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prey attraction as designing web

prey attraction as designing web - Behavioral Ecology Vol...

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Unformatted text preview: Behavioral Ecology Vol. 10 No. 5: 607–611 Prey attraction as a possible function of the silk decoration of the uloborid spider Octonoba sybotides Takeshi Watanabe Department of Zoology, Faculty of Science, Kyoto University, Kyoto, 606-8502, Japan Both laboratory experiments and field observations were used to examine the prey-attraction hypothesis for the function of the silk decoration on the orb web of Octonoba sybotides. The reflectance spectrum of the decorative silk showed that the decorations reflect relatively more ultraviolet (UV) light. Choice experiments were conducted using Drosophila melanogaster, a common prey species of the spider, to determine whether webs with silk decoration attract more flies than undecorated webs. The choice experiment showed that webs with silk decoration attract more flies in light that includes UV rays. However, flies choose their flight direction randomly in light without UV rays. This suggests that the silk decoration might attract prey insects that tend to fly toward UV-reflecting objects. Field observations comparing the prey capture rate between webs with and without a silk decoration showed that more prey are caught in decorated webs. In this study, no difference between the two forms of silk decoration, linear and spiral, was detected either in prey attraction in the choice experiment or in the prey capture rate in the field observations. Key words: choice experiment, Octonoba sybotides, prey attraction, prey capture rate, silk decoration, ultraviolet light. [Behav Ecol 10:607–611 (1999)] W eb design is an essential part of the foraging strategy of orb-web spiders. Although the orb web is made up of three fundamental elements—radial threads, frame threads, and catching spiral—some Araneid spiders are known to add silky structures to the center of their orb webs (Eberhard, 1990; Nentwig and Heimer, 1987). The orb web represents a behavioral and material investment in foraging by the spider (Eberhard, 1986). The energetic return depends largely on the prey-capture efficiency of the web, which is tightly related to its design, including factors such as mesh size (width between spiral threads) and web size (area of the catching spiral) (Eberhard, 1986; Rypstra, 1982; Sandoval, 1994; Uetz et al., 1978). Recent studies suggest that variation in orb web structure is associated with an individual spider’s allocation of energy to reproduction or prey capture (Higgins, 1990; Higgins and Buskirk, 1992; Sherman, 1994). However, the question of why spiders build web decorations, which must involve energetic investment, has been debated for more than a century. To date, various forms of the silk decoration, otherwise known as stabilimenta, have been described, and several possible functions for these silky structures have been proposed. These include adjusting web tension (Robinson and Robinson, 1970), strengthening the web (Robinson and Robinson, 1970, 1973), predator avoidance (Eberhard, 1973; Ewer, 1972; Lubin, 1975), and advertising the presence of the web (Blackledge, 1998; Eisner and Nowicki, 1983; Horton, 1980). The most recent and plausible hypothesis is the prey-attraction hypothesis (Craig and Bernard, 1990; Tso, 1996), which the authors examined in a study of orb-weaving spiders in the genus Argiope. They also showed that the stabilimentum silk of some Uloboridae species reflects more ultraviolet (UV) rays than visible rays. They suggested that the reflected light might attract prey insects that tend to fly toward UV-reflecting objects Address correspondence to T. Watanabe. E-mail: [email protected] zool.kyoto-u.ac.jp. Received 4 September 1998; revised 6 March 1999; accepted 12 March 1999. 1999 International Society for Behavioral Ecology and hence increase the prey-interception rate of the web. No additional studies have tested the prey-attraction hypothesis as an explanation of the function of the silk decoration on the webs of uloborid spiders. Octonoba sybotides is an uloborid spider that is widespread in eastern Asia. This spider is known to add conspicuous, white, linear or spiral silk decorations at the center of its orb web (Watanabe, 1999; Yoshida, 1980). Although many O. sybotides webs have linear or spiral-shaped decorations at their hubs, webs with a translucent disc sheet at the hub and webs without silk decorations are also found in the field (Watanabe, unpublished data). Although the latter two types are rarer, all four types can be observed in the same area. The function of each type of silk decoration is not yet clear. This study addressed two questions. (1) Do the stabilimenta of O. sybotides attract prey? (2) Does the prey-attraction hypothesis account for the different forms of stabilimenta? These questions were addressed with field observations and experiments. The prey-attraction hypothesis for the silk decoration on O. sybotides webs was examined by comparing the prey-capture rate in the field between webs with linear and spiral silk decorations and those without silk decorations. A choice experiment was also conducted to determine whether Drosophila fly to UV-reflecting silk decorations spun by O. sybotides. METHODS Choice experiment in the laboratory I collected mature female spiders from the Kyoto University Botanical Garden in Kyoto, Japan. The spiders were kept individually in cylindrical cases (10 cm diam, 8 cm high) and fed fruit flies. Within a few days, these spiders spun horizontal orb webs in their cases. I removed the silk decorations from the webs with a pair of tweezers under a binocular microscope and measured the reflectance spectra of the decorations alone with a Shimadzu UV-visible recording spectrophotometer UV240, which measures wavelengths from 240 to 700 nm with an accuracy of 0.3 nm. The light source for the spectropho- Behavioral Ecology Vol. 10 No. 5 608 periments. In the choice experiment, I used two types of light to determine whether the UV-reflecting property attracts prey insects; one beam contained short wavelength light (UV-plus; without the UV filter), and the other was a control without UV (UV-minus; eliminated UV component by the UV filter). Field observations Figure 1 Apparatus used to test the preference of Drosophila. A fly (D. melanogaster) was put into the connecting tube from the entrance. All the light entering the apparatus was shielded, except light from the lighting tube connected to the upper side of each chamber, into which rings with the central portion of webs were placed. tometer was dueterium, and barium sulfate (Merck) was used for white standard. I conducted a choice experiment similar to that of Craig and Bernard (1990) to determine whether fruit flies, a common natural prey insect of O. sybotides, are attracted to UVilluminated silk decorations. Two vinyl chloride cases, used for keeping spiders, were connected with a T-shaped pipe (2.0 cm diam; Figure 1). The central portions of webs with the two forms of silk decoration, or undecorated webs, were fixed to plastic rings 5 cm diam. The rings were placed in the cases at the opposite ends of the T, at a 45 angle to the central connecting pipe. Light tubes (2.0 cm diameter) were connected to the upper side of each case and focused on the centers of the rings. I introduced a single fruit fly (Drosophila melanogaster) through the central connecting pipe and then closed the entrance. The species of Drosophila used for this experiment was a species commonly caught by the spider in the field. One fly was introduced for each trial to avoid any effect of interaction between flies, and the same fly was never used again. The introduced fly could see both webs from the junction of the pipes. In the next trial, the two rings with webs were switched to eliminate site effects. After two trials, I replaced the two webs with two new webs. Each new web was taken from a different spider. A total of 90 mature (cephalothorax length 1.5 mm) female spiders supplied the webs (30 each linear, spiral, and no decoration) for the choice ex- To compare the prey capture rate between the different types of webs, I conducted field observations from late June until late September 1998 in the Kyoto University Botanical Garden. O. sybotides constructs a horizontal orb web at the base of trees, or between rocks, flower pots, or piled logs in dimly lit sites. The spiders tend to repair sections of their web, but infrequently renew the whole web before dawn. Although the spiders foraged both during the day and at night, they appear to be diurnal foragers and to catch prey insects mainly during the daytime (Watanabe, personal observation). Individual spiders build webs with linear or spiral decorations and sometimes build undecorated webs. I observed the number of prey trapped on the webs or consumed by the spiders at 3- to 8-day intervals. Although I did not identify individual spiders, no web was observed repeatedly because webs that had not been renewed since the previous observation could be distinguished by dust sticking to the spiral thread of the webs. Because it could not be predicted when a spider would form one of the three types of web, I randomly chose webs without silk decorations in the study area. Then one web with a linear decoration and one with a spiral decoration were surveyed within 1 m of each undecorated web. In choosing webs, I excluded those with repaired areas. Each trio of webs, an undecorated web and webs with linear and spiral decorations, was defined as an observational unit. If there were several fresh webs with decorations within 1 m of the undecorated web, the nearest ones were chosen. No web within a unit was nearer to webs in other units than to the webs in the same unit. Spiders within the same unit seemed to be in similar environments. I identified 16–21 units every observation period and counted the number of prey captured on each web between 1430 and 1600 h. The prey interception rate (number of insects per web per census) was calculated. By using observational units, in which spiders seemed to construct webs in similar environments, site effects never biased the observations for a particular type of web. Because the daily capture rate varied greatly, I log transformed the data on prey-capture rate to normalize their distribution. I used ANOVA to compare the rate of prey capture between the three types of webs. Webs with a translucent disc sheet at the hub were not included in the analysis because of a small sample size and the difficulty in distinguishing a typical disc sheet, which varied and sometimes included a linear or spiral silk decoration. Undigested prey were removed from the webs and identified later in the laboratory. In the study period, I also estimated the frequency of each type of O. sybotides decoration in the study area by counting all the spiders (total length 4.0 mm; including mature and immature spiders) with identifiable web types in a monitoring area (8 1 m) in the Botanical Garden. Because the structure of a web can affect how many insects it intercepts, I also estimated the mesh size and catching area of each web with reference to Sherman (1994). I measured the major and minor diameter of the orb web (from one outermost spiral to the opposite outermost spiral) to estimate the total web area, and also measured two cross-diameters of the hub and surrounding nonsticky spiral zone. I calculated the catching area of each web (total web area less the hub and nonsticky spiral zone areas). I counted the number of spirals Watanabe • Silk decoration as prey attractant 609 Table 2 Prey capture rate per census for three types of webs Web types Mean capture rate per census SD n With linear form decoration With spiral form decoration Without decoration 0.40 0.48 0.14 20 20 20 0.20 0.13 0.09 Data from 20 days are used. Daily sampling sizes varied from 16 to 21. Total number of webs was 362. Decorated webs intercepted more prey than undecorated webs (ANOVA; F2, 57 35.942, df 2, p .001) Figure 2 Normalized reflectance spectra of stabilimentum silk spun by Octonoba sybotides. Each point shows the mean of five measurements. in each web’s two major axes and then estimated the mean mesh size. RESULTS Choice experiments The silk decorations reflected relatively more UV light (Figure 2). The reflectance curve was nearly flat between wavelengths of 300 and 550 nm, with a slight peak around 300 nm. The reflectance decreased at wavelengths longer than 650 nm or shorter than 250 nm. The preference for decorated over undecorated was significantly different between UV-plus and UV-minus in experiments 1 and 2 (Fisher’s Exact test, p .0001, respectively). The fruitflies were more likely to choose a web with a silk decoration than an undecorated web under UV-plus light (Table 1); there was no significant difference in their preference for linear or spiral silk decorations (Table 1). Under UV-minus light, there was no significant difference in the flies’ choice of webs. Field observations The data from a total of 20 days were used for the ANOVA. The daily sample size (number of webs per census) of each type of web ranged from 16 to 21. Identifiable prey included midges (39.2%), mosquitoes (6.8%), crane flies (5.0%), other small (total length 4 mm) and large (total length 4 mm) flies (42.7%), and other insects (6.3%) including Ephemerop- 143). tera spp., Neuroptera spp., and Hymenoptera spp. (n The mean prey-capture rate at the webs with decorations was much higher than at undecorated webs (Table 2). There was a significant difference in the mean prey-capture rate between the three web types (ANOVA; F2, 57 35.942, df 2, p .001). Multiple comparison showed that (1) the prey-capture rate of the webs with a silk decoration was significantly greater than that of the undecorated webs, and (2) there was no significant difference in the capture rate between webs with linear and spiral silk decorations (post hoc Scheffe’s F test; linear ´ .001; spiral decoration decoration versus no decoration, p versus no decoration, p .001; linear decoration versus spiral .2611). decoration, p Although equal numbers of each type of web were observed to compare the prey-capture rate, the daily frequency of decorations in the study area varied greatly. The total number of webs in the monitoring area varied from 29 to 59 on 38 observation days. The mean frequency of undecorated webs was much lower than that of decorated webs: linear decoration, 57.3 11.3% (mean SD); spiral decoration, 32.9 13.7%; undecorated and others, 9.9 9.5%. The mean mesh sizes and catching areas of each type of web did not differ statistically in the 20 observations (all F ratios 1.302, p .15, for tests of between-day variation for all web types). Therefore, data from the 20 days were combined to compare the features of the three web types (Figure 3). ANOVA and multiple comparison tests (Scheffe’s F) ´ showed that the mesh size of webs with spiral decorations was significantly smaller than that of the other two types of web 146.65, p .0001). There was no statistical differ(F2, 1083 ence between the mesh size of the webs with a linear decoration and the undecorated webs. On the other hand, the catching areas of the webs with the spiral decoration and the undecorated webs were significantly larger than that of the 52.69, p .0001). webs with the linear decoration (F2, 1083 DISCUSSION The results of the laboratory experiments and field observations support the prey-attraction hypothesis for the function Table 1 Results of the choice experiments No decoration vs. linear No decoration vs. spiral Linear vs. spiral Light type n Linear p n Spiral p Linear Spiral p UV UV 3 24 57 26 2 28 58 32 .0001 ns 21 18 .0001 ns 19 22 ns ns Under UV-plus light, Drosophila were attracted to the chamber in which the web with silk decoration was placed more often than to the chamber with an undecorated web. Under UV-minus light, they chose the chamber randomly. P values calculated from binomial distribution. 610 Figure 3 (A) Mean ( SD) mesh size (mm) and (B) catching area (m2) for three types of O. sybotides webs. The results of the multiple comparison test (Scheffe’s F) are shown in the graph: **p ´ .0001. of the silk decoration on the web of O. sybotides. Webs with a silk decoration intercepted prey insects more frequently than undecorated webs, although there was no difference in the rate of prey intercepted by webs with different types of silk decoration. In the field, the prey species are mainly flying dipteran insects. Some dipteran insects (mosquitoes, fruitflies, and drone flies) are known to have photoreceptors that are very sensitive in the ultraviolet range (Bishop, 1974; Hu and Stark, 1977; Muir et al., 1992). There are several explanations for this sensitivity. UV-reflecting objects may attract insects searching for food resources, mates, oviposition sites, or escape routes. Insects might respond to UV light as a cue that indicates these objects, although it is uncertain if a single mechanism attracts prey to the web. Craig and Bernard (1990) showed that the catching silks of uloborid spiders also have a high UV reflectance (see also Craig et al., 1994). This poses a problem for the prey-attracting hypothesis, because a UV-reflecting catching silk might enable flying insects to avoid the web more easily. However, O. sybotides is found in the dim forest understory. Craig et al. (1994) showed that uloborid spiders generally forage in a diurnal forest or nocturnal light environment. Such light conditions reduce the visibility of the catching silks, although in dim light the effectiveness of prey attraction is also likely to Behavioral Ecology Vol. 10 No. 5 decrease. However, more light must be reflected from the silk decoration than from the catching strings because the surface area of the stabilimentum threads themselves, which are densely arrayed, is much greater than the surface area of the much thinner catching strings, even though the catching web covers a larger area. Therefore, the reflectance of the silk decoration that attracts prey insects might decrease less than the reflectance of the catching silk. One alternative explanation for the differences in the preyinterception rate between webs is the effect of web structure. Many studies suggest that mesh size, catching area, and the visibility of the web affect the interception of insects by webs (Craig, 1986; Rypstra, 1982; Sandoval, 1994; Uetz et al., 1978). However, this hypothesis might be rejected in O. sybotides because the prey-capture rate between webs with linear and with no decorations differed. Both had similar mesh sizes, but the catching area of undecorated webs was significantly larger than that of webs with linear decorations. In spite of their smaller size, the webs with linear decorations captured more prey insects than the undecorated webs. In addition, webs with a smaller mesh must be more visible than those with a larger mesh, so that a flying insect would be expected to avoid a web with a dense mesh more easily. Therefore, the low preycapture rate of an undecorated web is unlikely to result from a difference in the visibility of the web to prey insects. Tso (1996) also argues this point. If prey are attracted to decorated webs, why don’t all spiders decorate their webs? Several researchers argue that energetic constraints may limit decorating behavior (Eberhard, 1990; Hauber, 1998; Herberstein et al., 1997). In the field, however, O. sybotides spun undecorated webs less frequently than those with decorations, and in the laboratory O. sybotides that initially spun undecorated webs subsequently added spiral or linear decorations if they were not fed (Watanabe, personal observation). Therefore, undecorated webs might be part of the construction process, and there may be an interlude before the decoration is added. I cannot make any presumptions about the energetic state of the spiders on the undecorated webs. In this study, the prey capture rate did not differ significantly between webs with linear and spiral silk decorations. However, the mesh of webs with linear silk decorations was significantly larger than that of webs with spiral silk decorations, whereas the catching area of webs with linear decorations was significantly smaller than that of webs with spiral decorations. These structural differences should affect the quantity (number) and quality (size) of insects intercepted by the web. The energy gain of a web combines the quantity and quality of prey. The results of this study showed that there was no numerical difference in prey interception. To further examine the effect of structural differences on prey interception, it is necessary to take prey size into account and measure the qualitative difference in the intercepted prey. Further studies are needed to clarify the relationship between the form of silk decoration and the structural characteristics of webs in order to elucidate the functional significance of the decoration. I am extremely grateful to M. Yoshida, T. Miyashita, T. Masumoto, and the members of the Kansai Spider Study Group for encouraging me during the early stages of my work. M. Imafuku kindly allowed me to use the spectrophotometer. I am also grateful to K. Nakata, M. Urabe, E. Honda, and S. Ishida for comments and English corrections in drafts of this paper. Thanks are also extended to members of the Laboratory of Animal Ecology, Kyoto University, for their assistance and advice. Watanabe • Silk decoration as prey attractant REFERENCES Bishop LG, 1974. An ultraviolet photoreceptor in a dipteran compound eye. J Comp Physiol 91:267–275. Blackledge TA, 1998. Stabilimentum variation and foraging success in Argiope aurantia and Argiope trifasciata (Araneae: Araneidae). J Zool 246:21–27. 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