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1874
The Journal of Experimental Biology 214, 1874-1879
© 2011. Published by The Company of Biologists Ltd
doi:10.1242/jeb.055657
RESEARCH ARTICLE
Tarantulas cling to smooth vertical surfaces by secreting silk from their feet
F. Claire Rind
1,2,
*, Chris Luke Birkett
1
, Benjamin-James A. Duncan
1
and Alexander J. Ranken
1
1
School of Biology, Ridley Building, Newcastle University, Newcastle NE1 7RU, UK and
2
Centre for Behaviour and Evolution,
Institute of Neuroscience, The Medical School, Newcastle University, Newcastle NE2 4HH, UK
*Author for correspondence (claire.rind@ncl.ac.uk)
Accepted 24 February 2011
SUMMARY
Like all spiders, tarantulas (family Theraphosidae) synthesize silk in specialized glands and extrude it from spinnerets on their
abdomen. In one species of large tarantula, Aphonopelma seemanni, it has been suggested that silk can also be secreted from
the tarsi but this claim was later refuted. We provide evidence of silk secretion directly from spigots (nozzles) on the tarsi of three
distantly related tarantula species: the Chilean rose, Grammostola rosea; the Indian ornamental, Poecilotheria regalis; and the
Mexican flame knee, Brachypelma auratum, suggesting tarsal silk secretion is widespread among tarantulas. We demonstrate that
multiple strands of silk are produced as a footprint when the spider begins to slip down a smooth vertical surface. The nozzle-like
setae on the tarsi responsible for silk deposition have shanks reinforced by cuticular thickenings, which serve to prevent the
shanks’ internal collapse while still maintaining their flexibility. This is important as the spigots occur on the ventral surface of the
tarsus, projecting beyond the finely divided setae of the dry attachment pads. We also reveal the structure and disposition of
the silk-secreting spigots on the abdominal spinnerets of the three tarantula species and find they are very similar to those from
the earliest known proto-spider spinneret from the Devonian period, giving another indication that silk secretion in tarantulas is
close to the ancestral condition.
Key words: spider, spinneret, adhesion, spigot, tarsus, tarantula.
Miles and colleagues covered the tarantula’s spinnerets with a layer
of paraffin wax no silk was found on the glass slides (Pérez-Miles
et al., 2009).
The tarsus certainly seems an unusual location for silk secretion
in a spider, because silk is usually secreted from spinnerets at the
end of the abdomen (Coddington and Levi, 1991; Volrath and
Knight, 2001; Shultz, 1987; Shear et al., 1989). In fact, a defining
feature of all spiders is the production of silk, synthesized by
specialist glands inside the opisthsoma (abdomen) and then extruded
from the spinnerets on the abdomen. However, tarsal silk secretion
would provide strong support for the hypothesis that spider
spinnerets are derived from modified limbs (Shultz, 1987; Shear et
al., 1989; Damen et al., 2002; Selden et al., 2008), and direct silk
secretion from a setae on a limb without any complex storage organ
may represent the ancestral condition, with silk production from
abdominal spinnerets being the product of much later evolution.
The ability of spiders to secrete silk from their feet would show
that silk production is controlled by developmental modules able
to be expressed in a variety of body parts (Selden et al., 2008).
INTRODUCTION
Usually, when spiders adhere to smooth vertical surfaces they use
pads of finely divided hairs (setae) on the underside of their tarsi
(Kesel et al., 2003; Niedregger and Gorb, 2006; Artz et al., 2003;
Roscoe and Walker, 1991; Gasparetto et al., 2009); this type of dry
adhesion relies on the intra-molecular forces set up between the tips
of the hairs and the substrate (Kesel et al., 2003; Niedregger and
Gorb, 2006; Artz et al., 2003; Kesel et al., 2004; Roscoe and Walker,
1991; Gasparetto et al., 2009). These pads are known as scopula
pads and occur on the tarsi and meta-tarsi of all legs including the
pedipalps in female spiders. While the safety factor (maximum
adhesive force against body mass) can be as high as 160 for a small,
jumping spider (
Evarcha arcuata
) (Niedregger and Gorb, 2006),
the safety factor for the much larger, Costa Rican zebra tarantula,
Aphonopelma seemanni
, is much lower, making a fatal fall more
likely (Niedregger and Gorb, 2006). Controversy surrounds the
report of a novel mechanism demonstrated by the Costa Rican zebra
tarantula to increase the odds of it staying attached to a smooth
wall. Gorb and colleagues found silk deposits when they passively
elevated a tarantula sitting on a flat surface covered with glass
microscope slides towards the vertical and concluded that these
fibres had been secreted from the tarsi of the tarantula (Gorb et al.,
2006). However, Pérez-Miles and colleagues, studying the same
species of tarantula, this time free to roam in a glass slide-covered
arena, concluded that the silk fibres left behind were drawn from
the spinnerets by the legs before being deposited on the substrate
(Pérez-Miles et al., 2009). Two lines of evidence supported their
view. First, they found urticating hairs, which are commonly rubbed
off from the abdomen by the legs as would occur if the legs had
been used to pull the silk from the spinnerets. Second, when Pérez-
MATERIALS AND METHODS
Experiments were performed on captive-bred tarantulas obtained
from local pet shops (Newcastle upon Tyne, UK). To test for
physical evidence of tarsal silk secretion, an adult Chilean rose
tarantula,
Grammostola rosea
(Walckenaer 1837), was placed in a
glass tank that had first been cleaned with detergent and air dried
and its internal base layered with 7 pairs of Menzel-Glaser
®
pre-
cleaned 76
26mm microscope slides. Cleaning was important to
prevent dust contamination of the slides. The spider was carefully
positioned on the slides at one end of the tank and the positions of
THE JOURNAL OF EXPERIMENTAL BIOLOGY
Silk secretion from Tarantula feet
1875
all appendages, including the spinnerets, were recorded. The spider
was left to settle and all movements were recorded using a Canon
XM2 DV camera with a
10 Opteka HD2 macro lens. Slides
coming into contact with the spinnerets were excluded from the
results. The tank was then tipped so the spider was facing vertically
upwards. The camera was repositioned to view the base of the tank
and record movement of the legs and pedipalps. In order to induce
the legs to slip on the slides, the tank was gently shaken. A tarsal
slip was defined as a smooth movement of the tarsus down the
microscope slide under the influence of gravity. The tank was then
returned to its original orientation and the spider was removed, taking
care not to allow the abdominal spinnerets to come into contact
with any slides. The glass slides were then removed and inspected
under a binocular microscope and photographed. These experiments
were repeated successfully eight times with two different female
Chilean rose tarantulas. Trials in which the spiders moved and their
spinnerets came into contact with more than two slides were not
included in the study.
To study their anatomy, both freshly shed cuticle and exoskeletons
of recently deceased Chilean rose
G. rosea
, Indian ornamental
Poecilotheria regalis
(Pocock 1899) and Mexican flame knee
Brachypelma auratum
, Schmidt 1992, tarantulas were obtained from
captive-bred stock supplied by local pet shops. Some species of
tarantula such as the Mexican flame knee tarantula,
B. auratum
, are
listed under the Convention on International Trade in Endangered
Species (CITES; www.cites.org) index of endangered species and
cannot be taken from the wild. For examination under the scanning
electron microscope (SEM), material was fixed and dehydrated
before being mounted on a stub and coated with gold. Fine structural
details of tarsal setae were still present in moulted exoskeletons,
viewed after being coated with gold but without prior fixation or
dehydration. Being able to use moults increased the amount of
material available to us, as moulting occurs throughout the life of
a tarantula, which in female tarantulas can be up to 20years
(Coddington and Levi, 1991). Specimens were then viewed using
a Cambridge Stereoscan 240 SEM and digital images taken. Moults
were also viewed and photographed using a Wild M5a binocular
microscope fitted with a digital camera.
Fig.

1. Mexican flame knee tarantula (Brachypelma auratum) climbing a
vertical wall. (A)

Viewed from above, the tarsi of the walking legs
(numbered 1–4) and pedipalps are in contact with the Perspex wall. The
setae around the claws are extended (black double-headed arrow).
Black/white double-headed arrow, the scopula tufts. Insets show the tarsus
and pedipalp. (B)

Viewed from outside the tank, lighter shaded areas on the
tarsi and pedipalp indicate setae in contact with the wall (inset, white
double-headed arrow). Its tarsal claws are extended (inset, black arrow).
Ch, chelicerae; MT, meta-tarsal leg segment; P, pedipalp; T, tarsal leg
segment. Scale bars: A and B, 10

mm; insets, 1

mm.
RESULTS
Adults of all three tarantula species were recorded climbing on
smooth vertical surfaces. While climbing, the spiders’ scopula pads
on the tarsi of all eight walking legs were seen to be in contact with
the surface (Fig.1 and Fig. 2A). The tarsal claws (Fig.1 and inset
Fig.2B) and the scopula tufts around the claws (Fig.1B, Fig.2B,C
insets) were extended. To test whether silk was secreted by the tarsi
of these spiders, we placed a single female Chilean rose tarantula
in the bottom of a clean tank whose base was lined with microscope
slides (see Materials and methods), raised the tank to the vertical
position and shook it slightly. The spider remained attached to the
vertical wall (Fig.2A). We found tarsal silk secretions on the slides
in all eight experiments (Fig.2B,C). We always found traces of silk
where we induced a tarsal slip. In one example, we observed the
tarantula relocating the tarsi to a more stable position, as evidenced
by the new position being maintained while the spider was vertical.
Both the first and second right tarsi of the spider slipped and the
spider relocated them and regained grip. We found silk traces at all
sites where the tarsi of these legs came into contact with the slides
and slipped. Multiple silk threads, some up to 2.5mm in length,
were found at the location of the slip on later inspection of the slides
under the microscope, with the threads attaching individually to the
slide at the position where this sequence of events commenced
(Fig.2C). We concluded that the silk threads were attached to the
initial site and then drawn out throughout the period in which the
spider was slipping. We could make out at least 27 individual silk
strands over a 1mm width. For comparison, the total scopula pad
(area of densely packed attachment hairs) of the tarsus is around
3mm across in a walking tarantula, although the area of the pad in
direct contact with the wall is about 1mm across (Fig.1B and Fig.
2C inset), a similar width to the spread of silk deposits (Fig.2B,C).
To identify the structures on the ventral surface of the tarsi that
secreted the silk, we examined moulted skins from the female
Chilean rose tarantulas used above (Fig.2D). On the leg we saw
single strands of silk emerging from the tips of several setae
protruding above the scopula pads. The setae were in a regular array,
intermingled with the densely packed setae of the scopula pads. One
seta is shown with a single strand of silk emerging from its tip in
Fig.2D (lower inset).
A clearer picture of these structures was obtained using a SEM,
which revealed that silk-secreting setae were present on the ventral
THE JOURNAL OF EXPERIMENTAL BIOLOGY
1876
C. Rind and others
Fig.

2. Chilean rose tarantula
(Grammostola rosea) secretes silk
from its feet. (A)

A Chilean rose
tarantula on a vertical surface. (B)

Silk
secretions on a glass slide following a
tarsal slip. Inset, scopulae (arrow) and
claws (double-headed arrow). (C)

The
silk deposition area (strands 1–27)
matches the tarsal footprint. Inset,
tarsal area in contact with glass
(lighter area, white arrow). Claw,
black arrow. (D) Photomontage of
moulted pedipalp. Single silk strands
emerge from the tips of setae
protruding above the tarsal scopula
pads (white arrowheads). Lower inset,
a single silk strand (arrowhead).
Upper inset, tarsal tip. Scale bars:
1

mm, except A, 100

mm; C, 0.1

mm;
D lower inset, 100


m.
tarsi in moults and fresh tarsi from the three tarantula species (Figs3
and 4). Fig.3A is a SEM image of a Chilean rose tarsus showing
the claw and tarsal scopula pads. The brush-like setae of the scopula
pad are punctuated by unbranched setae terminating in tapering tips
(Fig.3B). These are the structures identified in the light microscope
as secreting strands of silk (Fig.2D). We refer to these structures
as spigots by analogy to silk-secreting setae on the spiders’
spinnerets (Coddington and Levi, 1991). The ribbed spigots,
550–750m in length, were dispersed through the scopula pad,
separated from each other by 50–150m. Fig.3B and inset are from
an animal that died while shedding its cuticle and the claws are
visible inside the partially shed cuticle. At the base of each spigot
is a 40m wide socket embedded in the cuticle; the sockets are
evenly distributed over the tarsus (Fig.3D).
The tarsi of the arboreal Indian ornamental tarantula have an
impressive multi-layered array of spigots, the tallest standing
100–200m above the forest of bent-tipped setae of the scopula
pad (Fig.4A). These tall spigots have regular angled bands of
thickening along their projecting parts and terminate in a small,
0.1–0.6m opening (Fig.4A inset). The thickened bands on the two
sides of the setae are offset relative to one another and only come
into alignment at its tip (Fig.4A). Shorter spigots stand less than
20m above the scopula pad (Fig.4C,D) and also have a prominent
ribbed structure at their distal ends and terminate in a pore
(Fig.4E,F). A broken spigot base reveals a reinforced central, 1.3m
channel. A third layer of ribbed blunt-tipped spigots terminates level
with the scopula pad (Fig.4G).
In contrast to the tarsi, the spinnerets of the Indian ornamental
and a Chilean rose tarantula were found to have silk-secreting spigots
in rows on the ventral midline intermingled with other setae
(Fig.5A–D). The spigots have prominent, 50–70m unornamented
rounded bases (Fig.5A,C). The short, 250m cuticular shank of the
spigot emerges from the base of the spigot and is ornamented with
small cuticular scales. The shank terminates in a small, 1–2m pore,
which can be open or occluded with globules of silk (Fig.5D,E).
DISCUSSION
One of the main objections raised to the secretion of silk from the
tarsus is a functional one: the silk-secreting nozzles would have to
be below the surface of the scopula pad for the spider to adhere to
the substrate and then how could the silk be attached to the substrate
(Pérez-Miles et al.
,
2009)? In all of the six tarantulas of the three
species we examined in this study, we found spigots projecting
beyond the finely divided attachment setae making up the bulk of
the scopula pads on the underside of the tarsi. We also observed
silk secretions coming from the spigots and found tarsal silk
secreted by Chilean rose tarantulas as they struggled to hold on to
a vertical surface. The three species of tarantula we studied were
large, with adult body masses of 15–50g and came from diverse
habitats: the Indian ornamental was arboreal, the Chilean rose and
the Mexican flame knee were ground-dwellers. We therefore
conclude that tarsal silk secretion is a feature shared by large
tarantulas and is used in addition to the dry attachment system to
prevent falls when climbing on smooth surfaces.
Why do tarantulas use silk when a dry attachment system gives
a safety factor (adhesive force/body mass) of 160 in the 15mg
jumping spider
E. arcuata
(Kesel et al., 2003)? The safety factor is
dependent on the mass of the spider; in the Costa Rican zebra
tarantula (17.9g) and the hunting spider
Cupiennius salei
(3.3g) the
adhesive forces generated by the tarsal and meta-tarsal pads on a
THE JOURNAL OF EXPERIMENTAL BIOLOGY
Silk secretion from Tarantula feet
1877
Fig.

3. Scanning electron microscope images (SEMs) showing spigot-like setae on the tarsi of both Chilean Rose (G. rosea) and Mexican flame knee (B.
auratum) tarantulas. (A)

Scopula pads on the ventral side of the Chilean rose tarsus (left leg number 2). (B)

The bent brushes of the dry attachment setae,
interspersed with taller, un-branched spigot-like setae (arrow). (C)

View into a partially moulted Chilean rose tarsus at the level indicated in by the arrow in A.
Lower inset, regular arrangement of spigot-like setae (arrows). Upper inset, spigot socket within cuticle. (D)

Internal view of spigot insertions (arrows) onto
the cuticle of an immature Mexican flame knee tarsal moult. Scale bars: A and C, 1

mm; B and D, 100


m; insets, 20


m.
smooth glass surface were equal in magnitude but the tarantula had
a 5 times greater body mass, reducing the safety factor for the
tarantula to around 1 (Niedregger and Gorb, 2006). In practice, we
found that the dry attachment forces of the scopula pads do not
always suffice in heavier tarantulas and silk is secreted when the
tarantula slips on a vertical smooth surface.
The tarantulas we examined had several lengths of tarsal spigots
that projected beyond the finely divided setae of the scopula pads,
and the spigot bases were inset into the tarsal cuticle so that only
the slim spigot shank interrupted the setae of the scopula pads. The
shank itself is adapted so it will not be crushed when the spider
walks or collapse during silk extrusion and it can bear the tension
of the silken tether anchoring the tarantula tarsus to the glass. The
linked, angled annular thickenings, offset across the width of the
shank are similar to the reinforcements seen in pressure vessels
(Karam, 2005; Wigglesworth, 1965; Rach, 1990) and suggest the
spigot’s role as a silk delivery and anchoring device. Can we explain
why no tarsal silk deposits were found by Pérez-Miles and colleagues
(
Pérez-Miles et al.
,
2009) when they blocked the spinnerets of the
zebra tarantula and allowed it to roam in a shallow tank? First, we
calculate that the tank may have had a wall height that allowed the
animal to partially support its weight on the ground so there was
no need for silk secretion. Second, no attempts were reported to
dislodge the spider from the vertical surface and make it slip (Pérez-
Miles et al.
,
2009). We think these factors could explain why no
evidence of tarsal silk secretion was found and suggest the
importance of a slip in causing silk to be secreted from the tarsi of
large tarantulas.
Our evidence of secreted tarsal silk suggests a pathway for the
evolution of spider silks. The earliest known silk-secreting animal
was the spider
Attercopus
from the Devonian period (Selden et al.,
2008), which secreted sheets of silk not from spinnerets but from
spigots arranged along the edges of an abdominal plate on body
segments 4 and 5. The spinnerets themselves originated later from
biramous appendages on the same segments (Damen et al., 2002;
Selden et al., 2008). In adult spiders the silk-secreting spigots on
the spinnerets do not resemble generalized mechanosensory setae
but have large bases important for the transition of the stored liquid
silk protein to a long silk fibre. Liquid silks are stored in glands in
the abdomen (Vollrath and Knight, 2001). We found that tarsal silk-
secreting spigots more closely resemble mechanosensory setae
(Foelix, 1970); they lack external silk-storage organs (Vollrath and
Knight, 2001) and could represent a missing link to their ancestral
condition. Interestingly, the first functional spigot on the
Antrodiaetus unicolor
spiderling’s spinneret also lacks a bulbous
base (Bond, 1994) but may still be connected to external silk-storage
organs. Alternatively, because the tarantula family Theraposidae is
a diverse group and includes many of the largest known spiders
THE JOURNAL OF EXPERIMENTAL BIOLOGY
1878
C. Rind and others
Fig.

4. SEMs showing spigot-like open-pored setae from the tarsi of Indian ornamental tarantulas (Poecilotheria regalis). The spigots project beyond the
numerous bent-tipped brushes of the dry attachment system. (A)

Long spigots on the ventral tarsal claw scopula pad. Inset, detail of the spigot tip with 1


m
diameter opening. (B)

Shorter spigots interspersed with setae of the dry attachment system. (C,D)

Multi-layered arrangement of spigots. A longer spigot is
indicated by the arrow. (E)

Tip of a spigot with a pore. (F)

A central channel is visible in the spigot base. (G)

Blunt-tipped spigot with a small pore (arrow).
Scale bars: A–D, 50


m; insets, 20


m.
(Coddington and Levi, 1991), tarsal silk may have evolved
independently several times within the group to enable tarantula
species to climb without slipping. The presence of tarsal silk
secretion in the three phylogenetically disparate (Pérez-Miles et al.
,
1996) tarantula genera examined, plus the Costa Rican zebra
tarantula, argues against such a piecemeal process. Both the different
suggestions for the origin of tarsal silk secretion are consistent with
the homology of legs and spinnerets as arthropod appendages
(Shultz, 1987; Shear et al., 1989; Damen et al., 2002; Selden et al.,
2008). As not all tarantulas are large, some being less than 1mm
Fig.

5. Silk-secreting spigots on
tarantula spinnerets (SEM images).
(A)

Chilean rose spigots with their
enlarged bases are in a mid-ventral
position on the spinneret. (B,C)

The
shank of the Chilean rose spigot
has scale-like cuticular
ornamentation and terminates in a
1.6


m pore. (D)

Distal segment of
the Indian ornamental spinneret
showing the arrangement of the
spigots along its ventral midline.
Inset, detail of an individual spigot.
(E)

Indian ornamental spigot shank
with scale-like ornamentation and a
silk droplet at its tip. Scale-bars: A
and D inset, 100


m; B, C and E,
20


m; D, 1

mm.
THE JOURNAL OF EXPERIMENTAL BIOLOGY
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