caruncle. The caruncle, when fully extruded and expanded, becomes a bilobed sucker, and when deflated, the entire caruncle is retracted into the basal stalk. The basal stalk of the pretarsus with the sucker fully retracted into it resembles an inverted cone with its narrow portion attached to the apex of the tarsus. The basal stalk consists of three large plates; two lateral and one median. The proximal end of each lateral plate bears a sclerotized claw-like structure which functions to support the expanded caruncle. The median plate possesses a long, narrow ridge process connecting the basal stalk with the caruncle, and functions to control retraction and protraction of the caruncle. The morphology and function of the basal stalk suggest that the claw-like structure are the ungues; the median plate is the unguifer, and the median ridge is the tendon of the retractor/depressor muscles of the pretarsus. The significance of the pretarsal suckers to the control of the mite is also discussed.Enperimental and Applied Acarology 12/1989; 8(1):105-114. · 1.85 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: The genus Brevipalpus has over 300 species worldwide. The three most important agricultural pest species in the genus, Brevipalpus californicus (Banks), B. obovatus Donnadieu, and B. phoenicis (Geijskes), have been consistently confused and misidentified for more than 50 years. The present study provides a discussion of the characters and character states used to separate these mites. Low-temperature scanning electron microscopy and traditional light microscopy techniques were used to illustrate the subtle morphological differences between these three species. Morphology of the dorsal propodosoma, opisthosoma, and leg chaetotaxy of all three species was examined and compared. The number of dorsal setae, the number of solenidia (omega) on tarsus II, and dorsal cuticular patterns were the most important characters in the identification of Brevipalpus species. B. phoenicis is similar to B. californicus in having two omega on tarsus leg II and different from B. obovatus which has only one omega on tarsus leg II and similar to B. obovatus in having only one pair of F setae (f3), but differing from B. californicus which has two pairs of F setae (f2-3). The dorsal opisthosomal and propodisomal cuticular patterns frequently used to distinguish between these three species are useful but one must be aware that age, feeding, and mounting techniques can affect the appearance of these characters.Enperimental and Applied Acarology 02/2003; 30(1-3):107-33. · 1.85 Impact Factor
Arachnids Secrete a Fluid over Their Adhesive Pads
Anne M. Peattie1*¤a, Jan-Henning Dirks1¤b, Se ´rgio Henriques2¤c, Walter Federle1
1Department of Zoology, University of Cambridge, Cambridge, United Kingdom, 2Department of Biology, University of E´vora, E´vora, Portugal
Background: Many arachnids possess adhesive pads on their feet that help them climb smooth surfaces and capture prey.
Spider and gecko adhesives have converged on a branched, hairy structure, which theoretically allows them to adhere
solely by dry (solid-solid) intermolecular interactions. Indeed, the consensus in the literature is that spiders and their
smooth-padded relatives, the solifugids, adhere without the aid of a secretion.
Methodology and Principal Findings: We investigated the adhesive contact zone of living spiders, solifugids and mites
using interference reflection microscopy, which allows the detection of thin liquid films. Like insects, all the arachnids we
studied left behind hydrophobic fluid footprints on glass (mean refractive index: 1.48–1.50; contact angle: 3.7–11.2u). Fluid
was not always secreted continuously, suggesting that pads can function in both wet and dry modes. We measured the
attachment forces of single adhesive setae from tarantulas (Grammostola rosea) by attaching them to a bending beam with
a known spring constant and filming the resulting deflection. Individual spider setae showed a lower static friction at rest
(26%62.8 SE of the peak friction) than single gecko setae (Thecadactylus rapicauda; 96%61.7 SE). This may be explained by
the fact that spider setae continued to release fluid after isolation from the animal, lubricating the contact zone.
Significance: This finding implies that tarsal secretions occur within all major groups of terrestrial arthropods with adhesive
pads. The presence of liquid in an adhesive contact zone has important consequences for attachment performance,
improving adhesion to rough surfaces and introducing rate-dependent effects. Our results leave geckos and anoles as the
only known representatives of truly dry adhesive pads in nature. Engineers seeking biological inspiration for synthetic
adhesives should consider whether model species with fluid secretions are appropriate to their design goals.
Citation: Peattie AM, Dirks J-H, Henriques S, Federle W (2011) Arachnids Secrete a Fluid over Their Adhesive Pads. PLoS ONE 6(5): e20485. doi:10.1371/
Editor: Corrie S. Moreau, Field Museum of Natural History, United States of America
Received March 7, 2011; Accepted April 26, 2011; Published May 26, 2011
Copyright: ? 2011 Peattie et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This research was supported by fellowships from the Royal Society (to AMP; http://royalsociety.org/Funding/) and the German National Academic
Foundation (to J-HD; http://www.studienstiftung.de/), and grants from the Biotechnology and Biological Sciences Research Council (http://www.bbsrc.ac.uk/;
Grant ID: BB/E004156/1) and the Cambridge Isaac Newton Trust (to WF; http://www.newtontrust.cam.ac.uk/). The funders had no role in study design, data
collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: email@example.com
¤a Current address: Berkeley, California, United States of America
¤b Current address: Department of Mechanical and Manufacturing Engineering, Trinity College Dublin, Dublin, Ireland
¤c Current address: Museu Nacional de Histo ´ria Natural, Lisboa, Portugal
Climbing animals use a vast array of attachment strategies to
scale vertical and inverted surfaces . Adhesive footpads, found
among arthropods, amphibians, reptiles and mammals, allow
strong, repeatable attachment to both smooth and rough, hard
and soft substrates . Geckos and anoles adhere by dry,
intermolecular adhesion [3,4], but other organisms secrete a fluid
over their adhesive pads, including insects [5–9], frogs  and
bats . Pad secretions are thought to enhance attachment forces
by contributing capillary and viscous adhesion, and increasing
overall contact on rough substrates [6,12]. Although fluids are
frequently used in industry to lubricate solid-solid interfaces,
wetted insect adhesive pads can resist significant shear forces
during climbing [5,6,13].
Arachnids have attracted little attention from researchers in
the field of biological adhesion, despite representing a wide
diversity of adhesive morphologies. Some mites have hairy, or
‘‘fibrillar’’, adhesive pads  while others have smooth ones
. Amblypygids, pseudoscorpions and solifugids are all
known to bear smooth pads on their feet and pedipalps [16–
18]. Spider adhesive pads most closely resemble the fibrillar
pads of geckos, with large arrays of branched hairs (‘‘setae’’)
terminating in flattened tips, called ‘‘spatulae’’ (Fig. 1a; [19–
21]). We would predict that the very small size of the spiders’
spatulae (200–300 nm wide; ) allows them to make close
contact with the substrate, attaching without the aid of a fluid as
the gecko does [3,4,23]. Indeed, this has been the consensus
among researchers who have studied spider adhesion [19–
Here we investigate the adhesive feet of selected arachnids
(spiders, mites and solifugids) for the presence of fluid secretions.
We used two independent approaches. First, we imaged the
footpad-substrate interface directly with interference reflection
microscopy (IRM). This technique allows the visualization and
characterization of very thin fluid films through transparent
substrates such as glass . Second, to investigate the functional
effects of a fluid in the contact zone, we compared the adhesive
performance of spider setae with that of gecko setae, which are
known dry adhesives.
PLoS ONE | www.plosone.org1 May 2011 | Volume 6 | Issue 5 | e20485
We found clear evidence of footpad secretions in every species
we studied: the spiders Grammostola rosea (Theraphosidae, Fig. 1a,b),
Salticus scenicus (Salticidae, Fig. 1d), and Cupiennius salei (Ctenidae,
Video S1), the mites Gromphadorholaelaps schaeferi (Fig. 1c) and
Balaustium murorum, and the solifugid Gluvia dorsalis (Fig. 1e,f).
Detailed characterization of the fluid deposited on glass showed
that arachnids left persistent, hydrophobic footprints much like
those of insects [5–9].
We measured the refractive indices and contact angles of
deposited fluid footprints in four of the study species (Table 1). For
all of them, the refractive index of the fluid was close to 1.5, similar
to previous measurements of hydrophobic insect footprints .
Both spider species as well as the mite secreted fluids with contact
angles on glass near 10u, while the solifugid fluid had a significantly
lower contact angle (4.0u60.46 SE). Visible fluid footprints and
trails were composed of extremely small droplets, ranging from less
than one-thousandth of a femtoliter to several femtoliters in
volume. Aggregate droplets imaged for analysis had a volume of at
most 100 femtoliters (Fig. 2a).
The fluid was very stable and remained on the glass for at least
48 hours without any apparent change in properties. The fact
that the secretion remained liquid over multiple days rules out the
possibility that we were observing haemolymph from a damaged
pad. Exposing the coverslip to water droplets failed to dissolve the
fluid, indicating its hydrophobic nature (Video S2).
IRM images of the Gromphadorholaelaps mite further showed large
hydrophilic droplets trapped between the smooth pad and the
glass (Fig. 2b; Video S3), closely resembling the volatile
hydrophilic fluid components seen in ants  and stick insects
As the spider Grammostola clung inverted to a glass cover slip,
fluid first appeared at individual spatulae, bridging the gaps
between them, until a continuous layer of fluid formed underneath
each seta, finally bridging the gaps between setae (Fig. 3; Video
S4). We noted that Grammostola setae continued to release fluid
even after they were isolated from the animal.
Single seta force measurements
Fluid had a significant effect on the attachment performance of
fibrillar adhesives. Individual setae from the tarantula (Grammos-
tola rosea) generated on average a higher shear force (290 mN630
41 mN66.0). Spider setae had more and larger spatulae
(1100657 SE spatulae; approx. area 0.03–0.05 mm2) than geckos
(480623 SE spatulae; approx. area 0.02–0.03 mm2), but even
when normalized by potential contact area (number of spatulae *
spatula area), the tarantula setae generated approximately twice
as much force.
A one-second pause in the shearing motion did not elicit a
significant decrease in force for Thecadactylus setae (Table 2),
whereas shear force immediately and sharply decreased in
Grammostola setae after the dragging motion ended, and continued
to decrease even further if the pause was extended beyond one
second. We observed that dragging an immobilized Cupiennius salei
Figure 1. Arachnids investigated for this study. (A) Tarantula (Grammostola rosea) setae. (B) Fluid trail left behind by a Grammostola tarsus. (C)
Mite (Gromphadorholaelaps schaeferi) clinging upside down to a polystyrene-coated glass coverslip, showing two adhesive pads in contact.
Footprints are indicated by arrowheads. A trail of fluid is also visible, lower left. (D) Jumping spider (Salticus scenicus) fluid trail from one tarsus. (E)
Solifugid (Gluvia dorsalis) tarsus, arolium situated distally, at base of claws. (F) Fluid footprint left by one Gluvia arolium.
Arachnid Adhesive Fluids
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spider foot across glass at different rates caused it to release more
fluid at higher velocities (Video S3).
We report here the discovery of fluid secretions associated with
arachnid adhesive footpads, from diverse arachnid species varying
many orders of magnitude in mass. We can only speculate as to
why arachnid fluid secretions went unnoticed for so long, but some
combination of the following reasons was likely at work:
researchers do not expect to see them; the droplets are very small;
their refractive index is close to that of glass; and fluids are not
continuously secreted. The only previous suggestion in the
literature of footprints associated with arachnid adhesive pads
was in the mite Tetranychus urticae , but no evidence was
Figure 2. Interference reflection microscopy of footprint
secretions. (A) Persistent, hydrophobic tarsal fluid from the tarantula
(Grammostola rosea) after aggregation of footprint for analysis, as
viewed under green light (546 nm). (B) Volatile, hydrophilic droplets
trapped under the foot of a mite (Gromphadorholaelaps schaeferi)
appear lighter than the surrounding pad.
Figure 3. Grammostola setae at various stages of wetting. (A)
Little or no fluid has accumulated underneath setae, and individual
spatulae are still visible. (B) Small fluid droplets, appearing here as dark
areas, bridge the gaps between spatulae. (C) A continuous fluid layer
forms underneath each seta. (D) Fluid accumulates, bridging gaps
between setae. See Video S4 for complete footage.
Table 1. Properties of arachnid footprint fluids from four species (mean 6 SE).
Contact Angle (6) Refractive Index
fresh48 h later fresh 48 h later
Salticus scenicus (n=3)10.3u60.5510.1u60.431.4960.001 1.5060.002
Grammostola rosea (n=3) 10.3u60.43 10.5u60.84 1.5060.0011.5060.001
Gromphadorholaelaps schaeferi (n=3)11.2u60.4210.2u60.39 1.4960.0011.4860.001
Gluvia dorsalis (n=3)4.0u60.46 3.7u60.25 1.5060.0011.5060.001
Arachnid Adhesive Fluids
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presented for the liquid nature of the footprint, its source or
Properties of Arachnid Tarsal Secretions
The properties of arachnid fluid secretions parallel those of
insect footpad secretions, where a persistent hydrophobic fluid is
left behind on glass [7,28–30]. There is no existing histological
evidence for glands or ducts to synthesize and transport fluid to
arachnid adhesive pads [19–21,24,31], but this was initially the
case for insects as well. The positive identification of pores for
secretion delivery in insects has only been achieved using
transmission electron microscopy, in targeted studies [8,29]. The
exact source of those secretions remains unclear, and multiple
glands may be involved [29,32]. Grammostola setae are hollow (pers.
obs.) and we observed that they continued to release fluid even
after being isolated from the animal. In this species, fluid may flow
through the setal stalk, but more work is needed to establish how
the fluid ultimately wets individual spatulae.
Volatile hydrophilic droplets appeared trapped between the
smooth pad of the Gromphadorholaelaps mite and the glass cover slip
substrate (Fig. 2b; Video S2), suggesting that the biphasic foot
secretions seen in ants and stick insects [5,13] are present in at
least one arachnid, and likely others. The stable hydrophobic
components of insect fluid are similar to cuticular lipids [7,28], but
we understand little about the nature of the hydrophilic
Adhesive Performance in Wet vs. Dry Systems
Comparative measurements of adhesive forces from fluid-
secreting spider setae and dry gecko setae demonstrate that there
are consequences for attachment performance when a fluid is
introduced into the footpad-substrate interface. We found that
individual Grammostola (spider) setae generated more shear and
adhesive force per potential contact area than Thecadactylus (gecko)
setae, and that individual gecko setae maintained higher levels of
static friction, whereas spider setae slid relatively easily.
However, this work is preliminary and future investigations
need to take into account several additional important factors.
First, the calculation of attachment force normalized by contact
area assumes that all spatulae make contact, which is not
necessarily true. Second, since the spider setae secreted fluid onto
the experimental substrate and were then dragged repeatedly
across the substrate during each trial, it is likely that those setae
were adhering in the presence of accumulated fluid. Accumulation
of fluid reduces attachment forces in stick insects , and this
effect almost certainly accounts for the low static force seen in
spiders. The ability to generate static friction has important
implications for locomotor control and maneuverability. Without
it, the animal would slip frequently, incurring high energetic costs
to maintain its position or trajectory. Video footage of a
Grammostola foot adhering to glass showed a significant amount
of fluid developing over time as the animal attempts to remain
attached, upside-down, to the cover slip substrate (Fig. 3; Video
S4). It is unlikely that, in natural conditions, spiders routinely
experience fluid accumulation to the degree we observed, and our
shear and adhesive force values for Grammostola may in fact
represent a lower bound on their performance capability.
Finally, our observations suggest that spiders may control fluid
release, either actively or passively, limiting the application of their
secretion to high-velocity situations (Video S1). If spiders are more
likely than geckos to experience low static forces due to
accumulation of fluid, it would be advantageous for them to
avoid secreting fluid during slow movements and instead adhere
via dry intermolecular forces as the gecko does. Similarities in their
morphology suggest that this is possible , and we plan to
investigate this hypothesis with future studies.
Theraphosid Tarsal Silk
Previous investigators observed a silk-like material secreted from
the tarsi of the theraphosid Aphonopelma seemanni , and this
observation was later questioned . We can confirm that
occasional strands of a silk-like substance were exuded from the
feet of Grammostola rosea, another theraphosid (Fig. 4; Video S5).
We did not observe tarsal silk or the associated silk-producing setae
in the jumping spider. Silk-producing setae on Grammostola tarsi
were substantially outnumbered by adhesive setae (ca. 50 adhesive
setae per silk-producing seta), and the silk did not appear to sustain
a tensile load (strands were not always stretched taut, and often
broke). We consider it unlikely that these silk-like secretions
contribute significantly to attachment force. It remains an open
and interesting question how the evolution and development of
these setae might be related to abdominal spinnerets.
Table 2. Direct comparison of single seta force in a gecko
and a spider (mean 6 SE).
Shear force41 mN66.0290 mN630
Adhesive force 12 mN62.0 33 mN65.1
Remaining friction 96%61.726%62.8
Remaining adhesion95%63.0 12%65.0
Spatulae per seta480 spatulae623*1100 spatulae657**
*n=16 setae from the same experiment.
**n=25 setae from an independent sample.
Figure 4. Evidence for tarsal silk in the tarantula Grammostola
rosea. Silk strands are indicated with arrows. Adhesive setae (as; lower
right) far outnumber silk-producing setae (ss) on the tarsus. Dark areas
indicate fluid or silk secretion; bright areas indicate thin layers of air
between the setae and the glass surface. See Video S5 for complete
time-lapse video footage of silk being secreted.
Arachnid Adhesive Fluids
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Arachnids remain a large, and largely understudied, group of
organisms from which we can draw biological and technical
insights into the general principles of adhesion. From an
evolutionary standpoint it is clear that tarsal secretions are
widespread among arthropods; such secretions can have important
ecological consequences, for instance in chemical communication
. Spiders represent the compelling possibility of a hybrid
fibrillar adhesive that takes advantage of both wet and dry
adhesive mechanisms. Whereas previous studies assumed a dry
adhesive system and interpreted the results as such, future
investigators will be able to take a more comprehensive approach
toward arachnid adhesive function.
Materials and Methods
Grammostola rosea spiders were obtained from a pet supplier
(Coast to Coast Exotics, Darlington, UK). Salticus scenicus spiders
and Balaustium murorum mites were collected in Cambridge, UK.
Gromphadorholaelaps schaeferi mites were collected from live Mada-
gascar hissing roaches kept in our laboratory in the Dept. of
Zoology, University of Cambridge, UK. Gluvia dorsalis solifugids
were collected in E´vora, Portugal. Cupiennius salei spiders were
provided by Dr. Friedrich Barth from his laboratory colony in the
Dept. of Neurobiology, University of Vienna, Austria. All spiders
and solifugids were individually housed and fed on a diet of
crickets and water. Mites were re-released after the experiments
Thecadactylus rapicauda geckos were obtained from California
Zoological Supply (Santa Ana, CA, USA) and cared for by the
Office of Laboratory Animal Care at the University of California,
Berkeley, where they were fed on a diet of crickets and water.
All experiments involving arachnids were performed in the
Zoology Dept. at the University of Cambridge, in accordance with
the Animals (Scientific Procedures) Act of 1986. Experiments
involving Thecadactylus were conducted in the Dept. of Integrative
Biology at the University of California Berkeley, in accordance
with Animal Use Protocol R137 (as approved by the Animal Care
and Use Committee).
We viewed three live specimens each from two species of spider
(Theraphosidae: Grammostola rosea, Salticidae: Salticus scenicus), one
mite (Laelapidae: Gromphadorholaelaps schaeferi), and one solifugid
(Daesiidae: Gluvia dorsalis) attaching to the underside of a glass
cover slip with an upright microscope. We filmed movies of the
contact using a high-speed HotShot PCI 1280 B/W digital video
camera (NAC Image Technology, Simi Valley, CA, USA), and a
10-bit B/W QICAM digital camera (QImaging, Surrey, BC,
Canada). In the event that animals were not able or motivated to
remain attached while inverted to the coverslip for the duration of
imaging, we allowed them to climb across an inclined coverslip for
a sustained period of time before imaging the fluid left behind.
Interference reflection microscopy (IRM) allowed us to measure
the refractive index, contact angle and volume of fluid left behind
on the glass. Fluid droplets as deposited by the animals were too
small to conduct IRM measurements, so we aggregated multiple
droplets from each footprint by dragging a fine glass rod with
spherical tip across the surface (Fig. 2a). Images of ten droplets per
individual (three individuals per species) were taken with green
(546 nm) epi-illumination using a Leica DRM HC series
microscope (Leica Microsystems, Wetzlar, Germany) and the 10-
bit B/W QICAM camera. We then used intensity line-plots to
measure the relative contrast of adjacent interference extremes. To
calculate the refractive indices of the deposited droplets, the
contrasts of the interference fringes [(Imax2Imin)/(Imax+Imin)] were
compared with the contrasts from similar-sized droplets of
calibration fluids (water-glycerol mixtures and immersion oil) with
known refractive indices. As the interference patterns of water-
glycerol droplets with steeper gradients are damped by the optical
resolution of the microscope , only droplet sections with
contact angles comparable to those of footprint droplets were used
for analyzing fringe contrasts. Contact angles of footprint droplets
were also measured from the intensity line-plots. A discussion of
the technique as applied to insect adhesion can be found in .
This process was repeated after 48 hours to confirm that the
droplets were still fluid and to discover if their properties or
volume changed over long timescales.
To determine whether the footprint secretions were hydropho-
bic or hydrophilic, we deposited small water droplets (5–30 mm in
diameter) onto the glass cover slip using an ultrasonic humidifier
(Honeywell, BH-860 E) and observed whether the footprint
droplets dissolved into them.
Single seta force measurements
Individual setae from the claw tufts and tarsi of Grammostola
rosea (N=18 setae; 4 individuals) were harvested from living
animals and mounted to insect pins using 5-minute epoxy
(Bondloc UK Ltd, Bewdley, UK). Each pin was clamped into a
pin vice and fixed to a three-dimensional DC motor stage (M-
126PD, Physik Instrumente, Karlsruhe, Germany), which drove
the seta through pre-programmed shearing motions 0.5–2 mm in
amplitude, at set velocities (0.5–4 mm/s). Setae were dragged
across a smooth glass substrate glued to the tip of a 58.9 mm
length of tungsten wire with a 0.1 mm radius. The spring
constant of this bending beam was determined to be 0.46 N/m.
The movement of the beam was captured using a Redlake PCI
1000 B/W high-speed video camera (Redlake, Tallahassee, FL,
USA), and digitized using ProAnalyst Lite (Xcitex Inc, Cam-
bridge, MA, USA) to yield peak shear and adhesive force,
remaining shear and adhesive force after a one-second pause, and
the velocity of the seta.
An analogous method was used to measure single seta forces in
the gecko Thecadactylus rapicauda, using a smooth silicon substrate
fixed to a 46 mm steel wire with radius 0.06 mm and spring
constant of 0.079 N/m, at velocities between 0.4–2.8 mm/s (0.5–
2 mm sliding distance). Details can be found in .
the seta is dragged across glass, it deposits fluid. More fluid appears
to be deposited during fast movements than during slow ones.
Cupiennius salei seta sliding across glass. As
exposure to water vapor. The secretion does not dissolve in
water, indicating its hydrophobic nature.
Grammostola rosea fluid before and after
Hydrophilic droplets underneath the foot of the mite Grompha-
dorholaelaps schaeferi evaporate quickly once they reach the edge of
the pad, much like the volatile hydrophilic component of biphasic
adhesive secretions seen in insects.
Evidence for a biphasic fluid in arachnids.
Arachnid Adhesive Fluids
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wetting. Close up of Grammostola rosea setae as the animal clings
to the underside of the glass. Dark areas are continuous layers of
trapped fluid. Camera initially shows distal setae completely
covered in fluid, then scans proximally along claw tuft to show
setae where less fluid has accumulated and individual spatulae are
Grammostola setae at various stages of
sliding across glass. Adhesive setae leave behind clusters of
minute fluid droplets, while other setae secrete silk.
Time lapse video of a Grammostola rosea foot
The authors would like to acknowledge the Insect Biomechanics Work-
group (Dept. of Zoology, Univ. of Cambridge) for helpful discussion and
logistical support, and Sujo Akoni for assisting in data collection. We are
grateful to Anja Klann for her solifugid expertise and to Friedrich Barth for
contributing Cupiennius salei.
Conceived and designed the experiments: AMP J-HD WF. Performed the
experiments: AMP J-HD WF. Analyzed the data: AMP J-HD. Contributed
reagents/materials/analysis tools: AMP J-HD SH WF. Wrote the paper:
1. Nachtigall W (1974) Biological mechanisms of attachment. Berlin, Germany:
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Arachnid Adhesive Fluids
PLoS ONE | www.plosone.org6 May 2011 | Volume 6 | Issue 5 | e20485