Table Of Content1998. The Journal ofArachnology 26:101-105
STABILIMENTUM-DECORATED WEBS SPUN BY
CYCLOSA CONICA (ARANEAE, ARANEIDAE) TRAPPED MORE
INSECTS THAN UNDECORATED WEBS
I-Min Tso': Museum of Zoology and Department of Biology, University of Michigan,
Ann Arbor, Michigan USA
ABSTRACT. In this field study, I tested the insect-attraction hypothesis as one of the functions of
stabilimenta spun by Cyclosa conica (Pallas 1772) by examining: (1) if stabilimentum-decorated webs
trapped more insects, (2) if a larger web diameter was responsible for the higher insect-trapping rate in
decorated webs and, (3) if the differential distribution of insects in spiders’ habitats was responsible for
the higher insect interception rates of decorated webs. The number of wrapped prey, web diameters and
presence ofstabilimenta was recorded daily from 13 web locations. The stabilimentum-decorated webs of
C, conicatrapped significantly moreinsects (150% more)thanundecoratedwebs,buttheyhadsignificantly
smaller mean web diameter (19% smaller). Among web locations, there was no significant difference in
their insect interception rates, whether the data were collected from decorated orundecorated webs. These
results suggest that the higher insect-trapping efficiency of decorated webs spun by C. conica resulted
from the presence of stabilimenta, instead of from larger web diameters or differential distribution of
insects.
Stabilimenta are silky structures on the Rogg 1988; Eberhard 1990), Most ofthe func-
webs of some diurnal orb-weaving spiders. At tional studies on silk stabilimenta have fo-
least 17 genera of ecribellate and cribellate cused on Argiope species, which spin linear
orb-weavers build various forms of stabili- silk bands arranged either vertically (e.g., A.
menta (Eberhard 1990). In most ofthe genera, aurantia (Lucas 1833) andA. trifasciata (For-
stabilimenta are made up entirely of bands of skal 1775)) or diagonally (e.g., A. argentata
silk that either encircle the hub (e.g., Lubinel- (Fabricius 1775)) around the hub (Levi 1983).
la morobensis Opell 1984 and Philoponella Investigators have proposed and tested many
sp., see Lubin 1986) or are located at various hypotheses about stabilimenta’s possible func-
positions around the hub (e.g., all Argiope tions, such as web advertisement, predator de-
species, see Levi 1983). Some spiders also in- fense, web tension adjustment andproductun-
corporate egg sacs, prey remains and/or detri- der stress (see review in Nentwig & Rogg
&
tus into the silk bands (e.g., Cyclosa octotu- 1988; Eberhard 1990; Schoener Spiller
berculata Karsch 1879, see Yaginuma 1986), 1992 and Kerr 1993).
which make the spiders difficult to detect Recently, insect-attraction has been dem-
among those objects. onstrated to be one ofthe functions ofArgiope
The function of silk stabilimenta have long spiders’ silk stabilimenta. Diagonally arranged
been a focus of study for arachnologists. For silk stabilimenta of Argiope argentata were
those genera that incorporate other objects found to reflect ultraviolet-light, and the sta-
into the silk bands, the function ofstabilimen- bilimentum-decorated webs of those spiders
ta has generally been hypothesized as cam- intercepted more insects than undecorated
ouflaging (Eberhard 1973). As to the silk sta- webs (Craig & Bernard 1990; Craig 1991).
bilimenta, ever since Simon introduced this The webs of Argiope trifasciata decorated
term in 1895 suggesting a web-stabilizing with vertically-arranged stabilimenta were
function (Robinson & Robinson 1970), many also found to trap more insects than undecor-
functional hypotheses have beenproposed and ated webs (Tso 1996). These findings lead me
tested (Nentwig & Heimer 1987; Nentwig & to hypothesize that insect-attraction may also
be one ofthe functions ofthe silk stabilimenta
'Current address: Dept, of Biology, Tunghai Uni- built by other orb-weaving spiders, such as
versity, Taichung 407, Taiwan, R.O.C. Cyclosa species. The functions of the linear
101
102 THE JOURNAL OF ARACHNOLOGY
stabilimenta build by Cyclosa species have orated webs. However, in addition to stabil-
not been investigated. Although Rovner imenta, the size and the location ofa web may
(1976) studied how the position of silk stabi- also affect its DIIR. Previous studies suggest-
limenta and gravity affect wrapped prey ed that larger webs may potentially trap more
placement on the web of Cyclosa turbinata insects than smaller webs (Brown 1981; Craig
&
(Walckenaer 1841), he did not provide an- 1989; Higgins Buskirk 1992). Because of
swers regarding the possible functions of the the heterogeneous distribution of insects, the
silk stabilimenta of those spiders. To deter- location of a web may also greatly affect its
mine the insect attraction ability of Cyclosa insect trapping ability (Craig 1989). There-
stabilimenta, I conducted a field study exam- fore, I tested the insect-attraction hypothesis
ining whether or not the presence of silk sta- by comparing (1) the DIIRs between stabili-
bilimenta increases the insect-interception of mentum-decorated and undecorated webs, (2)
webs spun by Cyclosa conica (Pallas 1112). the difference in web diameter between dec-
METHODS orated and undecorated webs, and (3) DIIRs
of the same type of web (decorated or unde-
Cyclosa conica (see Levi 1977) builds webs corated) collected from different web loca-
between dry tree branches in the dim forest tions. The insect-attraction hypothesis can be
understory. Only adult female spiders were supported if (1) the decorated webs intercept-
used in this study since mature males C. con- ed more insects, (2) the decorated webs were
ica do not build as full a web as did females no largerwebs than undecorated webs, and (3)
(Kaston 1948). Sometimes spiders added a the average insect trapping rates of the same
stabilimentum made of white silk band on type of webs did not differ between various
their webs, making the webs relatively easy to web locations. —
be located by researchers. Cyclosa conica Census methods. Web locations of C.
have been reported to load the stabilimentum conica were marked by fastening green tape
with prey pellets, plant detritus or, in subse- on the tree trunk a meterbelow the web. Webs
quent webs, egg sacs (Comstock 1913; Mar- from all locations {n = 13) were monitored
pies & Marples 1937). However, the C. conica each day between 0800-1800 h. The number
population at this study site seldom retained of days those web locations remained occu-
the old stabilimenta. Instead, for all the spi- pied ranged from 5-13 days. Web diameter
ders, stabilimentum-decorated webs werebuilt (cm) and presence of stabilimenta were re-
interspersed with undecorated webs. Cyclosa corded once at 0800 h. The number of
conica might have recycled their orb each day wrapped prey per web per 10 hours of obser-
because the web diameters as well as number vation (between 0800-1800) was used as an
and location of wrapped prey recorded from estimate of DIIR, and the webs were moni-
the same web site varied from day to day. Cy- tored three times a day. I also mapped the po-
closa conica have also been reported to in- sition ofwrapped prey on webs to check ifthe
corporate wrapped prey into stabilimenta spiders reused the old web, thus recounting of
(Marples 1969; Levi 1977); but those in my the previously wrapped prey was avoided. In
study frequently left the wrapped prey where most cases there was no wrapped prey on
the insects were intercepted on the web. The webs at the time of web diameter measure-
recorded position and number of wrapped ment. A total of 93 DIIRs was collected,
prey on webs indicated that spiders seldom among them 24 were from decorated webs
retain wrapped prey and stabilimenta. Among (from seven locations; webs from six other lo-
the 24 decorated webs recorded, only four of cations were all undecorated) and 69 were
them contained wrapped prey in the stabili- from undecorated webs (from all 13 loca-
menta. tions).
—
Tests.- This study was conducted in June Statistical analysis.—To examine the ef-
and July, 1992, at the University of Michigan fect of stabilimenta, I used a Mann-Whitney
Biological Station near Pellston, Michigan. I f/-test to compare the DIIRs collected from
tested insect-attraction as one ofthe functions decorated and undecorated webs. I also used
of stabilimenta spun by C conica by compar- a Mann-Whitney U-test to compare the web
ing the daily insect interception rates (DIIRs) diameters between two types of webs. Rrus-
between stabilimentum-decorated and undec- kal-Wallis one way ANOVAs were used to
m
TSO—STABILIMENTA OF CYCLOSA CONICA 103
Decorated Undecorated
Decorated Undecorated
Webtypes Webtypes
— —
Figure 1. Means (± SF) of daily insect inter- Figure 2. Means (± SF) ofweb diameters (cm)
ception rates (number ofwrapped prey per web per of the stabilimentum-decorated and undecorated
10 hours ofobservation) ofthe stabilimentum-dec- webs spun by Cyclosa conica (Pallas).
orated and undecorated webs spun by Cyclosa con-
ica (Pallas).
suited from the presence of stabilimenta, rath-
er than from the difference in web diameters
determine if differences existed between (1) or the differential distribution of insects
DIIRs ofdecorated webs collected from seven among web locations.
locations and (2) DIIRs of undecorated webs DISCUSSION
ANOVA
collected from 13 locations. In both
analyses, web locations were used as catego- The hypothesis that presence of stabilimenta
ries to sort DIIRs collected. By performing increased insect interception of webs spun by
two Kruskal-Wallis ANOVAs on DIIRs col- Cyclosa conica was supported by the results.
lected from two types of webs, the effect of Decorated webs intercepted almost 150% more
web location on insect interception can be insects than did undecorated webs. The higher
separated from the effect of the stabilimenta, DIIR of decorated webs seemed to result from
since only one category was used in each presence of stabilimenta, instead of from size
ANOVA analysis. variation between two types of webs or from
differential insect distribution between web lo-
RESULTS
cations. Moreover, the average web diameter of
Decorated webs spun by Cyclosa conica in- decorated webs was significantly smaller than
tercepted significantly more prey than undec- that ofundecorated webs, and prey interception
orated webs (Mann-Whitney U statistic = did not differ between different web locations.
389.0, P — 0.000, Fig. 1). Although decorated Compared to similar studies on other stabili-
webs contained almost 150% more wrapped mentum-building taxa such as Argiope argen-
prey than undecorated webs, their web diam- tata (31.3%, Craig & Bernard 1990) midArgio-
eters were significantly smaller by 18.9% pe trifasciata (72% more flying insects, Tso
(Mann-Whitney U statistic = 1156.0, P = 1996), this gain in insect interception is exceed-
0.004, Fig. 2). Although the web location was ingly high.
known to affect its trapping efficiency, deco- The higher trapping efficiency and the
rated as well as undecorated webs at different smaller diameter of decorated webs spun by
locations trapped similar numbers of insects. Cyclosa conica provides an important insight
There was no difference in DIIRs ofdecorated to the foraging ecology of this orb-weaving
webs collected from seven web locations spider. The size of an orb web, in addition to
(Kruskal-Wallis statistic = 9.363, df= 6, P = other web characteristics, is known to affect
0.154), nor was there difference in DIIRs of its insect trapping efficiency. Studies on sev-
undecorated webs collected from 13 web lo- eral orb-weaving spiders demonstrated that
cations (Kruskal-Wallis statistic = 1.727, df= larger webs tended to trap more prey (Brown
12, P = 0.806). These results suggested that 1981; Craig 1989; Higgins & Buskirk 1992).
the difference in the number of wrapped prey Recent studies further demonstrated that some
between decorated and undecorated webs re- orb-weaving spiders may manipulate their orb
104 THE JOURNAL OF ARACHNOLOGY
size when prey intake varies. Sherman (1994) may allow approaching insects less time to
reported that Larinioides cornutus (Clerck avoid the web, therefore leading to higher in-
1757), while maintaining same mesh size, de- sect interception.
creased web diameters afterfood-satiation and The results from this study suggest that the
increased web diameter when experiencing a presence ofstabilimentacanpotentially increase
&
long period of hunger. Higgins Buskirk the foraging efficiency ofCyclosa conica. How-
(1992) demonstrated that Nephila clavipes ever, one important question still remains un-
(Linnaeus 1767) built larger webs in habitats answered. That is, given the gain in prey intake
of lower prey abundance. Although some of generated by stabilimenta, why do C conica
the studies did notconsiderthe potential effect and Argiope spiders not always build stabili-
of other web characteristics, they did indicate mentaontheirwebs?The study by Craig (1994)
that orb size must be considered when eval- on Argiope argentata provided an evolutionary
uating the prey interception of orb-webs.
solution to the riddle of inconsistency in stabi-
However, in this study the average web di-
ameter of decorated webs was almost 20% limentum-building. Craig (1994) demonstrated
that the highly unpredictable pattern and build-
smaller than that ofundecorated webs, but the
average prey interception rate was 150% ing frequency of stabilimenta could prevent hy-
menopteran insects from learning from past ex-
more. This result suggested that in the future
study of foraging ecology using orb-weaving perience to associate stabilimenta with danger.
spiders, in addition to the commonly known Craig (1994) suggested that a consistent build-
web characteristics such as orb size, mesh size ing of stabihmenta (in both shape and frequen-
and web location, silk stabilimenta (if exhib- cy) was disadvantageous to Argiope spiders be-
ited by the taxa studied) should also be in- cause some insects could learn from past
cluded in the analysis. experience to associate stabilimentawith danger
The effectiveness of stabilimenta built by and would actively avoid decorated webs in fu-
Cyclosa conica in attracting prey greatly ex- ture encounters. Craig (1994) thus provides a
ceeds that of Argiope spiders investigated so possible evolutionary explanation (an ultimate
far, which may result from the different types factor) for the inconsistency in stabilimentum-
of habitats occupied by the spiders. Cyclosa building. However, although Eberhard (1973)
conica typically build their webs in the dif- and Lubin (1986) provided evidence that light
ferentially shaded forest understory in which intensity may affect stabilimentum-building of
the light intensity is dim (Marples & Marples nocturnal uloborids, how stabilimentum-build-
1937; Levi 1977). In contrast,—Argiope spiders ing is proximately controlled in diurnal orb
tend to choose an—open field a very bright weavers is not clear. Edmunds (1986) and Nen-
light environment as web sites (Levi 1968). twig & Rogg (1988) examined the effect ofmi-
C
The insects available to conica are mostly croclimatic conditions, habitat type, web char-
small dipterans and hymenopterans (collected acteristics, presence ofmales, illumination, prey
from sticky traps, Tso unpubl. data), charac- abundance, ecdysone and even heredity on sta-
terized by high flight maneuverability and the bilimentum-building ofvariousArgiope spiders.
capability of detecting and avoiding spider
But none of the factors examined could signif-
webs (Craig 1986). However, those insects re-
icantly affect the building of silk stabilimenta.
spond quite differently to spider webs hanging
in different light environments. Webs in the Although this study demonstrates that silk sta-
dim forest understory are less visible to those bilimenta may increase spiders’ foraging, the
insects, making the webs more difficult to lack of knowledge regarding ecological factors
avoid than those in the bright open field controlling the building of stabilimenta (which
(Craig 1988). The dim light environment, plus result in the role stabilimenta play in the ecol-
the extremely fine silk characteristic of C. ogy of spiders unclear) greatly reduces the va-
conica (Comstock 1913; Marples & Marples lidity of prey-attraction hypothesis. Therefore,
1937), may make the webs difficult to detect the identification of proximate factors control-
by those insects (Craig 1986). Although the ling stabilimentum-building is essential to fully
decorated webs of both Argiope and Cyclosa realize how silk stabilimenta is involved in the
spiders can attract insects to orient toward ecology ofmore than 17 genera oforb-weaving
them, the lower web visibility of the latter spiders.
TSO—STABILIMENTA OF CYCLOSA CONICA 105
ACKNOWLEDGMENTS & A
Higgins, L.E. R.E. Buskirk. 1992. trap-build-
This study was funded by a University of ing predator exhibits different tactics for differ-
ent aspects of foraging behavior, Anim. Behav.,
Michigan Biological Station Grant-in-Aid. I 44:485-499.
thank the staff of the University of Michigan Kerr, A.M. 1993. Low frequency of stabilimenta
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plies and information about study sites. Drs. neidae) from Guam: an indirect effect of intro-
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