CAUTION, WEBS IN THE WAY! POSSIBLE FUNCTIONS OF SILK STABILIMENTA IN GASTERACANTHA CANCRIFORMIS (ARANEAE, ARANEIDAE)

2006·8.6 MB·English

by  Rodolfo Jaffé

2006. The Journal of Arachnology 34:448-455
CAUTION, WEBS IN THE WAY! POSSIBLE FUNCTIONS OF
SILK STABILIMENTA IN GASTERACANTHA CANCRIFORMIS
(ARANEAE, ARANEIDAE)
Rodolfo Jaffe^ William Eberhard^, Carlos De Angelo^, Diana Eusse^,
Adrian Gutierrez^, Sandra Quijas^, Antonio Rodriguez^ and Mayanin Rodriguez^:
dnstitut fiir Biologic, Martin-Luther-Universitat Halle^Wittenberg, Hoher Weg 4, #
125, D-06099 Halle/Saale, Germany. E-=mail: [email protected]™halle.de;
^Smithsonian Tropical Research Institute, and Escueia de Biologfa, Universidad de
Costa Rica, Ciudad Universitaria, San Jose, Costa Rica; Tundacioe Vida Silvestre
Argentina, Iguazu, Argentina; "^Universidad del Valle, Cali, Colombia; ^Universidad
Autonoma de Mexico, Mexico, D.F., Mexico; ^Centro de Investigacion en
Ecosistemas, Universidad Autonoma de Mexico, Yucatan, Mexico; ^Grupo de
Ecologia de Vertebrados, Facultad de Biologia, Universidad de la Habaea, La
Habana, Cuba; ^Universidad de los Andes, Merida, Venezuela.
ABSTRACT. We tested three hypotheses concerning the function of stabilimenta in the orb weaver
Gasteracantha cancriformis: 1) warning to large animals that might accidentally destroy the web; 2) prey
attraction; and 3) camouflage. One prediction of the warning hypothesis (but not of the others), that less
exposed webs should have fewer stabilimentum tufts, was verified: very few tufts occurred on the pe-
ripheral lines of small webs. On the other hand, a prediction of the prey attraction hypothesis, that webs
withro.ore stabilimentumtufts shouldalsohavemorecapturedprey, was onlyconfirmedinonesubsample,
and further analysis indicated that spider size rather than number of stabilimentum tufts best explained
the presence of prey. An additional observation not in accord with prey attraction was that resting webs,
which lacked sticky silk for prey capture, nevertheless had abundant stabilimentum tufts. Finally, the
number of stabilimentum tufts was lower in the webs of white (as opposed to yellow or orange) spiders,
contradicting a prediction of the camouflage hypothesis. The strongest conclusions from our results are
support for the warning function, and lack of support for the prey attraction function.
RESUMEN. Pusimos apruebatres hipotesis concernientes alafuncion de los estabilimentos en laaraea
Gasteracantha cancriformis: 1) advertencia a animates grandes que podrian destruir accidentalmente la
red; 2) atraccion de presas; y 3) camuflaje. Se cumplio una prediccion de lahipotesis de advertencia(pero
no de las demas) de que las redes menos expuestas deberfan presentarmenos estabilimentos: seobservaron
muy pocos estabilimentos en las Imeas perifericas de redes pequenas. Por otra parte, la prediccion de la
hipotesis de atraccion de presas, de que redes con mas estabilimentos deberfan presentar mas presas
capturadas, solo se cumplio en una submuestra y analisis posteriores mostraron que el tamano de la arana
y no el numero de estabilimentos explico mejor la presencia de presas. Otra observacion que no apoya la
hipotesis de atraccion de presas fue haberencoetradoredes de descanso, las cuales carecierondeunespiral
de captura, con un importante numero de estabilimentos. Finalmente, el numero de estabilimentos fue
menor en las redes de arafias blancas (en relacion a las amarillas o las anaranjadas), contradiciendo la
prediccion de la hipotesis de camuflaje. Nuestros resultados apoyan la hipotesis de advertencia y no la de
atraccion de presas.
Keywords; orb webs, silk stabilimenta, spiders, warning, prey attraction, camouflage
The function of silk stabilimenta on the berstein et aL 2000; Eberhard 2003). The hy-
webs of diurnal orb-weaving spiders has long potheses which are currently best supported
been debated (e.g., Hingston 1927; Marson include prey attraction, camouflage frompred-
1947; Marples 1969; Edmunds 1986) and re- ators, and web advertisement to warn offlarge
mains controversial (see summary by Her- animals which might damage the web (several
448
JAFFE ET AL.—POSSIBLE FUNCTIONS OF SILK STABILIMENTA 449
other hypothesized functions, such as provid- web (presumably an orb with a sticky spiral)
ing shade for the spider, a path for the male it rests at a central point where several threads
to find the female, physical stabilization ofthe bearing tufts converge. Comstock (1967)
web, and a deposit of excess silk have little speculated that the tufts of G. cancriformis
support at present). Most of the recent debate might deceive midge-eating insects, which in
regarding silk stabilimentum function has cen- their efforts to capture the supposed midges
tered on the stabilimenta in the genusArgiope, accidentally fly into the web. Muma (1971)
but silk stabilimenta have also evolved inde- stated (probably somewhat imprecisely, as
pendently in several other lineages of orb will be shown below) that the “webs of im-
weavers (Scharff and Coddington 1997; Her- matures are different from those of sub-adult
berstein et al. 2000). Evidence from other or adult females in lacking . . . distinct tufts
groups is likely to be useful in understanding of silk on the radii or foundation lines ...”
possible functions. In this note we test predictions made by
The araneid orb weaver Gasteracantha can- each of the three major hypotheses for stabi-
criformis (Linneaus 1767) is widespread in limentum function. The warning hypothesis
the New World, ranging from the southern (but not the others) predicts that stabilimen-
USA to northern Argentina (Levi 1978). tum tufts should be more abundant in orbs
Bright body colors have been said to attract which are larger and span larger spaces, and
prey in another species of Gasteracantha are thus under greater risk ofbeing destroyed
(Hauber 2002). It is highly variable in color by large animals passing by. The fact that
and shape (Levi 1978). The orbs of mature birds have been seen to actively avoid orb
females of G. cancriformis occur in fairly webs (Blackledge & Wenzel 1999), and that
m
open situations, up to more than 6 above birds which have flown through spider webs
the ground, and are relatively large: anchor show signs ofintense discomfort, immediately
lines extend up to 2-4 m from the hub to sup- preening themselves extensively (Robinson &
ports, and the diameter ofthe area covered by Robinson 1976), support the assumption of
m
the viscid spiral can be up to 0.6 (Marples this hypothesis that visual signals from spiders
1967; Muma 1971). or their webs might be used by birds to avoid
Stabilimenta apparently evolved indepen- webs. The prey attraction hypothesis (but not
dently in this group (Gasteracanthini) from the others) predicts that spiders with more sta-
other araneids such asArgiope (Herberstein et bilimenta should be more often found feeding
al. 2000). The stabilimenta on G. cancriformis on prey. Einally, the camouflage hypothesis
webs consist of multiple short tufts of white (but notthe others) predicts that spiders whose
silk. Most tufts are on the frame and anchor abdomens are white and which thus match the
lines, though they also occur on one or more color ofthe stabilimenta (in contrast to yellow
radii near the hub (Comstock 1967; Marples or orange, colors which also occur in the same
1967; Muma 1971; Levi 1978). Tufts of sta- population), should produce more stabilimen-
bilimentum silk are added to frame and anchor ta.
lines while the spider is reinforcing a line al- METHODS
ready in place (Marples 1967); the spider
pauses, pulls a loose swath of white silk from On 20 January 2004 (early dry season) in
its spinnerets with strokes of its hind legs, a plantation of African oil palm {Elaeis gui-
dabs its spinnerets to the line one or more neensis) located near Parrita, Costa Rica, we
times to attach the swath, and then moves on. followed transects defined by lines of palm
Each tuft consists ofmany fine threads. Newly trees, covering an area of about 1500 m^. We
made stabilimentum tufts blow out to the side measured the following variables in easily ac-
m
ofthe main thread in the wind (thus indicating cessible spider webs (approximately 1-2.5
that the lines in the tufts are under no stress, above the ground): 1) Spider size: estimated
and unlikely to provide any physical support by multiplying the length (cephalothorax plus
for the web). The stabilimentum silk tends to abdomen) by the maximum width of the ab-
stick to or entangle itself with the line, and domen (mm); 2) Maximum radius of the vis-
becomes less conspicuous as time goes by cid spiral: the maximum distance between the
(Marples 1967). Marples (1967) noted that center of the hub and the outer border of the
sometimes when a spider lacks a “complete” viscid spiral; 3) Maximum span of the web:
450 THE JOURNAL OF ARACHNOLOGY
24
3 20
H
E
3 16
c
E
S 12
Spider Size (mnf)
8
TD A A
DCCS 4 A A4 AA4a4A a a^ aa 4
‘o 4 4 4AM 4 4 4
d M 4 A A A
0 ArlAWAiAAAl4*l4A>AA,..«A 1 case
50 100 150 200 2-4cases
5-8cases
Maximum WebSpan (cm) >8cases
—
Figure 1. Number of radial stabilimentum tufts in relation to the maximum web span. The frequency
histogram at the upper right shows the distribution of spider sizes (n = 220).
the maximum distance between points where 54.9 cm for maximum span; Mann-Whitney
anchor lines were attached to the substrate; 4) U-Test: U = 1008 and 1232 respectively; P <
Number of radial stabilimentum tufts: the 0.001 in both cases; = 71, ^2 147).
number of tufts of silk on radial lines inside Interpretation of these patterns is compli-
the viscid spiral; 5) Number ofperipheral sta- cated by the correlations between spider size
bilimentum tufts: the number of tufts of silk and web size, and thus the possibility of in-
on peripheral and anchor lines, i.e. all tufts direct effects. Small spiders had fewer radial
outside the viscid spiral; 6) Prey: silk-wrapped and peripheral stabilimentum tufts than large
prey near the hub; and 7) Color: the color of spiders (Mann-Whitney U-tests: U “ 894 and
the dorsal surface of the abdomen, classified 424 respectively; P < 0.01 in both cases;
as white, yellow or orange. Although there = 94, ^2 = 126). Smaller spiders were also
was a certain overlap in some colors, like light more likely to not have any stabilimentum
yellows, it was relatively easy to assign all tufts (median = 0 for both radial and periph-
spiders to one of these three categories. We eral tufts; min-max = 0--5 and 0-30 respec-
measured 220 capture webs with spiders, and tively), while the large spiders typically had a
in addition, 7 resting webs that lacked viscid few radial stabilimentum tufts, and a larger
spirals. number ofperipheral stabilimentum tufts (me-
RESULTS dian = 4 and 24.5; min-max — 0-21 and 0-
—
Warning Hypothesis. As predicted, 62 respectively).
smaller webs had smaller numbers of stabili- The agreement with the predictions of the
mentum tufts. The numbers of radial and pe- warning hypothesis was not due, however, to
ripheral stabilimentum tufts were positively secondary effects of the correlation between
correlated with both the maximum viscid spi- web size and spider size. When we performed
ral radius (R's = 0.70 and 0.77 respectively; partial correlations of the total number of sta-
P < 0.001 in both cases; n = 218) and max- bilimentum tufts with the maximum span of
imum span of the web (i?’s = 0.62 and 0.76 the web, the maximum radius of the viscid
respectively; P < 0,001 in both cases; n = spiral, and spider size, the maximum span had
=
220) (Figs. 1, 2). Similarly, webs that com- the highest correlation coefficient {R 0.35,
pletely lacked peripheral stabilimentum tufts 0.25, and 0.16 respectively; ^214 ^ 5.51, 3.81,
were smaller than those that had them (3.6 ± and 2.42 respectively; P < 0.01—in all cases).
2.0 cm vs. 1 1.3 ± 4.7 cm for maximum radius Prey Attraction Hypothesis. In general,
of viscid spiral; 43.6 ± 24.2 cm vs. 111.0 ± spiders with prey in their webs had higher
^
JAFFE ET AL.—POSSIBLE FUNCTIONS OF SILK STABILIMENTA 451
70
1
H
60
E
c 50
E
5 40
CO
CD
30
2
.c 20
Q. A
'v.
0)
a. 10
•o+—
z6 0 .. ^ ^ ^ ^ . . . A^ 21-c4acsaeses
J 50 100 150 200 250 A 5-7cases
Maximum Web Span (cm) >7 cases
—
Figure 2. Number of peripheral stabilimentum tufts in relation to the maximum web span.
numbers of radial and peripheral stabilimen-
tum tufts than spiders without prey in their
webs (Mann-Whitney U-test: U = 3222 and
2939; P = 0.036 and 0.005 respectively). Be-
35
cause the array of potential prey is probably
30 different for small and large spiders, and be-
Tufts
cause the distribution of spider sizes (inset in
26
Fig. 1) suggested the separation of spiders in
20 two size categories (small < 15 mm^, and
Stabilimenlum large > 15 mm^), we repeated this analysis
15
separately for both categories.
of
10 When the webs of small spiders with prey
No.
were compared with those of small spiders
Total 5 without prey, the number ofradial stabilimen-
0 82 tum tufts was not different (Mann-Whitney U-
WithoutPreWyebs WithPrey test: U ~ 392; P ~ 0.26), but the numbers of
both peripheral and total stabilimentum tufts
70 were higher in webs with prey {U = 269 and
272, respectively; P = 0.01 in both cases;
Tufts 60 “ 12, «2 “ 82). On the contrary, in large spi-
50 ders the numbers of radial, peripheral and to-
tal stabilimentum tufts did not vary between
Stabilimentum 40 webs with and~without prey (U = 1508, 1538
30 and 1556; P 0.76, 0.88 and 0.97, respec-
of tively; Wj == 34, «2 ^ 92) (Fig. 3).
20 The higher number of prey in the webs of
No.
Total 10 small spiders that had more stabilimentum
tufts could have been an indirect effect of the
0 relationship between the number of stabili-
WithoutPrey With Prey
Webs mentum tufts and web size (which is likely to
Figure 3.—The median numbers of stabilimen- affect capture success) (see Figs. 1, 2), or be-
tum tufts in webs of small (<15 mm^) and large tween stabilimentum tufts and spider size
spiders (>15 mm^) with and without prey (mini- (which is proportional to both web size and to
mum and maximum values arerepresentedbywhis- the strength of silk lines (Craig 1987)) (Fig.
kers). Sample sizes are above the columns. 4) rather than to the stabilimenta themselves.
452 THE JOURNAL OF ARACHNOLOGY
a)
-25
CO
20
T3
15
10
I
I 0 1case
25 50 75 100 2-5cases —
6-10cases Figure 5. Median numbers of stabilimenta in
SpiderSize(mm'' A >10cases webs ofspiders with differentcolors (minimumand
maximum values are represented by whiskers).
b) Sample sizes are above the columns.
300
cm), had few radii, and completely lacked vis-
e 250 cid spirals. Contrary to expectations of the
prey attraction hypothesis, they all had many
2.200 stabilimentum tufts (23.4 ± 17.1). In these
CO resting webs, the number of stabilimentum
-
150 tufts did not correlate with spider size (i?s
i 100 ‘ * i 0(/;?sP==0.13),6;orPth=e 0m.a42x)i.mSupmidesrpasnizoefwtahse walesbo
B 50 tka A i ‘A i Pnot=related to the span ofthe web {Rs = 0.53;
0.22). —
Camouflage Hypothesis. The number of
* 1case
25 50 75 100 A 2-4cases radial stabilimentum tufts did not vary among
A 5-8cases differently colored spiders (Kruskal Wallis
— SpiderSize(mm' A >8cases (KW) test: = 5A\P = 0.078). Contrary
Figure 4. Relation between maximum radius of to the prediction of the camouflage hypothe-
wPthiet<hv0iss.pc0ii0dd1ersipsniirzbaeolt(h(Rasc)a=saens0d;.8m6a=axnid2m1u0%.,m78wree=bsp2es2cp0ta)in.vel(yb;) lsiis,metnhteunmumtubfetsr wofasperliopwheerralinanwdhittoetalspsitdaebris-
than in yellow or orange individuals (KW:
In an effort to separate these possible effects, ^re(s2p.2e20c)ti=vel1y0)-4(Faingd. 95.)1.; SFma-ll0e.r00s5pidaenrds0w.e0r1e0
we performed a logistic regression using pres- more likely to be white, so we repeated this
ence/absence ofprey as the response variable, analysis for small and large spiders separately.
and the number of radial stabilimentum tufts, This eliminated all significant differences with
the number of peripheral stabilimentum tufts, respect to spider color (small KW: ~
spider size, maximum radius ofthe viscid spi- 0.9; 0.9 and 0.6; P = 0.64; 0.64 and 0.75 re-
draelp,eannddenmtavxairimaublmes.spIannsomfaltlhespwiedebrsa,stthheespiin-- s1p.e0;ctPive=ly;0.5l2a;rge0.5K1W:andH^02..\6260) r^esp1e-c3t;iv1e.l4y)a.nd
der size was the only variable to explain prey
DISCUSSION
presence (Final loss = 33.23; = 5.3 and P —
= 0.021), while in large spiders prey presence Warning Hypothesis. The strong posi-
was not explained by any of the variables. tive correlations of the number of stabilimen-
We also found seven spiders (44.3 ± 34 tum tufts with the maximum web span and the
mm^ in size) on “resting” webs. These were viscid spiral radius are in accord with the pre-
relatively small (maximum span 72.1 ± 30 diction of the advertisement hypothesis. Her-
JAFFE ET AL.—POSSIBLE FUNCTIONS OF SILK STABILIMENTA 453
—
bersteie et aL (2000) claimed that the warning Prey Attraction Hypothesis. Our results
function is unlikely in the stabilimenta ofGas- give little support for the prey attraction hy-
teracantha because the spiders build their pothesis. First, given that all spiders need to
webs in “shrubs and understory where birds feed, under this hypothesis all spiders should
are unlikely to fly through and damage the present stabilimenta. The fact thatthe majority
web'' (p. 665). Both our observations and of small spiders did not have stabilimenta in
those of previous authors (Comstock 1967; their webs is not easily explained by the prey
Marples 1967; Muma 1971) show that this attraction hypothesis unless ad hoc modifica-
characterization of G. cancriformis web sites tions are added. For instance, the prey attrac-
is incorrect. Nor is it likely to be true for the tion hypothesis could explain the reduction of
African Gasteracantha species (G. curvispi- stabilimenta in small webs ifthe prey utilized
na) that builds apparently similar stabilimenta by large spiders but not by small spiders are
and on which they apparently based their de- attracted to stabilimenta, or if the cost-benefit
scription (they cite it in their table as building balance (cost of predator attraction vs. benefit
“under bushes”). The complete description of prey attraction) is different for small vs.
from the reference they cite for this species large spiders. We know of no evidence favor-
(Edmunds & Edmunds 1986) is “The web ing these ideas, nor do we know of any data
was spun in open spaces, between or under suggesting that they are incorrect.
bushes, or attached to buildings” (p. 78). Second, we did not find the expected rela-
It is worth noting that the number of pe- tion between prey and number of stabilimen-
ripheral stabilimentum tufts increased approx- tum tufts. In webs of larger spiders there was
imately linearly with maximum web span no significant relation. In small spiders we
(Fig. 2). Use of such a linear increase rule found the predicted correlation betv/een high-
may be the mechanism by which spiders pro- er numbers of peripheral stabilimentum tufts
duced the adjustment in numbers of stabili- and prey, but when the analysis took into ac-
mentum tufts in larger webs that was predict- count the possibility of indirect influences of
ed by the warning hypothesis. In contrast, other variables, the only variable that ex-
radial stabilimentum tufts differed in being plained prey presence was spider size. This
generally lower in number and in increasing implies that the effect on prey attributed to the
less sharply with web size (Fig. 1). If stabili- number ofstabilimentum tufts in small spiders
menta serve as warning devices, the periph- may have been due to a relation between prey
eral stabilimentum tufts may mark the edges capture and spider size. It is important to note,
of the area occupied by the web, while the however, that the arrival of prey is probably
radial stabilimentum tufts may simply mark highly stochastic, so larger sample sizes than
which side of the well-marked periphery of ours might be needed to document prey cap-
the web is to be avoided (because it has the ture effects,
dense array of radial and sticky lines). This A third, especially strong type of evidence
could explain the lower numbers ofradial sta- against the prey attraction hypothesis comes
bilimentum tufts and their weaker relation to from resting webs. These lacked viscid spi-
web size that were observed. rals, and thus did not function to capture prey,
It is also interesting that the stronger and but were nevertheless well equipped with sta-
stickier silk lines of larger spiders that a bird bilimentum tufts. Stabilimenta were also ob-
would contact if it flew into an orb are prob- served on webs that were apparently of this
ably more disturbing than the silk lines of type by Marples (1967). These resting webs
smaller spiders, as they may be more restric- strongly suggest that stabilimenta are not used
tive and more difficult to clean off. So, ifbirds to attract prey. If these webs serve as molting
learn from experiences with webs and distin- platforms (one newly molted individual was
guish those made by spiders ofdifferent sizes, observed with its shed cuticle on one ofthese
larger spiders should gain a greater advantage webs, Eberhard pers. comm.), the presence of
from having peripheral stabilimentum tufts. stabilimenta even on these relatively small
This gives a second reason under the warning webs could be explained by the warning hy-
hypothesis for expecting more stabilimentum pothesis. The impact of a large animal might
tufts on the webs of larger spiders, such as we be especially dangerous to a spider in its rel-
found. atively defenseless condition around the time
454 THE JOURNAL OF ARACHNOLOGY
ofmolting. This consideration makes the pres- 2004-2). We thank the OTS and all members
ence ofwell developed stabilimenta on resting of the course, specially the coordinators Dr.
webs even more difficult to explain under the Alejandro G. Farji-Breeer and Dr. Gilbert Ba-
prey attraction hypothesis, as the impact of rrantes, and two referees for valuable com-
prey could also be dangerous for the spider. ments. We also thank Doe Pedro Caspar for
The prey attraction hypothesis could be saved permitting us to work in his palm plantation.
from these problems by ad hoc adjustments LITERATURE CITED
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mentain orb webs attractprey ordefend spiders?
spider). — Behavioral Ecology 10:372-376.
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be confused with them (white spiders). The interdependence between web architecture and
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Eberhard, W.G. 2003. Substitution of silk stabili-
However, the logic of this test depends on the menta for egg sacs by Allocyclosa bifurca (Ara-
spider’s ability to adjust its behavior in terms neae: Araneidae) suggests that silk stabilimenta
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an araneid has been shown to be able to sense vipalpis and Argiope trifasciata in West Africa,
its own color. This test also depends on the with a discussion ofthe function ofstabilimenta.
colors detectable by potential predators; for Pp. 61-72. In Proceedings of the Ninth Interna-
dsiofmfeereanntifmarlosmtthhee wwhhiittee sciollkortaufttisonmoafytaheppsepair- t(iWo.nalEbCerohnagrrde,ssY.ofLuAbriacehn&oloB.gy,RobPiannsaomna, e1d9s8.3)..
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clude that our data do notjustify a certain re- mechanisms of orb weavers (Araneae: Aranei-
jection of this hypothesis. dae) in Ghana, West Africa. Pp. 73-89. In Pro-
The camouflage hypothesis might explain ceedings of the Ninth International Congress of
the reduction of stabilimenta in small webs, if Arachnology, Panama 1983. (W. Eberhard, Y.
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Hauber, M.E. 2002. Conspicuous colouration at-
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JAFFE ET AL„--POSSIBLE FUNCTIONS OF SILK STABILIMENTA 455
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