EFFECTS OF PREY CAPTURE, WEB DESTRUCTION AND HABITAT PHYSIOGNOMY …
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Enders, F. 1976. Effects of prey capture, web destruction and habitat physiognomy on web-site
tenacity of Argiope spiders (Araneidae). J. Arachnol. 3:75-82.
EFFECTS OF PREY CAPTURE, WEB DESTRUCTION AND
HABITAT PHYSIOGNOMY ON WEB SITE TENACITY
OF ARGIOPE SPIDERS (ARANEIDAE)
Frank Enders1
North Carolina Division of Mental Health Services
Research Section
Raleigh, 27611
ABSTRACT
Both in the laboratory and in the field prey capture did not have a strong influence upon web-site
tenacity of Argiope aurantia. But experimental web destruction increased the probability that A.
aurantia changed m web-site, perhaps only due to the physical displacement of the spider. Removal ^
of vegetation near the web of immature A. aurantia resulted in most of these spiders leaving their
web-sites, especially in areas less sheltered from the wind. Argiope trifasciata, in contrast, did not leave
web-sites after removal of nearby vegetation.
INTRODUCTION
Spiders often remain at the same web-site from one day to the next (McCook, 1889;
Enders, 1975). Yet, little is known which factors might influence the probability that a
spider will stay at a particular site. “Web-site tenacity” is defined as the per day proba-
bility that a spider remains at the same web-site, or the number of changes of web-site
divided by the number of observations of webs from one day to the next (Enders, 1975).
The total number of changes of web-site includes animals found again nearby and also
those which both take up their web and disappear from view. Thus animals which appar-
ently have died are excluded from the calculation, since mortality of Argiope spiders is
normally marked by the disappearance of the spider coupled with the persistence of the
old web.
The initial and subsequent choices among habitats by the web spider Argiope aurantia
(Araneidae) have been described (Enders, 1973). And some speculation is available
regarding the use of prey and habitat as resources by various araneid orb web spiders
(Enders, 1974,1975b). Turnbull (1964) reported a strong effect of prey abundance on
web-site tenacity of Achaearanea tepidariorum (Theridiidae). But other studies (Arane-
idae: Cherrett, 1964; Coleboume, 1974; spiders in general: Duffey, 1966) have empha-
sized the greater importance of habitat structure (physiognomy or architecture) for selec-
tion of web-sites by spiders. Field observations of Argiope aurantia (Enders, 1975a)
revealed no marked influence of prey capture on web-site tenacity. Here, I report my
experimental studies which estimate the relative importance of prey capture, web destruc-
tion and habitat physiognomy on web-site tenacity of Argiope aurantia. I include a few
Present address: Biology, California State University, Fresno, 93740
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observations on A. trifasciata for comparison with a species which selects less densely
vegetated habitats than A. aurantia.
FIELD EXPERIMENTS ON THE EFFECT OF FEEDING
Methods—The study areas used were the edges of road cuts, where large numbers of
Argiope aurantia occurred, near Raleigh, North Carolina. Areas were dominated by the
herbaceous perennial plant Lespedeza cuneata and are described in greater detail else-
where (Enders, 1972; 1974).
Different feeding rates were maintained for three groups of spiders in the field: (a) “no
prey,” by removing any prey noticed in the web; (b) natural feeding rate, or whatever
entered the web by itself; and (c) prey always present in the web, by adding to what the
spider captured, roughly tripling the intake of prey biomass from group b. Spiders were
originally assigned to treatments alternately. As the original spiders disappeared from
particular treatment groups, the nearest available unmarked spiders were used as replace-
ments.
Treatments were applied twice a day, late morning and late afternoon (eve-
ning). Insects added to the web were usually grasshoppers slightly longer than the
spiders, or else several houseflies. Since most natural prey (the most abundant, bees,
grasshoppers and chauliognathid beetles) were kept in the web at least half a day, and
since virtually no prey was taken during the night by Argiope spiders, the treatment
schedule should have been effective to influence spider feeding rates. Two replicates of
this experiment were performed, one during the period 22 to 25 June 1970 (using middle
stage immatures) and the other 6 to 11 September 1970 (adult spiders). One additional
experiment was done feeding spiders water sweetened with table sugar (Bays, 1962), but
the negative results of that feeding replicate might be due to insufficient caloric uptake
by spiders, even though the sugar water was accepted by them.
Results—Different levels of feeding could not be maintained every day because spiders
occasionally refused to attack any insects offered. This occurred primarily in the June
replicate. Analysis of results using only the actual feeding status of the spider did not
change the conclusions. Only one statistically significant effect was found in eight statis-
tical comparisons made (by chi square test, Snedecor and Cochran, 1967). The extreme
comparison between prey removed and prey added groups for the September experiment
indicated a 7% increase of web-site tenacity (Table 1), with p between 0.05 and 0.025.
LABORATORY EXPERIMENT ON THE EFFECT OF FEEDING
Methods—A cage was made 2.3 m high, 2.3 m wide and 4.6 m long from translucent
plastic sheets stapled onto an exterior 5 cm X 5 cm wood frame. This cage was sealed by
plastic tape along the seams, and the only entrance was a zipper sewn into one edge of the
cage. The zipper was opened only once a day, in order to give the spiders water from a
syringe and to feed them. The room containing the cage had a photophase of 16 hours,
and an air conditioner running for three hours during the morning to provide a regular
cycle of temperature.
Four marked (with fast-drying paint) A. aurantia taken from the field were released on
successive days in different corners of the cage, starting on 1 July 1970. The spiders
climbed to the top of the cage along the tape and built webs in the upper corners of the
cage. Two spiders could and sometimes did build webs in the same corner.
ENDERS-WEB-SITE TENACITY OF ARGIOPE
77
Table 1.-Summary of feeding experiments with Argiope aurantia        in the field. Web site tenacity is
the percentage probability a spider remains at the same web-site from one day to the next.
Web-site tenacity
Treatment    From initial day to the    % of all observations
Group    second day of observation    of which animals remained at
(% of individuals)    same site
Prey removed    90 (n=l)    90 (n=90)
Whatever spider caught
by self (control)    96 (n=28)    87 (n=87)
Prey added    95 (n=20)    93 (n=93)
The cage was centered below the lighting fixture which had 320 watts of flourescent
lighting. The entrance of the cage was away from the single boarded-up window, but near
the door of the room. Only those spiders which built webs in the front right or back left
comers were fed, a housefly a day. This arrangement neutralizes the effect of any gradi-
ents of light, noise, etc., which might have influenced preference for the corners in
consequence of the location of door, light, window, and window air conditioner.
Results—Not even a small increase of web-site tenacity with prey catching was ob-
served. Additional spiders in individual cages and a second four-spider replicate in the
large cage which lasted only 20 days also revealed no differemce in web-site tenacity of A.
aurantia in fed and in unfed corners. Instead, spiders moved out of corners in which they
had been getting flies, as well as moving into them. In the course of the completed
four-spider experiment, one spider was eaten by another, two emaciated spiders starved
to death, and one well-fed spider died after several months on its web. In addition those
spiders, including two A. trifasciata, that were starved but watered regularly did not show
any decrease in web-site tenacity with time. Starvation did result in a reduction of
frequency of renewal of webs as animals were near death.
EFFECT OF WEB DESTRUCTION AND OF DISTURBANCE IN ARGIOPE AURANTIA
Methods—This experiment was performed at the edges of lespedeza-covered road
cuts. The treatment was total destruction of the web each day, while the spider was left
wherever it went. The spider’s dragline which had been attached to the web was
destroyed, so that no silk spanned the original web-site, but the spider was left on the
vegetation whenever possible (most instances). The disturbance treatment is that certain
nearby spiders were placed into individual jars, carried to the laboratory, taken from the
jars, weighed, transported back to the web-site, and released in their original webs. Treat-
ments were applied just after dark, and the spiders of the disturbance group were
returned to their webs after 2-3 hours. Every third spider found was placed in the same
treatment group (web destroyed, disturbance and control). Each spider was marked with
an individual pattern of rapidly drying paint, and was retained in its treatment group if it
could be found the following day, at the old web-site or at a new one (web sites were
marked with masking tape). Due to the disappearance of the original members of the
groups, more spiders were added to each group on subsequent days. All spiders used in
this experiment were females, mostly fully adult, from 21 September to 7 October 1969.
Chi square not corrected for continuity (Snedecor and Cochran, 1967) was used to test
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for statistical significance of treatment effects.
Results—No significant effect of the handling disturbance was found (Table 2). Those
animals whose webs were destroyed left web-sites significantly more often than the
controls, both the night following destruction of the web, and also on subsequent nights
when webs happened not to be destroyed.
Table 2.—Web-site tenacity of Argiope aurantia in the field after web destruction and after han-
dling disturbance (** = difference with control group statistically significant at 0.01 level; * =
difference with control group statistically significant at 0.05 level).
Web-site tenacity
Treatment group From    initial i    day to the    ‘■ % of all observations
second    day of    observation    which animals remained at
Eli    of individuals)
Web destroyed    33    (24)**    50    (54)**
Undisturbed (control)    91    (ID    71    (52)
Animal handled, web not
destroyed    82    (17)    78    (45)
Dates on which web-destroyed
animals were not disturbed    40    (10)*    54    (13)
Dates on which handled animals
were not disturbed    88    ( 8)    88    ( 8)
EFFECT OF VEGETATION DENSITY ON ARGIOPE SPIDERS
Methods—Enders (1973) hypothesized that it was the density of the nearby vegetation
and plant density in the plant community as a whole (habitat physiognomy) which
controlled the occurrence of A. aurantia immatures, but not the occurrence of A.
trifasciata. To test this, in July 1971 all vegetation was cut away in a band from 20 cm to
100 cm around the webs of spiders in the field. Bushes and branches of large trees to a
distance of 4 m were also removed. Vegetation to which silk was attached was not
removed, and, as in other experiments, I made a particular attempt not to disturb or
damage the web or Ss inhabitant. As in other experiments, animals were used as they
were found, with no exclusions. After initial experiments indicated color-marking to be
superfluous, spiders were left unmarked. The location of the web was marked with
masking tape, and the experimental site was also quite noticeable, in consequence of
vegetation removal.
Results were planned to be compared with the known web-site tenacity of 80+ per
cent (Enders, 1975a). In addition, three Argiope aurantia were left undisturbed at one
study site to check that high web-site tenacity of undisturbed animals. The spiders used
in this experiment were middle stage immatures, mostly being the sixth and seventh
instars.
Two main study sites were used, one an old-field planted with pine trees and the other
the center of a lespedeza-covered road cut. Within the old-field site two subsites were
used, one a location with sparse vegetation with the nearest trees 5 m away; the second
subsite had pine trees within 5 m of one another, that is, roughly four times the density
of vegetation.
The old-field subsite with less vegetation probably had the greatest exposure to
wind. The old-field subsite with more trees was expected to have less wind, and the road
ENDERS-WEB-SITE TENACITY OF ARGIOPE
19
cut could be assumed to be the most sheltered at the height where immature A. aurantia
build webs (Enders, 1974). The latter study area was entirely protected from wind on
one side by the upward slope of a hill; and this site was also sheltered even on the
downhill side by vegetation which was considerably denser than the old-field vegetation
present at the other experimental site. Some trees were present at about 10 m further
uphill.
Results—Removal of vegetation greatly reduced web-site tenacity of the immature A.
aurantia. This reduction of web-site tenacity was statistically significant, whether one
used as control the three animals observed the same year (none of which changed web-
site), or the 83% web-site tenacity for Argiope aurantia in the lespedeza area in July of
the previous year (Enders, unpublished data). Casual observation of untreated animals
nearby and of post-treatment spiders also indicated a high web-site tenacity of animals
living in the old-field site.
The effect of physiognomy of the study site was also statistically significant and of
large magnitude: none of 13 experimental animals in the weedy old-field remained on the
following day, 44% of nine remained in the old-field with denser trees, and 63% of 19 in
the lespedeza-covered road cut. Since the old web of spiders which disappeared could not
be found and since several spiders which left experimental web sites were found nearby
after the experimental treatment, those spiders which did not remain had apparently left
the web-sites for other locations.
Finally, there was also a statistically significant difference between the species A.
aurantia and A. trifasciata: records showPMt seven immature A. trifasciata had vegeta-
tion removed from around their web-sites at lespedeza (three animals) and old-field (four
animals) areas, and no spider changed web-site or disappeared.
DISCUSSION
Different ecologically definable groups of spiders have various manners of hunting, but
most spiders are sit-and-wait predators (Enders, 1975b). Exceptions are known primarily
in errant, non-web spiders (chiefly clubionids and salticids; also smaller lycosids). Though
web spiders are restricted to the web, even such species may effectively search for prey if
they change web-site until they encounter a web-site with sufficiently high prey capture
rate (Turnbull, 1964). My results detailed above suggest that prey capture has no such
effect in the orb-weaving spider Argiope aurantia: field experiments do indicate the
possibility of small 7% (but compounded daily) increase in web-site tenacity of mature A.
aurantia, as a consequence of a range in prey capture rate equal to three times normal
feeding rates, compared to virtually zero in the comparison group. This effect, while
statistically significant (0.05 level) may be a purely random statistical effect (p actually
only 0.4, considering eight separate statistical contrasts made by me using 0.05 level of
probability as criterion), or the result of partial destruction of webs (see below) during
removal of prey items from webs of the comparison group. I emphasize that field obser-
vations (three summers) and laboratory experiments (detailed above) give no support to
the idea that web-site tenacity might be related to prey capture rate in Argiope aurantia
(Araneidae). In other species of orb web spiders, Cherrett (1964) and Eberhard (1971)
found, respectively, no relation of prey capture to web site use (several araneid species),
and a negative effect of prey capture of web-site tenacity (one uloborid species; uses orb
web made of different type of sticky silk). Therefore, it appears that these orb web
spiders are not normally limited by prey abundance, so that they have not evolved a
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positive behavioral response to capture of prey. In contrast, Achaearanea tepidariorum
(Theridiidae, three-dimensional web) appears to live in areas where prey are sometimes
locally limiting, since most houses (natural habitat for this species) apparently have a high
variance and low mean of insect abundance (potential spider prey). Houses probably also
offer a lower density of potential predators on the spiders, when the spider is off the web
and moving from one web-site to the next. The level of selectivity for web-site, I argue, is
determined by a balance between increased predation rate on web spiders off the web and
the increased prey intake possible at a better web-site (see Morton, 1971; Safriel,
1972). Increased predation on web spiders when they are off the web is logically
deduced from the many anti-predator advantages afforded by the use of a web, such as
early warning of attack, familiarity with terrain, and the greater number of escape
maneuvers compared to non-web spiders (shaking the web, running on the web, and
jumping off the web). Robinson and Robinson (1973) have indicated they consider the
use of a web by molting spiders to be related to the anti-predator advantages of a
web. Thus, the increased rate of change of web-site in unfed immature Achaearanea
tepidariorum (Turnbull, 1964) may be a special case among web spiders. Data on this
species’ unusually high metabolic rate (Anderson, 1970) and large clutch size (relative to
the female’s length, and relative both to other Theridiidae and to most spiders; Enders,|
1976b), seem to support this interpretation.
Removal of nearby vegetation here resulted in a greatly reduced web-site tenacity of
immature Argiope aurantia, but not of A. trifasciata. This response is part of the
behavior, apparently in response to wind (Enders, 1972) which brings the former species
to the immature’s species-typical web-site near the ground (Enders, 1974) in dense vegeta-
tion (Enders, 1973). Wind reduction near ground level is discussed by Gloyne (1964).
Witt and Reed (1968) and Anderson (1974) find web spiders able to conserve web size
for several weeks and to live several months, without food. Therefore, it appears that
habitat structure rather than food is the proximate factor involved in selection of web-site
by most web spiders. Turnbull (1964) did not investigate the influence of physical struc-
ture (physiognomy) of the habitat. My results support the idea that habitat physiognomy
may be as important to predatory arthropods (Duffey, 1966; Elton, 1966; Coleboume,
1974) as it is to (predatory) vertebrates (Klopfer, 1965; Wecker, 1963; Sale, 1969). Wind
may be an important proximate factor for vertebrates also, as in the selection of patches
of habitat by grassland birds (Cody, 1964, pp. 25, 70).
After experimental destruction of the webs of Argiope aurantia, the spiders moved to
some place on the vegetation and remained still thereafter for a long time, often till
dusk. “Natural” apparent destruction of webs of this species in the field was observed
very rarely. My observations to date indicate that web-site selection by araneid spiders is
a fairly undirected process, and may be unguided by previous experience with a
site. Specifically, Argiope spiders removed by me from their webs (or placed in a differ-
ent web from which the occupant had been removed) to only a meter away were never
found to return to the original web and web-site. Burrowing spiders (mygalomorphs and
lycosids) seem to have some knowledge of their surroundings (Kuenzler, 1958), and
araneid spiders are able to find their way about on the web itself (Le Guelte, 1969) to
their retreat. But since araneid spiders cannot be said to have any home range beyond the
confines of the web, the simplest interpretation of the effect of web destruction upon
web-site tenacity of A. aurantia is that the spider simply did not happen upon the same
web-site when it began to put down the frame of the new web. Curiously, one web
spider, Agelenopsis aperta (Agelenidae; Riechert, Reeder and Allen, 1973) can be flushed
ENDERS-WEB-SITE TENACITY OF ARGIOPE
81
some distance from its web. But that family of spiders is also intermediate to the non-
web spiders both in clutch size and Dyar’s constant (growth increment at the molt;
Enders, 1976a).
It is interesting that Argiope aurantia shows a reduction of web-site tenacity after web
destruction, for the period of a few days after treatment has stopped (Table 2). Perhaps
the spiders are unable to locate a suitable web-site the first night, but that is doubt-
ful. The time scale of a reduction of web-site tenacity seen after molting is also several
days (Enders, 1975a). An errant spider, Lycosa nordenskioldi (Lycosidae) also shows
increased locomotion, but for only a few hours, and after being merely picked up
(Enders, unpublished data). Thus, it appears that spiders show increased locomotion as a
response to appropriately serious disturbances, such as web destruction (but not han-
dling) in the case of a web spider, and handling, in the case of a lycosid species; and this
response is on the order of days, for web spiders which can change web-site once a day, or
hours, for errant spiders, which can walk at any time during their period of activity.
ACKNOWLEDGMENTS
This research was in part supported by NSF Grant GB-6246 to P. N. Witt, and is part
of a thesis carried out under his guidance for the Ph.D. degree in Zoology at North
Carolina State University. During preparation of the manuscript, the author was sup-
ported by NSF Grant GB-27152 to W. F. Blair. H. W. Levi, B. S. Gerschman de Pikelin
and R. D. Schiapelli identified species mentioned herein. S. E. Riechert and J. S. Rovner
made helpful comments on the papers.
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