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Unformatted text preview: Behavioral Ecology Vol. 10 No. 4: 372–376 Do stabilimenta in orb webs attract prey or
Todd A. Blackledge and John W. Wenzel
Department of Entomology, Ohio State University, Columbus, OH 43210, USA
Orb-weaving spiders are ideal organisms for the study of conﬂict between behavioral investments in foraging and defense because
their webs provide physical manifestations of those investments. We examined the impact of including stabilimenta, designs of
bright-white noncapture silk, at the center of orb webs for foraging and defense in Argiope aurantia. Our ﬁndings suggest that
stabilimentum building is a defensive behavior, supporting the ‘‘web advertisement’’ hypothesis that the high visibility of stabilimenta can prevent birds from ﬂying through webs. Yet, spiders often do not include stabilimenta in their webs, indicating that
a serious cost is associated with them. We also show, through comparison of paired webs with and without stabilimenta, that
stabilimenta reduce the prey capture success of spiders by almost 30%. This demonstrates the potential impact that defensive
behaviors of spiders can have on their foraging success and suggests that much of the variation in stabilimenta may be accounted
for by a cost–beneﬁt trade-off made when including stabilimenta in webs. Key words: aposematic signal, Argiope, foraging–defense
trade-offs, predator–prey, silk, spider webs. [Behav Ecol 10:372–376 (1999)] C onﬂict between foraging and predator avoidance can
have a profound impact on the behavior of organisms
(Lima and Dill, 1990; Sih, 1980; Stephens and Krebs, 1986).
Animals may forage in lower energy patches that have reduced risks of predation (Gilliam and Fraser, 1987; Holomuzki, 1986; Lima, 1985; Lima et al., 1985) or engage in defensive
behaviors that reduce their foraging efﬁciency within patches,
such as vigilance or hiding (Rothley et al., 1997; Schmitz et
al., 1997; Sih et al., 1992; Skelly, 1995). Ultimately, this conﬂict
results in a suite of foraging and defense strategies, each of
which may be selectively advantageous in different environments. This may lead to selection for the ability of organisms
to actively manipulate the trade-offs they make in changing
environments (Rothley et al., 1997; Turner, 1997). Before the
adaptive value of varying strategies in different environments
can be studied, it is essential to identify the costs and beneﬁts
of the behaviors when organisms adopt those strategies.
Orb-weaving spiders provide an ideal model for the study
of conﬂict between behavioral investment in foraging and defense because their webs are physical manifestations of their
behaviors. The orb web is clearly a tool used in foraging
(Eberhard, 1990), but the sticky silk and additional silk structures such as barrier webs can also serve as defenses against
predators (Cloudsley-Thompson, 1997; Edmunds and Edmunds, 1986; Higgins, 1992; Rayor and Uetz, 1990; Tolbert,
1975). Unlike the transient behavioral trade-offs between foraging and defense made by animals engaging in vigilance or
hiding, making a web is unique because the trade-off it represents is constant over the course of a single day. Yet spiders
can alter that investment between days when webs are rebuilt.
Stabilimenta are conspicuous lines or spirals of silk, included
by many spiders at the center of their otherwise cryptic webs.
They provide an example of how extreme variability in investment can occlude the functional role of web structures
because their high degree of variation in shape and frequency
often seems incompatible with existing functional hypotheses Address correspondence to T. A. Blackledge, Museum of Biological
Diversity, Ohio State University, 1315 Kinnear Road, Columbus, OH
43212-1192, USA. E-mail: Blackledge.firstname.lastname@example.org.
Received 3 June 1998; revised 30 November 1998; accepted 17 December 1998.
1999 International Society for Behavioral Ecology (Blackledge, 1998a; Eberhard, 1990). We examine the functional role of stabilimenta in webs and how predator–prey
conﬂict can explain their variability.
The reﬂectance of ultraviolet (UV) light by stabilimenta has
been used to argue that they attract prey to webs (Craig, 1991,
1994b; Craig and Bernard, 1990; Elgar et al., 1996; Hauber,
1998; Tso, 1996, 1998a,b). Craig and Bernard (1990) and Tso
(1996, 1998b) used correlations between high prey capture
and presence of stabilimenta in webs to support this hypothesis. But Blackledge (1998b) demonstrated that high prey capture causes spiders to build stabilimenta more often, creating
this same pattern. He proposed that spiders with low foraging
success did not build stabilimenta because insects could use
them to avoid webs. Furthermore, a consideration of the reﬂective properties of stabilimenta across the entire insect visual spectrum, rather than only UV wavelengths, suggests that
the silk is cryptic to insects, compared to more primitive silks
(Blackledge, 1998a). Thus, the role of stabilimenta in the attraction or repulsion of prey to webs remains to be tested in
a manipulative experiment.
Stabilimentum-building spiders are largely diurnal (Eberhard, 1973; Scharff and Coddington, 1997) and rest at the
center of their webs where they are exposed to visual predators, as opposed to nocturnal spiders or those species resting
in retreats (Eberhard, 1973, 1990). Horton (1980) demonstrated that stabilimenta can prevent predation by captive
birds, and Eisner and Nowicki (1983) found that stabilimentum-like designs of paper reduced the rate of damage to webs,
presumably from birds. Decreased frequencies of stabilimenta
have also been associated with absence of bird predators in
island populations of Argiope spp. (Kerr, 1993; Lubin, 1975).
These studies suggest that one defensive function of stabilimenta is to warn birds and prevent damage to webs from accidental bird ﬂy-through or even predation of spiders. Yet no
ﬁeld test of the ‘‘web advertisement’’ function has been conducted using webs of actual stabilimentum-building spiders
and natural populations of birds.
We directly examined the effect of stabilimenta on the prey
capture success of the yellow garden argiope, A. aurantia (Araneae: Araneidae). We also conducted the ﬁrst test of the web
advertisement hypothesis (Eisner and Nowicki, 1983) to use
real stabilimenta and natural populations of birds. Finally, we
discuss the implications of our results for a cost–beneﬁt model
to explain variation of stabilimenta. Blackledge and Wenzel • Stabilimenta in webs METHODS
Stabilimenta and prey capture
We collected adult and subadult female A. aurantia along a
drainage culvert in Gainesville, Florida, USA, during mid-July
and immediately transported them back to Ohio. This allowed
us to begin the experiment before native A. aurantia were
mature. The experiment was conducted in a ﬁeld adjacent to
the Rothenbuller Honeybee Laboratory at Ohio State University. The ﬁeld had a vegetation structure similar to the typical
habitat of A. aurantia, and both A. aurantia and A. trifasciata
(an ecologically similar species) occurred there naturally. Approximately 200 beehives were scattered to the north, south,
and west, most within a 0.5 km radius and provided a large
population of visually proﬁcient, ﬂying insect prey.
Eight stations were haphazardly placed throughout the
ﬁeld. Each station consisted of a pair of square wooden frames
(75 75 12 cm) with the large sides being removable plastic
sheets. This allowed us to conﬁne spiders to the stations while
they built webs overnight, yet let them forage freely once the
sides were removed. The two frames at each station were adjacent to one another and were oriented in the same direction, though we varied orientation haphazardly between stations. Therefore, both webs at a station experienced similar
We placed a single female A. aurantia in each frame, making an effort to pair similarly sized spiders. Each day on which
both spiders at a site built webs, one was randomly designated
as an ‘‘experimental’’ web and its stabilimentum was removed
by using a wire heated by a small butane blowtorch to cut the
two radii to which the stabilimentum was attached. The stabilimentum was then easily pulled from the web using forceps.
We also performed sham removals on the other ‘‘control’’
web by cutting radii immediately adjacent to the stabilimentum, thus creating a similar-sized hole in the web. The random removal of stabilimenta controlled for variation in total
web area, web height, and mesh size of webs, which would
otherwise be important variables affecting prey capture (Eberhard, 1990; Higgins and Buskirk, 1992; Sherman, 1994).
Prey capture was observed over foraging trials lasting 3 h
each, beginning between 0830 h and 1000 h. Because the
trials ran into the afternoon, stabilimenta were exposed to a
wide range of light conditions under which Argiope spp. forage (Endler, 1993). We collected all prey in webs and all prey
on which spiders were actively feeding every half hour and
stored the prey in ethanol for later identiﬁcation. Very small
prey could be consumed between collection periods so, although there is no reason to expect a bias between treatment
groups, we restricted our analysis to prey larger than 3 mm.
We identiﬁed prey to family under a dissecting scope after
dissolving the swathing silk with chlorine bleach (Vetter et al.,
Cages were kept closed outside of the foraging trials; therefore each spider was fed a single large mealworm (Tenebrio
molitor) daily. This also helped standardize foraging motivation and size of stabilimenta (Blackledge, 1998b) between spiders. Spiders occasionally built new stabilimenta where they
had been removed or over an existing one. These new stabilimenta were excised from the webs only in the experimental
treatment. Any prey captured during a half-hour period in
which a new stabilimentum was built were excluded from the
analysis for both webs at that station.
To compare capture rates between web treatments, we categorized each station as to whether the majority of paired
comparisons at that station had experimental webs catch more
prey than control webs. We then used a G test to compare the
number of stations in which experimental webs captured the 373 most prey, compared to control webs, in greater than 50% of
the paired comparisons.
Stabilimenta and defense
To examine the interaction of birds with stabilimenta, at two
sites we used setups which consisted of a dark blue plastic dish
containing bird seed, surrounded by a triangular array of
three frames (the same frames as described in experiment 1
above). Birds were allowed to acclimate to the setups containing empty frames before the experiment began. The west
campus site was in a small ﬁeld in a grassy forest clearing
(approximately 15 m diam) which contained natural populations of both A. aurantia and A. trifasciata. The museum site
was on a mown lawn adjacent to a bird feeder at the Museum
of Biological Diversity, Ohio State University, an area which
would not normally have Argiope spp.
For each trial, two of the three empty frames were randomly
replaced, one by a frame containing a web with a stabilimentum (and sham operation as in experiment 1) and one by a
frame containing a web with the stabilimentum removed. The
third frame was left empty to provide birds with a ‘‘web-free’’
access route to the station. We conducted 12 trials at each site
using webs without spiders. Then we conducted an additional
eight and nine trials at the west campus and museum sites,
respectively, using webs with spiders left in them. Comparison
of the two sets of trials allowed us to determine whether the
spider itself had any inﬂuence on avoidance of webs by birds.
Frames were put out at mid-morning and observed periodically until the ﬁrst sign of bird impact, at which time the
trial was ended, or until dusk if neither web was damaged.
Bird impact was quite distinct from insect damage, as it consisted of destruction of entire pie-shaped sectors of the web
or even collapse of part or all of the web. Occasionally both
webs were damaged by the time of the ﬁrst observation period
and were therefore both scored as ‘‘damaged.’’
Data from both sites were combined for this analysis, and
the frequency with which experimental webs were damaged
ﬁrst was compared to that of control webs using chi-square
tests. Comparisons between trials for webs containing spiders
were made separately from comparisons between trials for
webs without spiders.
Stabilimenta and prey capture
Prey capture was not normally distributed, but the mean capture rate for spiders in webs without stabilimenta was higher
than that for spiders in webs containing stabilimenta (mean
SE, 2.9 0.3 versus 2.0 0.3 prey/3-h trial; n 55). Spiders in
webs without stabilimenta caught the most prey in more trials
than spiders in control webs, at a majority of stations (G
.025; Figure 1).
At least 31 families of prey were captured. The most common prey were Apidae (32%, mostly Apis mellifera) and Muscidae (22%, mostly Stomoxys calcitrans; Table 1). The capture
of ﬂies (Muscidae and Calliphoridae) was strongly inﬂuenced
by stabilimenta (a 56% and 100% reduction, respectively).
The reduction of capture of Apidae (40%), miscellaneous
(33%), and unidentiﬁed (38%) taxa in webs containing stabilimenta were all similar to the overall reduction in prey capture of 34%.
Stabilimenta and defense
Webs without stabilimenta were damaged signiﬁcantly more
often than webs with stabilimenta during both the trials when Behavioral Ecology Vol. 10 No. 4 374 Table 1
Families of prey captured by A. aurantia in 55 pairs of webs, with
and without stabilimenta
Taxa Figure 1
The distribution of differences in prey capture for 55 paired
comparisons at 8 stations (difference
prey capture at webs
without stabilimenta prey capture at webs containing
stabilimenta). Seven of eight stations that had webs without
stabilimenta caught more prey than webs containing stabilimenta
for 50% of the trials at the station (G
The mean ( SE) prey capture rate over 3 h was 2.9 0.3 for spiders
in webs without stabilimenta and 2.0 0.3 for spiders in webs with
stabilimenta. spiders were removed from the webs (p .001; Table 2) and
the trials when spiders were present in webs (p .005; Table
2). There was no signiﬁcant difference in the distribution of
damage between the trials with and without spiders ( 2
The ﬁtness costs of behavioral responses to predation risk can
be substantial due to the reductions in foraging efﬁciency,
alterations of patch choice, or modiﬁcation of life histories
which can be associated with those defensive behaviors (Lima
and Dill, 1990; Schmitz et al., 1997; Scrimgeour and Culp,
1994; Sih, 1992; Skelly, 1995). Our study suggests that one
function of stabilimenta is as a behavioral defense against
birds because webs without stabilimenta are damaged more
often by ﬂying birds (Table 2). However, the defensive behavior of including stabilimenta in webs results in a serious reduction in the ability of A. aurantia to function as predators
(Figure 1). Because predation pressure and prey density vary
spatially and temporally, the trade-off that A. aurantia and
similar stabilimentum-building spiders must make between
the defensive beneﬁts and foraging costs of including stabilimenta in webs may account for much of the variation seen in
stabilimentum production both within and between Argiope
spp. No stabilimentum Stabilimentum Apidae
163 Miscellaneous taxa are those families for which fewer than three
individuals were captured. actual number of prey captured by spiders rather than inferring it from web damage. This gave us a direct measure of the
effect of stabilimenta on spider foraging success. Thus, our
data provide a better indication of the impact stabilimenta can
have on the ﬁtness of spiders by altering their foraging success.
One explanation for the reduction in prey capture caused
by stabilimenta is that insects learn to avoid webs containing
them (Craig, 1994a,b). However, all but 2 of the 31 families
of prey were captured so infrequently that it is unlikely that
individuals of those taxa ever encountered more than a single
web. We also conducted our experiment early enough that
native A. aurantia were not yet mature; thus prey were essentially naive to stabilimenta. Therefore, the effect of stabilimenta on prey capture we demonstrate is likely the result of
ﬁrst-time interactions of insects with webs, rather than a
The taxa of prey captured by A. aurantia in our experiment
is similar to that found in other studies of temperate and tropical Argiope spp. where Hymenoptera often constitute 50–90%
of the diet of Argiope spp. (Brown, 1981; Horton and Wise,
1983; Howell and Ellender, 1984; McReynolds and Polis, 1987;
Robinson and Robinson, 1970a), and Apis spp. may account
for more than 15% of prey captured by Argiope bruennichi
(Nyffeler and Breene, 1991) and Argiope amoena (Murakami,
Number of days on which webs were damaged by birds Stabilimenta and prey capture
Our results contradict the hypothesis that stabilimenta attract
prey to the webs of spiders (Craig and Bernard, 1990; Craig,
1994b; Hauber, 1998; Tso, 1996, 1998a,b) because we found
that webs containing stabilimenta caught 34% fewer prey. Previous studies used web damage (Craig and Bernard, 1990;
Hauber, 1998; Tso, 1996) or infrequent censuses (Tso, 1998b)
as indices of prey interception rates and found correlations
between the presence of stabilimenta in webs and high prey
capture success. However, Blackledge (1998b) demonstrated
that this same pattern is caused when spiders that catch more
prey increase their frequency of stabilimentum construction.
We controlled for this effect through direct manipulation of
the presence of stabilimenta. Furthermore, we measured the 37
Webs without spiders
Webs with spiders
1, p Not damaged 9
5 .001 .005 Chi-square values were computed from the expectation that webs
with stabilimenta would be damaged at the same frequency as webs
with no stabilimenta. Blackledge and Wenzel • Stabilimenta in webs 1983). However, the large percentage of Diptera captured by
spiders in webs without stabilimenta is unusual (Table 1). Diptera are often less common than expected in the webs of Argiope spp. when compared to the diets of other co-habiting
spiders (Olive, 1980) or when compared to the distribution
of available prey in the environment (Bradley, 1993; Murakami, 1983). Because webs without stabilimenta caught many
more ﬂies than webs containing stabilimenta, our data suggest
that at least some of the specialization on nondipteran taxa
by Argiope spp. might be attributed to the common inclusion
of stabilimenta in their webs.
Stabilimenta and defense
Our data corroborate the hypothesis that stabilimenta can
function as a defense against birds (Eisner and Nowicki, 1983;
Horton, 1980; Kerr, 1993; Lubin, 1975) because we found that
stabilimenta can reduce the frequency of damage to webs
from ﬂying birds by 45% (Table 2). We observed several instances where house sparrows (Passer domesticus), carolina
chickadees (Poecile carolinensis), and goldﬁnches (Carduelis
tristis) ﬂew toward webs with stabilimenta but abruptly halted.
They then hovered brieﬂy in front of the stabilimenta before
entering the stations through open frames or ﬂying away. Yet,
we never saw birds actively avoid webs that did not contain
stabilimenta. Our data also suggest that the bright black-andyellow color pattern of A. aurantia does not itself function as
an aposematic warning (Horton, 1980; Nentwig and Rogg
1988), at least to ﬂying birds, because webs were damaged no
less frequently in trials with spiders than in trials without spiders (Table 2).
Damage to webs in the ﬁeld by birds is rare (Blackledge
and Wenzel, personal observations) and alone is unlikely to
account for inclusion of stabilimenta in webs, given their cost
to foraging success. In addition to destroying webs, birds can
be important predators of spiders (Edmunds and Edmunds,
1986; Marples, 1969). Horton (1980) demonstrated that the
stabilimenta of A. aurantia can function as an aposematic
warning to predatory blue jays (Cyanocitta cristata), signaling
that an otherwise palatable spider was in an orb web containing irritating sticky silk. We saw no instances of predation by
birds, but two A. aurantia disappeared during the experiment
on prey capture and were likely eaten by birds. In both cases,
the orb webs were almost completely destroyed with single
spider legs remaining; in one case the leg was even hanging
in the tattered web remains. Adult A. aurantia are too large
to be prey for most temperate North American wasps and
salticid spiders, no vertebrate predators other than birds were
seen during the experiment, and Argiope do not normally
abandon a web without ﬁrst consuming it. Interestingly, one
of the two spiders that disappeared was in an experimental
web with the stabilimentum removed, and the other web had
been excluded from the experiment because it had an abnormally short and thin stabilimentum that was barely visible
Conﬂicts in stabilimentum building
Many spiders vary their behaviors in response to changes in
predation risk and foraging success (Rayor and Uetz, 1990;
Whitehouse, 1997). Tolbert (1975) has suggested that changes
in stabilimentum shape as spiders mature are responses to
changes in predation risk as spiders increase in size. Our study
supports the hypothesis that stabilimenta can help defend spiders against birds (Eisner and Nowicki, 1983; Horton, 1980)
by demonstrating that webs containing stabilimenta are 45%
less likely to be damaged by ﬂying birds. Kerr (1993) and
Lubin (1975) found correlations between reduced densities 375 of bird predators of Argiope spp. and reduced frequency of
stabilimentum building, suggesting that spiders can respond
to variation in predation risk by modifying stabilimentum
building. Yet, it can be difﬁcult for organisms to track changes
in risk of predation accurately over short periods of time (Sih,
1992). They are therefore expected to be conservative in their
estimation of predation risk, and such risk cannot alone account for stabilimentum variation.
We also found that stabilimenta cause a 34% reduction in
prey capture by A. aurantia, and Blackledge (1998b) demonstrated that A. aurantia and A. trifasciata alter their investment in stabilimenta based on variation in foraging success.
Variation in foraging success can also be more reliably assessed by most organisms than can risk of predation. Thus,
much of the variation in stabilimentum frequency, particularly
that observed within populations, is more likely to be attributed to behavioral responses of spiders to ﬂuctuating prey
availability. This model also explains investment in stabilimenta in noncapture webs by several genera of spiders which increase the frequency of stabilimentum building just before
molting or egg laying (Eberhard, 1973; Nentwig and Heimer,
1987; Robinson and Robinson, 1970b, 1973). Spiders do not
feed at these times, and the costs of including stabilimenta in
their nonsticky webs are therefore minimal. Future research
should focus on modeling the relative contributions of predation risk and prey capture success to the control of intraand interpopulation variation in stabilimentum production.
Such study will help elucidate the importance of behavioral
responses to predation risk on other aspects of the life history
of spiders. Our results further support the importance of dynamic behavioral responses by organisms when they confront
conﬂict between foraging strategies and predation risk, particularly in a variable environment.
We thank Brian H. Smith and the Rothenbuller Bee Laboratory for
their tolerance while our spiders feasted upon their honey bees. Richard A. Bradley, William G. Eberhard, Thomas C. Grubb, Linda S.
Rayor, and anonymous referees provided insightful comments on statistical analyses and on the manuscript. Christopher L. Caprette
helped construct the frames used in our experiments. Mark K. Stowe
kindly helped locate Argiope in Florida. This research is based on work
supported under a National Science Foundation Graduate Research
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This note was uploaded on 01/27/2012 for the course ECOLOGY 300 taught by Professor Zumdahli during the Spring '11 term at St. Mary NE.
- Spring '11