Avshalom Konigswald, Yael Lubin, and David Ward.
The effectiveness of the nest of a desert widow spider, Latrodectus revivensis, in predatory deterrence.
Psyche 97(1-2):75-80, 1990.

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THE EFFECTIVENESS OF THE NEST OF A
DESERT WIDOW SPIDER, LATRODECTUS REVIVENSIS, IN PREDATOR DETERRENCE
Most nocturnally-active arthropods rest during the day in con- cealed and protected locations. The nests or retreats of nocturnal spiders are often in vegetation or beneath stones. The widow spider Latrodectus revivensis of the Negev desert of Israel (Levy and Ami- tai 1983), builds a conspicuous, cone-shaped nest in bushes up to 1 m high (Shulov 1948, Lubin et al. in prep.). The nest is attached by bridging threads to a horizontal capture platform, which in turn is attached to nearby stones or shrubs (Fig. 1). The most notable feature of the nest is the array of extraneous material, mainly arthropod carapaces, snail shells and feces, and dried plant material, placed on the outside. This material obscures the spider from view even in the early stages of nest construction when the dense silk layer of the upper portion of the cone (Fig. 1) has not yet been laid down.
There may be several functions of this layer of extraneous mate- rial, namely:
(a) to shade the spider, and its eggs and young, from the high insolation experienced in the desert in summer, (b) to provide mechanical protection from predators by strength- ening the nest,
(c) to make the nest cryptic,
(d) to obscure the spider from the gaze of predators. We have examined elsewhere (Lubin et al., in prep.) the function of the extraneous material on the nest in the regulation of tempera- ture inside the nest. We found that this material plays a significant Department of Biology, Ben Gurion University of the Negev, Beer Sheva, Israel. 'Mitrani Center for Desert Ecology, Jacob Blaustein Institute for Desert Research, Ben Gurion University of the Negev, Sede Boqer Campus 84993, Israel. *To whom correspondence should be addressed. Manuscript received by the editor December 8, 1989. Pachc 97:75-80 (1990). hup Yipsychc einclub orgOTi97-075 html



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Figure 1.
Web of Latrodectus revivensis consisting of (a) nest with dense, debris-covered upper portion and open-mesh, lower portion, (b) capture platform, and (c) vertical capture threads which are sticky near their attachment to the ground. At night the spider sits under the capture platform to monitor the sticky threads.



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19901 Konigswald, Lubin & Ward-Latrodectus 77 role in reducing the temperature of the nest, and of the spider, during the summer by shading the nest.
We then turned to the role of the nest covering in predator avoid- ance and deterrence. Natural history observations of the nests of L. revivensis indicated that the visually-orienting Great Grey Shrike, Lanius excubitor, was an important predator of this spider. We set out to test the ability of the Great Grey Shrike to detect and capture the spiders in nests with different amounts of extraneous material. We captured 4 adult Great Grey Shrikes in the vicinity of Sede Boqer, Israel (30å¡52'N,34057'E and placed them individually in aviaries (4 m X 1.5 m by 2.5 mhigh). In addition, two juvenile shrikes were captured from a single brood and reared in captivity until fledging and then used in the experiments in the same manner as the adults. Each aviary was separated from the adjacent ones by black plastic sheeting to prevent the shrikes fromlearning from the behavior of their neighbours.
Bushes containing nests of widow spiders were removed from the same study area and placed on the floor of the aviaries in positions simulating those in the field. As adult L. revivensis were not abun- dant enough for repeated trials, we used large mealworms (Tenebrio molitor larvae) which were hung on a fine wire hook in the top of the nest cone (in the position where the spider would normally rest during the day). Immediately after placing a bush in the aviary, the observer withdrew to a position about 10 m from the aviary and recorded the subsequent behavior of the shrike for at least 20 min (or until the prey was captured). We allowed 5 min between succes- sive trials with the same bird, during which time the shrub was removed from the aviary.
All birds were tested once initially with shrubs containing real nests of L. revivensis. The four adult birds were then given shrubs with artificial nests (nest-sized cones made of opaque white parachute cloth), each containing a mealworm, daily for a period of 5 days. Testing with real widow-spider nests was resumed after this initial period of training. Juveniles were not trained on artificial nests. A total of 38 presentations were made of nests with two densities of cover (about 10% and 90%).




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78 Psyche [vo~. 97
Differences between treatments were tested with Kruskal-Wallis non-parametric analyses of variance, with the exception of the binomial data for capture success which were compared with a Wil- coxon signed-ranks test. The 95% confidence level was the signifi- cance level chosen for all tests. All means are represented +S.E. The shrike first observed the bush from its perch about 1.5 m above the floor of the aviary, then flew down to the ground beneath the bush. The bird then attempted to peer into the widow spider's nest from below, often changing position on the grond below the bush to gain a better angle. If no prey was detected, the shrike returned to its perch. Upon detecting prey in the nest, the shrike hopped up to the branches of the bush and attempted to grab the prey, either through the nest opening (n=4) or by inserting the bill through the mesh of the side of the nest cone (n=12). The prey was seized and swallowed on the spot.
The degree of cover of the nest significantly influenced capture success (Wilcoxon signed-ranks test, Z = 3.38, p <0.001). The birds detected and captured the prey in all trials with nests with little debris cover (n = 20), but were successful in only one of 17 trials with densely-covered nests. Significantly more capture attempts were made (Kruskal-Wallis test, H = 17.68, p <0.001) with densely- covered nests (x = 4.3 +0.54) than with lightly-covered nests (x = 1.65 k0.34). However, repeated attempts to capture prey in densely- covered nests may have reflected conditions in captivity, where alternative prey items were not available. For lightly-covered nests, the mean time to capture a prey item was 44.5 k9.9 s from the time the shrike landed on the ground beneath the shrub (i.e. recognition + handling time). The sequence of presentation to the shrikes of nests with different amounts of cover influenced neither the number of attempts nor capture success, nor was there significant variation among the birds (p >0.4 in all tests).
Hand-reared juvenile shrikes treated the widow spider nests in the same manner as the adults. It is possible that, prior to fledging, these juveniles may have observed their parents capture widow spiders. However, they would have had no experience of their own of locat- ing and capturing spiders. The approach and attack behaviour of



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19901 Konigswald, Lubin & Ward-Lutrodectus 79 adult and juvenile shrikes was invariable, although the young birds appeared less adept at removing the prey from the nest once it was detected.
Chew (1961) suggested that the predominance of hunting spiders (particularly Thomisidae and Salticidae) over web-spinning spiders in deserts was probably due to the sparseness of the vegetation, which provides few sites for webs and leaves the latter group exposed to predators. Many desert-dwelling web-builders conceal themselves in vegetation, in burrows or in nests (Cloudsley- Thompson 1983). While most widow spiders (genus Lutrodectus) construct retreats under stones or in burrows, two desert-inhabiting species occurring in the Middle East, L. revivensis and L. pallidus, place their nests in exposed locations near the tops of scattered bushes (Shulov 1948, Szlep 1965). These spiders, nonetheless, are hidden from the gaze of visually-orienting predators by the debris- covered walls of their nests. We have suggested that desert widow spiders build nests in bushes to allow cooling by convection, where wind speeds increase with height above ground (Lubin et al. in prep.). However, by doing so, the spiders expose themselves to visually-orienting predators. The results of these experiments sup- port the hypothesis that the debris cover of L. revivensis nests has an anti-predator function, providing both concealment and mechanical protection against a visually-orienting diurnal predator, the Great Grey Shrike.
Many spiders are known to use extraneous material in their webs or nests. Prey exoskeletons and plant material are incorporated in stabilimenta of some orb-web spiders and appear to conceal the spider at the hub or at its resting place near the web (Hansel1 1984 and references therein). Prey remains are incorporated into the nest wall in other Latrodectus species, including those with concealed retreats (e-g. L. tredecimguttatus, pers. obs.). The function of the extraneous material in concealed nests is not known. In L. reviven- sis (and possibly L. pallidus), the role of concealment and protection may have evolved secondarily with the adoption of above-ground nest-construction.




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80 Psyche [vo~. 97
The debris layer on L. revivensis webs increases in length and thickness, both with the duration of nest occupation (which ranges from a few days to more than three months) and with increasing spider size (Lubin et al., in prep.).
Widow spiders in newly-
constructed nests are most susceptible to such predators. Indeed, of 40 widow spiders at a field site near Sede Boqer that relocated their webs and constructed new nests, 27.5% disappeared within 10 days. Another period when widow spiders may be very vulnerable to predation is in the hot parts of the day in the summer months when they leave the dense, upper portion of the nest and sit in the open, lower part of the nest. This behaviour occurs in order for the spider to cool convectively. The upper part of the nest is impervious to wind and is significantly hotter than the lower, open part of the nest entrance, whenever there is direct insolation (Lubin et al. in prep.). The nests of widow spiders have significantly larger areas covered by debris in summer thanin winter (Lubin et al. in prep.). We hypothesize that this increase in debris covering of the nest is related to the increased risk of predation from visually-orienting predators during behavioral thermoregulation.
This is contribution no. 96 of the Mitrani Center for Desert Ecology. The research was supported by U.S.-Israel Binational Science Foundation grant no. 8600092. We thank Fiona Lawes for the drawing (Fig. 1).
CHEW, R. M.
1961. Ecology of the spiders of a desert community. J. New York Ent. Soc., 69: 5-41.
CLOUDSLEY-THOMPSON, J. L.
1983.
Desert adaptations in spiders. J. Arid Environ., 6: 307-317. HANSELL, M. H.
1984.
Animal Architecture and Building Behaviour. Longman, London. LEVY, G. AND P. AMITAI
1983.
Revision of the widow-spider genus Latrodectus (Araneae: Theridiidae) in Israel. Zool. J. Linn. Soc., 77: 39-63. SHULOV, A.
1948.
Latrodectus revivensis sp. nov. from Palestine. Ecology, 29: 209-215. SZLEP, R.
1965.
The web-spinning process and web-structure of Latrodectus tredecimgut- tutus, L. pallidus and L. revivensis. Proc Zool. Soc. (Lond.), 148: 75-89.



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