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Water Repellent Properties of Spiders: Topographical Variations and Functional Correlates

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Biological surfaces, depending upon their physical structure and chemical composition, can fall anywhere on a spectrum of wettability that runs from strongly water repellent to forming strong adhesive bonds with water. When the surfaces in question are those at the interface between the organism and its environment, these wettability characteristics have profound consequences for function. For example, in semi-aquatic plants, water repellent surfaces near the stomata are important for preserving the ability to exchange gasses with air (Schönherr and Ziegler 1975), and the wettable ventral surfaces of gyrinid beetles provide the intimate contact with water that is necessary for their style of aquatic locomotion (Fish and Nicastro 2003; Fish 1999).
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Functional Surfaces in Biology, Edited by S. Gorb
Chapter
WATER REPELLENT PROPERTIES OF SPIDERS:
TOPOGRAPHICAL VARIATIONS AND FUNCTIONAL
CORRELATES
Gail E. Stratton
Robert B. Suter
Gail E. Stratton: Department of Biology, University of Mississippi,
University, Mississippi 38677 USA. e-mail: byges@olemiss.edu
Robert B. Suter: Department of Biology, Vassar College, 124 Raymond
Avenue, Poughkeepsie, New York 12604 USA. e-mail: suter@vassar.edu
Corresponding author: Gail E. Stratton
Department of Biology
University of Mississippi,
University, Mississippi 38677 USA.
e-mail: byges@olemiss.edu
phone: 662-915-5786
FAX: 662-915-5144
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1. WATER REPELLENT SURFACES
Biological surfaces, depending upon their physical structure and
chemical composition, can fall anywhere on a spectrum of wettability that runs
from strongly water repellent to forming strong adhesive bonds with water.
When the surfaces in question are those at the interface between the organism
and its environment, these wettability characteristics have profound
consequences for function. For example, in semi-aquatic plants, water
repellent surfaces near the stomata are important for preserving the ability to
exchange gasses with air (Schönherr & Ziegler 1975), and the wettable ventral
surfaces of gyrinid beetles provide the intimate contact with water that is
necessary for their style of aquatic locomotion (Fish & Nicastro 2003; Fish
1999). Likewise, the water repellency of some plant surfaces drastically
reduces the adhesion of particles of dust, and particles allowing these surfaces
to be effectively self cleaning (Barthlott & Neinhuis 1997).
1.1 Importance of shedding water from surfaces
The ability to shed water is important for arthropods at a microscopic
level, where microorganisms reside, as well as at the scale of the whole animal.
In the first instance, an organism’s interactions with small particles such as
viruses, fungal spores, and bacteria are strongly influenced by the interplay
between among the particles, the arthropod’s surface, and water — when liquid
water has a higher affinity for the particles than for the surfaces on which the
particles have lodged, contacts with water (e.g., raindrops or dew) can
efficiently clean the arthropod’s exposed surfaces (much as is seen in plants, as
in the so called “Lotus-effect”, see Barthlott & Neinhuis 1997; Neinhuis &
Barthlott 1997). At the macroscopic level, the ability to repel water facilitates
air breathing by submerged animals such as water scorpions (Hemiptera,
Merritt & Cummings 1984) or aquatic dipteran larvae and also makes possible
the style of water surface locomotion employed by water striders (Hemiptera,
Anderson 1976) and fishing spiders (Pisauridae, Suter et al 1997; Suter &
Wildman 1999).
Many spiders have some ability to locomote on the water surface, and
those that move there most effectively do so while staying entirely dry, relying
for propulsion on the interaction of their legs with the depressions their legs
make in the surface (Suter et al. 1997; Bush & Hu 2006). In some cases, as in
the semi-aquatic Dolomedes (Family Pisauridae), the movement is quick and
effective and the animals are clearly well adapted for their lives close to water.
In a comparative study of locomotion on water by a wide variety of spiders
(249 species, 42 families), Stratton et al. (2004) found that water surface
locomotion was mostly limited to the superfamily Lycosoidea, although some
salticids and tetragnathids were also quite adept at this task:
(http://faculty.vassar.edu/suter/1websites/comparisons). A prerequisite for this
kind of locomotion is that the animal’s surface (cuticle or cuticular hairs) must
remain dry, thereby allowing the animal to be supported by but not trapped in
the surface tension. In the same study, Stratton et al. (2004), noted wide
variability between species: in some species, the legs would get wet while the
body remained dry, in other cases, the legs stayed dry but the ventral surface of
the abdomen was wet and adhered to the water surface, and so forth. An
interest in understanding some of the inter-specific variability in water
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repellency, as well the functional variation in the water repellency of different
body regions, motivated the present study.
Although spiders in only a few families routinely need to move on the
water’s surface (Suter et al. 2003), spiders in many families may be exposed to
flooding, wetting by large raindrops or, in the case of very small spiders,
immobilization from the accumulation of dew drops (Decleer 2003; Rovner
1986). In these cases, some degree of water repellency would enhance
survivorship and fitness.
Many aquatic or semi-aquatic insects have regions of their bodies that
are conspicuously more water repellent than other parts —for example the
antennae of some insects (Hix et al. 2003), the tarsitasrsi of some gerrids
(Wichard et al. 2002), and hairs around the entrances to the respiratory
structures of water scorpions and larval and pupal mosquitoes (Wichardt et al.
2002). In aquatic insects, these water repellent regions surrounding openings
into the tracheal systems give the animals access to atmospheric oxygen while
the rest of the body remains submerged. In addition, many insects and some
arachnids can make a plastron, a non-collapsible film of air that is held in place
by densely-packed nonwettable hairs that surround some or all of the body and
allow the animal to extract oxygen from the surrounding water (e.g. Thorpe &
CrsCrispip 1947; Crisp & Thorpe 1948; Thorpe 1950; Hebets and Chapman
2000, and Braun 1931 in Foelix 1996). Without the hairs, an air bubble would
shrink and eventually collapse either from changing water pressure or from
losing volume as nitrogen diffuses from the bubble into the water. Insects that
routinely move on water, such as water striders, have many hairs on their tarsi
and legs; and increasingly, there is evidence that the microstructure of the hairs
is important in maintaining water repellency (Andersen 1977; Cheng 1973 and
Gao & Jioang 2004). In a comparative study of spiders, Rovner (1986) found
support for the assertion that hairiness, with the implicit ability to form a
plastron when submerged, may allow various spiders to survive being flooded
for a period of time
We were interested in understanding the functional implications of
differences in the resistance to wetting across individual spiders’ topographies
for representative species. We approached this problem via the close
inspection of the ventral surfaces of spiders in five families. We hypothesized
(1) that the openings to the book lungs and the trachea would be particularly
water repellent in spiders likely to become submerged, because of the role of
these openings in supporting gas exchange; (2) that in the same spiders, much
of the soma would be covered with hydrophobic hairs at a high enough density
to support the establishment of a plastron; and (3) that in spiders in general,
areas such as leg joints and genital openings would be surrounded by water
repellent hairs to limit the intrusion of small particles, especially pathogenic
microorganisms.
2. METHODS
2.1 Spiders
We examined in detail females of 5 species representing 5 families.
Three come from families that are generally associated with water, living either
associated with the surface of water [e.g., the fishing spider, Dolomedes triton
(Walckenaer 1837), in the family Pisauridae], or making webs above or near
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water [e.g., Tetragnatha elongata Walckenaer 1842 in the family
Tetragnathidae and Larinioides cornutus (Clerck 1757) in the family
Araneidae]. We chose the other two examples because they represent families
not usually associated with water and differ from each other both in habit and
in conspicuous surface characteristics [Platycryptus undatus (De Geer 1778),
in the family Salticidae, is cursorial and covered by hairs; Xysticus ferox (Hentz
1845), in the family Thomisidae, is a sit and wait predator found in leaf litter,
that appears nearly hairless.] All specimens except Dolomedes were mature
females when tested; the Dolomedes was immature and judged by its size to be
1-2 molts from maturity. Spiders were collected from Lafayette and Marshall
County in Mississippi (USA) and are preserved in the personal collection of
GES. Except for the Xysticus, which was held in the lab for several weeks,
specimens were used within a day of their capture.
2.2 Determining water repellency
Two attributes contribute to the water repellency of a particular area of
an arthropod’s body: hair density and the molecule-level physical interaction
between the hair or cuticle surface and water (Suter et al. 2004). In the present
study we modified the procedures in Suter et al. (2004) in order to investigate
patterns of water repellency across the topography of each of the 5 spiders
examined.
We anaesthetized each animal using CO2 and cooling, and then
attached it with epoxy (Liquid Nails Perfect Glue 3, Epoxy Adhesive), ventral
surface upward, to the center of a Petri dish. We then treated each specimen
with a three-step process that started with examining the ventral surface of the
animal microscopically while it was dry. We took a series of images of each
specimen (Olympus SZX12 Dissecting Microscope; digital images captured
with a Nikon D100 attached to the microscope). Images for each included the
whole ventral aspect followed by much closer views of the stigma or spiracle
(the opening to the spider’s tracheal system, located anterior to the spinnerets),
the opening of the book lungs (located on the anterior region of the venter), the
epigynal area, the coxal/trochanter/femur joint, and the mid-ventral region
(Figs. 1-5). The second step involved misting the regions of interest using a
stream of microscopic droplets of distilled water, created by an ultrasonic
humidifier (ReliON Humidifier ®). The mist was conveyed through a 0.5 m
tube and directed at the target through a glass nozzle. The appearance of the
mist was similar to steam, but was at room temperature (21-23ºC). Each region
of each specimen was again digitally photographed. The third and final step
was to completely submerge the specimen in distilled water and observe which
portions of the animal’s ventral surface were wetted and which remained dry.
Digital images were imported into Photoshop (Photoshop CS or
Photoshop Elements) and examined on a Macintosh G4 computer. Hair
density at each of the above regions was determined by counting hairs in a 0.1
mm X 0.2 mm rectangle oriented (where possible) with the hairs lying at right
angles to the long axis of the rectangle. All of the hairs within that box were
counted, and the average diameter of the hairs was estimated. To calculate
percent cover (C) of the cuticle by the hairs we assumed that each hair
traversed the measuring grid (that is, its’it’s length was 0.1 mm). With these
assumptions we calculated percent cover as
C(%) = 100 • (Nd(mm) • l(mm)) / a(mm2)
5
where N is the number of hairs, d is the average diameter of the hairs, l is the
average length of the hairs, stipulated as the width of the sample quadrat, and a
is the area of the sampled quadrat. Because of the assumption that each hair
completely traversed the width of the grid, this method overestimated the
coverage of the cuticle by hairs and it was possible to calculate more than
100% coverage. In those cases, we set the coverage to 100% for our analyses.
Suter et al. (2004) were able to infer the molecule-level physical
interaction between the hair or cuticle surface and water by measuring the
contact angles of water droplets on the legs of spiders (see also McHale et al.
2004 and Barthlott & Neinhuis 1997). This technique was not possible in the
current study of body surfaces because we could not get a microscopic lateral
view of each misted body region. Instead, we captured images and made
qualitative observations on droplet formation. Functional assessment of
resistance to wetting came from observations of surface wetting when the
whole animal was submerged.
3. WATER REPELLENT SURFACES IN SPIDERS
The representative species differed widely in how much of their
surfaces were covered by hairs (Table 1). When different regions were
compared within a species, the percent cover of hairs was highest for the
spiracular region in 3 of the 5 species (Dolomedes, Larinioides and
Platycryptus), and was as high or nearly as high for at least a portion of the
book lung region in two species (Larinioides and Platycryptus). As expected
based on superficial examination, Xysticus had very low percent cover of hairs
for all the regions examined.
Table 1. Summary of percent cover of hairs for different anatomical regions for 5 species of
spider. Percent cover was calculated by taking the number of hairs counted in an .020 m2 area x
estimated diameter of a hair (assuming the length of an individual hair could cover the areas
measured.
Dolomedes
Tetragnatha
Platycryptus
Xysticus
1.00
0.48
1.00
0.17
1.00
0.67
0.25
0.19
0.38
0.35
0.09
0.00
0.55
0.35
1.00
0.00
na
0.61
0.50
0.00
na
0.61
0.53
0.00
na
0.44
1.00
0.22
0.86
0.08
0.42
0.04
0.25
0.00
0.50
0.36
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3.1. Dolomedes
Although we expected that the semi-aquatic Dolomedes would
exemplify the extreme of water repellent properties, we were still impressed
with the extent of water repellency as seen both when the spider’s surfaces
were misted and when the spider was submerged. When Dolomedes was
misted, numerous fine, spherical droplets were formed on hairs all over the
ventral surface (Fig. 1 C’,D’). Clearly, the large and small hairs that covered
much of the surface of the animal (Table 1) had hydrophobic surfaces (Fig. 1).
The areas near the spiracle (Fig. 1A), the book lung (Fig. 1C,C’) and the coxae
(Fig. 1D, D’& E) were all well protected by hairs that repelled water. A large
bubble of air formed when the spider was submerged, covering the abdomen,
sternum and legs with only the fangs and a few long hairs or spines outside of
the bubble (compare Fig. 1A with 1B). The extent of the bubble surrounding
Dolomedes, together with the fact that it enveloped both the spiracle and the
openings to the book lungs, suggests that this animal is equipped with a
functional plastron.
A closer examination of the spider’s coat of hair suggested that
Dolomedes has both an overcoat of larger hairs and an undercoat of very fine
hairs. When the spider was submerged, the larger hairs held the plastron well
away from the body to the extent that it was possible to see droplets on the
smaller hairs inside the bubble (visible both in Fig. 1B & 1E). In addition, the
arrangement of the hairs in some regions appeared to accentuate the ability to
repel water. For example, the hairs near the coxae form a basket-like structure
over the membrane connecting the trochanter and the coxae in a way that
appeared to help to keep the cuticle of the joint membrane itself completely dry
when the animal is submerged (compare Fig. 1D,D’& E).
3.2. Tetragnatha
Members of the genus Tetragnatha, family Tetragnathidae, are well
known for making their webs near water (Foelix 1996; Ubick et al. 2006), and
spiders in the genus Tetragnatha can move easily and quickly on the surface of
water (Suter et al. 2003, see also Ehlers 1939 in Foelix 1996). Nevertheless,
the water repellency of T. elongata’s surfaces varied extensively across its
topography. For each region of the body examined, Tetragnatha had a smaller
percent hair coverage than Dolomedes (Table 1) and had two areas that had no
hairs (the trochanter and the mid region of the book lung). When Tetragnatha
was misted, the lower density of hairs as compared to Dolomedes was evident
(Fig. 2C& D). However, the epigynal region and spiracular region were
surrounded by hairs that bore spherical droplets under our misted condition.
When the animal was submerged (Fig. 2B), an air bubble formed around its
abdomen including the book lung and spiracle which, as with the Dolomedes,
could possibly function as a plastron. The opening and closing of the entrance
to the book lung (Fig. 2F&F’) while the spider was submerged probably
constitutes breathing and supports our contention that the spider uses the
bubble as a true plastron. The spiracle (Fig. 2D) and the epigynal area (Fig. 2C)
were well protected with hairs that collectively provided these areas with water
repellency. As indicated above, the coxa and trochanter of the legs had very
few hairs and submersion showed that these areas were entirely wetted.
7
3.3. Larinoides
Larinioides is a genus of orb-weaving spider that also often builds
webs near water. Its legs and the lateral sides of its venter were quite hairy
(Fig. 3; Table 1), but except for right near the epigynum (Fig. 3C), the mid
region of its venter (including the spiracle; Fig. 3D) had few hairs and they
were very short. The epigynal region had a higher coverage of hairs on its
posterior and lateral edges (Table 1, Fig. 3C). The coxae had hairs that, similar
to Tetragnatha, appeared to curve over the coxal joint (Fig. 3F). When misted,
the book lung covers were wettable (note flattened droplets, Fig. 3E). When
the epigynal area was misted, spherical droplets formed on the hairs suggesting
they had hydrophobic surfaces. When Larinioides was submerged, parts of the
ventral surface were covered with air (Fig. 3B), but the mid-region of the
venter, including the spiracle, appeared quite wet and outside of the bubble.
The bubble extended to the base of the legs (Fig. 3B).
3.4. Platycryptus
The salticid representative, Platycryptus undatus, (Fig. 4) appeared to
be as hairy as the pisaurid, Dolomedes, a trait reflected in relatively high
measures of percent cover of hairs for most of the regions examined (Table 1).
When misted, fine droplets formed on the hairs all over the body, and it
appeared that the epigynal area (Fig. 4C’), the book lungs (Fig. 4C), and the
spiracle (Fig. 4D), as well as the coxae (Fig. 4B&B’), were well protected from
wetting. Curiously, the midline of the venter had a relatively small percent
coverage of hairs. When the animal was submerged, a thin layer of air was
maintained, keeping the book lung opening dry. However, the air layer
surrounding much of the abdomen disintegrated quickly (1-5 minutes), leaving
only a small area over which gas exchange could occur if the layer of air were
to function as a plastron.
3.5. Xysticus
Finally, the extreme of lack of water repellency was illustrated by the
crab spider Xysticus (Fig. 5). With the exception of the trochanter, Xysticus
showed the smallest % coverage of hairs for any of the species examined
(Table 1). When misted, fine bubbles formed around the spinnerets and
spiracular opening (Fig. 5), but when the Xysticus was submerged, it was
almost completely wetted (Fig. 5D).
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Fig. 1. Ventral surfaces of an immature Dolomedes triton (Pisauridae). In air (A), nearly all surfaces appear to
bear hairs, although the fact that the cuticle is clearly visible on the leg segments, mouth parts, sternum, and
anterior abdomen indicates that hair density per se is not particularly high in those regions. Under distilled water
(B), the entire ventral surface is seen to be swathed in a layer of air, indicating the presence of hairs at a high
enough density, and with sufficiently hydrophobic surfaces to support a plastron. The glistening, bespeckled
appearance of the sternum and legs in this image is due to refraction from the numerous spherical water droplets,
deposited during misting, that are on hairs between the cuticle and the water enveloping the spider. The presence
of spherical water droplets on the area surrounding a book lung opening (C’) and on the basal segments of the legs
(D’) emphasizes the hydrophobicity of the surfaces of the hairs. Some structures, in this case the coxa-trochanter-
femur joint of leg IV (E), are protected from water intrusion by a combination of the hydrophobicity of the
surrounding hairs and their orientation (see also Fig. 6).
9
Fig. 2. Ventral surfaces of an adult female Tetragnatha elongata (Tetragnathidae). In air (A), the surfaces appear
hairless or nearly so, but under distilled water (B) it is clear that at least some of the spider’s surface is covered
with a film of air, evidence that those parts of the surface effectively repel water. When the otherwise dry spider
is bathed in a fine mist of water, the areas indicated by rectangles in (A) give evidence of the presence of small
hydrophobic hairs that cover the epigynum (C) and the region around the spiracle just anterior to the spinnerets
(D). In these two images, it is the appearance of spherical droplets that suggests that the hair surfaces are
hydrophobic. On the submerged specimen, the basal segment of leg II (E) can be seen to bear hairs but they, the
underlying cuticle, and the more distal portions of this and other legs are entirely wet. This indicates that either
the surfaces here are not hydrophobic or the hairs and other structures are too dispersed to support a film of air.
Under the same conditions, the slit-like opening to a book lung (F, upper arrow) is entirely within the plastron and
periodically opens (F’), a change that gives the book lung access to the air stored in the plastron. The lower arrow
in F points to the corner of the slit-like epigynum. This and other black and white images in this chapter can be
viewed in color at faculty.vassar.edu/suter/1websites/surfaces.
10
Fig. 3. Ventral surfaces of Larinioides cornutus (Araneidae). In air (A), most of the spider can be seen to be
covered with hair, the exceptions being the fangs, some parts of the legs, and the area surrounding the ventral
midline of the abdomen. On closer inspection, the region of the book lungs and the epigynum (C) has patches of
hairs but the epigynum and the entrances to the book lungs are entirely exposed. The region of the spiracle (D),
just anterior to the spinnerets, bears numerous short hairs but the percent cover by these hairs is low (see Table 1).
After misting the ventral surface with distilled water, the book lungs and their entrances (E) can be seen to be
wetted, bearing well-spread patches of water quite different from the spherical droplets seen on water repellent
surfaces. Under the same conditions, the proximal joints of the legs (F) have regions that are hirsute and regions
that are entirely hairless, but the hydrophobicity of the hairless regions cannot be assessed from these images.
Under water (B), the ventral surface of the abdomen supported an air bubble that did not extend to cover the
epigynum or the entrances to the book lungs but did cover the region posterior to those structures and included the
spiracle and the spinnerets. Lateral surfaces of the abdomen, though hairy, did not support a plastron, and only
some parts of the proximal leg segments bore a layer of air.
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Fig. 4. Ventral surfaces of an adult female Platycryptus undatus (Salticidae). In air (A), the spider is
conspicuously hairy over most of the abdomen and on the distal portions of the legs. On closer examination,
even the proximal segments of the legs are well supplied with hairs (B) and, judging by the spherical droplets on
them after misting (B’), the hairs’ surfaces are relatively hydrophobic. The same can be said of the region around
the epigynum and a book lung (C, C’) and the environs of the spinnerets and spiracle (D). Despite these
properties, during submersion in distilled water only a small region of the ventral surface (E), the part
encompassing the epigynum, the two book lungs, and a small area just posterior to the epigynum, retained a film
of air constituting a plastron. The remainder of the spider’s ventral surface was wet (E, lower right). This and
other black and white images in this chapter can be viewed in color
at faculty.vassar.edu/suter/1websites/surfaces.at faculty.vassar.edu/suter/1websites/surfaces.
12
Fig. 5. Ventral surfaces of an adult female Xysticus ferox (Thomisidae). In air and at low magnification (A), the
surface of this crab spider appears to be nearly hairless with the exception of stiff hairs or spines on the legs.
Closer views reveal that the spider has numerous short hairs, at low density, over much of its body including the
area that includes the spiracle and the spinnerets (B), and they are at least somewhat hydrophobic judging by the
spherical droplets that form on them during misting (B’, C). When the spider is submerged (D), no plastron is
formed and the entire animal is wet, although bubbles can be seen near the tip of the spinnerets and at the medial
edge of the left booklung (arrows), and where trapped between appendages and other surfaces (D, upper right).
This and other black and white images in this chapter can be viewed in color
at faculty.vassar.edu/suter/1websites/surfaces.
13
4. FUNCTIONAL CORRELATES OF WATER REPELLENT
SURFACES ON SPIDERS
Arthropods that are frequently exposed to submersion or that occasionally
encounter heavy dews, flooding, or torrential rain, can be expected to have
adaptations that increase their ability to survive under those conditions. Such
adaptations are commonly seen in semi-aquatic insects (Merritt & Cummings
1984) and should be present in spiders as well when water exposure has been
common in the lineage under study. The results described above allow us to
evaluate this general assertion and the following more specific hypotheses:
(1) the openings to respiratory organs, the book lungs and the
trachea, should be protected by water repellent hairs or other
hydrophobic structures in spiders likely to become submerged or
otherwise challenged by excess water;
(2) in the same spiders, much of the soma should be covered with
hydrophobic hairs at a high enough density to support the
establishment of a plastron; and
(3) in spiders in general, areas such as leg joints and genital openings
should be surrounded by water repellent hairs to foster the rinsing
away of small particles and thereby, in turn, limit the intrusion of
pathogenic microorganisms and other debris.
4.1. Respiratory organs
Mostany of the spiders in the Aranaeomorphae have two separate
respiratory systems: book lungs and tubular tracheae (Foelix 1996; see also
Schmitz & Perry 2002) leading to what some authors call bimodal breathing
(Schmitz & Perry 2001, 2002). The extent of the tracheal system in spiders
Fig. 6. Hair orientations associated with critical structures. The regions of the spiracle in Tetragnatha elongata
(A), of the epigynum in Platycryptus undatus (B), and of the coxa-trochanter-femur joint in Dolomedes triton (C),
share the property that they bear long, curved hairs whose orientations contribute to the maintenance of an air
space over the respective morphological structures. This and other black and white images in this chapter can be
viewed in color at faculty.vassar.edu/suter/1websites/surfaces.
14
varies dramatically (Leviy 1967; Schmitz & Perry 2002; Schmitz 2005) and the
variability presumably is related to the efficacy of the book lungs, to the size of
the spider, and to the relationship between physiological demand and
environmental supply of oxygen. For example, Argyroneta aquatica, the only
spider known to live underwater, has very small book lungs and extensively
developed tubular tracheae (Braun 1931 in Foelix 1996) and can apparently
exchange gases through its integument (Crome 1953 in Foelix 1996). In a
study in which the author experimentally blocked book lungs and or tracheal
openings in a wolf spider and a jumping spider, Schmitz (2005) showed that
for the wolf spider, the tracheae compensated for lost lung capacity but for the
jumping spider, the main role of the tracheae was to supply oxygen to organs
not associated with running, suggesting that the relative role of book lungs and
tracheae varies with the family of spider. Indeed, the relative importance of
book lungs versus tracheae has been explored for only a few species (Schmitz
& Perry 2001, 2002; Schmitz 2004).
Our hypothesis that species most likely to be exposed to wetting would
have respiratory openings well protected by water repellent hairs is partially
supported. For both Dolomedes and Tetragnatha, the lung slit and the spiracle
have numerous small hairs that functionally repel water: When misted, small
droplets formed on lung slits and spiracles (compare Figs. 1C & 1C’ for
Dolomedes and Fig. 2D for the spiracle of Tetragnatha). In addition, both lung
slits and spiracles were enclosed in the plastron when individuals of these two
species were submerged (Figs. 1B, 2B). Larinioides was less well protected:
the lung slit as well as the cover of the book lung formed flattened droplets
(compare spherical droplets on hydrophobic hairs at bottom of Fig. 3E with
flattened droplets near lung slit and book lung cover). In the salticid we
examined (Platycryptus), both respiratory regions appeared well protected by
water repellent hairs (Fig. 4C’, 4D). That hydrophobic hairs protect both
respiratory structures is consistent with the study by Schmitz (2005): for at
least some salticids, if the book lungs were blocked, the spider compensated by
using its tracheal system. Perhaps for a particularly active group of spiders such
as the salticids, there may have been selection pressures to protect both
openings with hydrophobic hairs. Finally, Xysticus ferox, a species that lives on
the ground in leaf litter, appears the least adapted to wetting; when submerged,
it was nearly entirely wet (Fig. 5D) although the hairs around the spiracle
(compare Fig. 5B’ with 5B) did form spherical droplets when wetted. Crab
spiders in general are sit and wait predators; perhaps they have a lowered
metabolism that allows them to “wait out” a flooding situation (suggested for
other families in Rovner 1986).
4.2. Formation of a plastron
We also predicted that in species most likely to be wetted, their soma
would be covered with hydrophobic hairs at a high enough density to support
the establishment of a plastron. Although the presence of a plastron is thought
to be important in respiration (Thorpe 1950; Hebets & Chapman 2000), it may
also be true that water repellency to the extent of forming a plastron may allow
animals to recover from falling onto the surface of water and be able to move
on water. As an example of the former, we have observed that when they are
disturbed, Tetragnatha can escape capture by plummeting to the surface of the
water below their webs. Typically, they land on their dorsal side, and after a
second or two on the water, they climb up their dragline, clearly not impeded
15
by being caught in the surface tension. Likewise, in a related species that is
found near streams in the western United States, females of Glenognatha
emertoni can escape unwanted males by dropping into a stream and easily
climbing out, once they are several meters down stream (Danielson-François In
Press.).
Of the species we compared, the plastron of the Dolomedes was most
extensive (note the sheen from the bubble formed when submerged, Fig. 1B).
The abdomen of the Tetragnatha also had a mostly intact plastron (Fig. 2B).
Both the Larinioides and the Platytcryptus had an incomplete and poorly
formed plastron and the structure was completely lacking in the Xysticus.
Consistent with the well-formed plastron, the hair covering of Dolomedes was
the most extensive, with many small hairs close to the body and large hairs
extending out faurther. Both the under-hairs and over-hairs were hydrophobic
(as evidenced by the droplets visible on the smaller hairs within the plastron
formed by the larger hairs; Figs. 1B & 1E). This appears to be functionally
similar to the two layers of hairs noted by Andersen (1976) for the semi-aquatic
Hemiptera. He describes a “macro-hair layer of long, flexibly inserted hairs,
and a micro-layer of minute, stiff cuticular outgrowths (microtrichia).”
Interestingly, although abundant. the hairs of the Platycryptus were not nearly
as hydrophobic as was seen in the Dolomedes.
The formation of a plastron would also allow an animal to move on the
water’s surface. Both Dolomedes and Tetragnatha move easily and quickly on
water, albeit with different gaits (Suter et al. 2003; Stratton et al. 2004, see also
Ehlers 1939). Salticids were much less adept at moving on water, but could do
so, a trait consistent with the poorly formed plastron. And, not surprising given
the results of the current study, Xysticus was hydrophilic and could not
effectively move at all on water (Stratton et al. 2004).
A curious example of the importance of hairs, combined with an
indentation of the abdomen, has been found by Simon Pollard (summarized in
Pain 2005). Pollard has studied Misumenops nepenthicola, a crab spider that
specializes in preying on insects trapped underwater in pitcher plants found in
Malaysia. Pollard found that the crab spiders maintain a bubble around their
book lungs via both long, water repellent hairs on their abdomen and also a pit
or depression on the venter of the abdomen. The air in this bubble is somewhat
like holding a glass of air underwater by keeping the open side of the glass
down. These crab spiders can stay submerged for 40 minutes or longer. The
ability to break though the surface tension is made easier by the fact that the
rest of the body is not particularly water repellent. The plant secretes materials
that lower the surface tension of the water, making its capture of insects more
probable. This lowered surface tension possibly allows the crab spider to move
more easily through the air- water interface, and because the bubble of air is
held in the depression on its ventral side, it does not affect the spiders ability
to breathe underwater.
4.3. Protection against particle intrusion
We expected leg joints and genital openings to be at least somewhat
water repellent or to be surrounded by hairs that were water repellent in all of
the spiders tested, although our expectation was heightened for the spiders
most apt to be in frequent contact with water. It appeared that leg joints
without hairs were not particularly water repellent (e.g. Xysticus, Fig. 5D) but
that, for several of the species examined, the presence and orientation of the
16
hairs provided for water repellency. This water repellency, most evident in
Dolomedes, could both contribute to the ability to move on water and allow for
the possibility of cuticular respiration. A similar arrangement of hairs is seen in
the spiracular area of Tetragnatha (Fig. 6A), epigynal area of Platycryptus
(Fig. 6B) and the coxal area of Dolomedes (Fig. 6C). A similar arrangement of
hairs near the epigynum is seen in a wide variety of spiders. Might these
arrangements also confer resistance to the intrusion of pathogenic organisms?
Water repellent surfaces in plants have been shown to reduce the
adhesion of contaminating particles. When wetted, contaminated surfaces of
plants would form water droplets that would roll off the leaf, carrying
contaminants (Barthlott & Neinhuis 1997). In that context, it seems likely that
the hydrophobic hairs surrounding openings like the epigynum and spiracles,
especially when water is present in quantity, may function to protect the
animals from infection. Of course, structures such as the epigynum have
primary functions for which surface characteristics may be important (e.g., in
sperm conduction/retention or in facilitating the passage of the male’s
intromittent organ), and the requirements of these primary functions may, in
evolution, have superseded such secondary functions as the repulsion of
foreign particles. For the species studied here, it appears that except for the
presence of hairs, the epigynal area was easily wetted (in Larinioides and
Platycryptus, and also in Latrodectus mactans and Geolycosa rogersi, unpub
data.), suggesting other selection pressures on the internal surfaces.
4.4 Conclusions
Although all spiders have hairs, those whose hairiness is conspicuous
evoke curiosity about the functions of these structures. Many of the hairs are
innervated (Foelix 1996), suggesting a primary function of sensation;, others
are conspicuously colored or borne in tufts and ridges, indicating a visual
signaling function (Stratton 2005; Maddison & Hedin 2003);, and still others
are used in defense (e.g., Marshall 1992). In the current study, we sought to
expand upon our earlier work on water surface locomotion (Suter et al. 1997,
Suter & Wildman 1999, Stratton et al. 2004) by investigating the non-
locomotor functions of the usually hairy water repellent surfaces in spiders.
Building upon the earlier work of Rovner (1986), we have shown that water
repellency not only varies widely among spider species but also within an
individual across its ventral topography, and that the support of respiratory and
other functions (e.g., defense against pathogen intrusion) by hair-bearing
cuticle is likely to have played an important role in the evolutionary history of
spiders.
Acknowledgements
We thank Pat Miller and Kasey Fowler-Finn for help collecting the
spiders used in the study. We also thank Pat Miller for identifications. The
study was supported in large part by Vassar College's Class of '42 Faculty
Research Fund.
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... For example the legs of the water strider are covered with hairs to help it to float on the water surface and to leave the water easily as it jumps. The water spider has a body covered with stiff hairs to trap a bubble of air, the plasteron, which it uses for respiration [21]. Moreover, devices have been constructed which exploit meniscus-dominated buoyancy. ...
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