Stabilimenta on the Webs of Uloborus Diversus (Araneae: Uloboridae) and Other Spiders
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J. Zool., Lond. (1973) 171, 367-384 Stabilimenta on the webs of Uloborus diversus (Araneae: Uloboridae) and other spiders WILLIAMG. EBERHARD* Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts, U.S.A. (Accepted 12 June 1973) (With 4 plates and 3 figures in the text) Uloborus diversus places extra silk (“stabilimenta”) near the hubs of its webs, preferentially on short radii ending near anchor threads. Spiders probably distinguish these radii from others by their relatively low extensibility. The stabilimentum probably functions as a camouflage device, and the orientation of the stabilimentum lines probably aids disturbed spiders in making quick exits from webs. Turning responses at radius-frame junctions during these exits are influenced by thread angles at the junctions (and possibly by other factors), and also enable disturbed spiders to reach hiding places quickly. The available data on other stabilimentum-building spiders suggests that they also use stabilimenta to provide defence against visually-hunting predators. Contents Page Introduction. ........... .. .. .. .. .. 361 Methods .......... .. .. .. .. .. .. 369 Stabilimentum patterns ........ .. .. .. .. .. 369 Orientation of stabilimenta in the field .... .. .. .. .. .. 312 Orientation of stabilimenta in the laboratory . .. .. .. .. .. 313 Movements preceding stabilimentum construction .. .. .. .. .. 313 Stabilimentum construction ...... .. .. .. .. .. 314 Possible stimuli directing stabilimentum placement .. .. .. .. .. 316 Spiders’ positions on webs in nature .... .. .. .. .. .. 311 Escape behaviour .......... .. .. .. .. 318 Discussion ............ .. .. .. .. .. 380 References ............ .. .. .. .. .. 383 Introduction Many spiders which remain on their webs during the day rest near objects they have fastened to the web. These objects are usually called stabilimenta when found in orb webs. In these webs (made by spiders in the families Uloboridae and Araneidae), stabilimenta consist of bands or tufts of silk, egg sacs, and detritus (Table I). Although egg sac and detritus stabilimenta are generally thought to serve as camouflage (e.g. Gertsch, 1949), the function of silk stabilimenta has been controversial. Silk stabili- menta were first thought to stabilize orbs, supposedly by strengthening the connections between hub threads (McCook, 1889; Comstock, 1940). Other authors suggested they were reserve cables for use in swathing prey (Vinson, 1863 quoted in Wiehle, 1927), * Present address: Universidad del Valle, Departamento de Biologia, Cali, Colombia, South America. 361 368 W. G. EBERHARD TABLEI Orb weaving spiders which spin stabilirnenta, and their daytime resting sites Spider at hub Species during the day? References SILK STABILIMENTA Argiope (apparently all species) Yes McCook, 1889; Wiehle, 1927, 1928; Hingston, 1927, 1932; Marson, 1947a; McKeown, 1952; Levi, 1968; Robinson & Robinson, 1970; Ewer, 1972 Uloborus (apparently all except U.gibbosa McCook, 1889; Wiehle, 1927, 1931; which often omits the stabilimentum) Hingston, 1927, 1932; B. J. Marples, 1955, 1962; Kullmann, 1972; this paper Araneus adiantus Wiehle, 1929 armatus Wiehle, 1931 redii Wiehle, 1927 Aculepeira ceropegia (= Aranea ceropegia) Wiehle, 1927 Cyclosa conica McCook, 1889; Marples & Marples. 1937 ginnage Yaginuma, 1966 inssulana Kullmann, 1972 sedecuolata Yaginuma, 1966 Cyrtophora moluccensis Yaginuma, 1966 Gasterucantha brevispina Marson, 19476 cancriyormis McCook, 1889; Comstock, 1940 pallida Hingston, 1932 versicolor Ernerit, 1968 Landana cygnea Eberhard, in prep. Micrathena armatus Hingston, 1932 gracilis McCook, 1889 sagittata Comstock, 1940 Neoscona benjamina McCook, 1889 Singa haemata Wiehle. 1929 EGG SAC STABILIMENTA (GENERA) Cyrtophora Kullmann, 1961 Cyclosa McCook, 1889; Wiehle, 1927, 1928; Kaston, 1948 Micrathena (1Acrosoma) McCook, 1889 Uloborus Emerton, 1883; McCook, 1889; Scheffer, 1905 DETRITUS STABILIMENTA (GENERA) Gasteracantha Hingston, 1927 Cyclosa McCook, 1889; Hingston, 1927; Wiehle, 1927 Landana Eberhard, in prep. deposits of superfluous silk left over after web construction (Wiehle, 192Q or “love paths” to guide the male to the female (Wolfram, 1924 quoted in Wiehle, 1927). Hingston (1927, 1932), and later Marson (1947a, b) showed the great variety of patterns of silk stabilimenta, and, noting a general correlation between the shape of the stabilimentum and the posture of the spider as it rested at the hub, concluded that stabilimenta function as defence against visually orienting predators by obscuring the outline of the spider or STABILIMENTA ON ULOBORUS DIVERSUS WEBS 3 69 by distracting the attention of such predators from the spider. Several recent authors (e.g. Gertsch, 1949; Robinson & Robinson, 1970) are not convinced of the defensive function of stabilimenta, at least in certain cases, and propose other possible functions such as “tuning” or strengthening the web. The data of this study, which is the first detailed examination of stabilimenta on the webs of Uloborus diversus, suggest that the stabilimenta on these webs function as camou- flage devices, and that they are oriented so the resting spider is in position to follow a short escape route off the web when disturbed. Methods Webs were studied both in the field and in the laboratory. Approximately 325 webs, each spun by a different spider in a different location, were observed in high Sonoran desert near Cave Creek, Arizona (elevation c. lo00 m) between 20 July and 5 August 1966. Webs of spiders of all ages except the first instars were studied. More than 600 webs and more than 30 stabilimentum constructions made by a stock of about 50 spiders were observed between September 1965 and March 1966 in a laboratory in Cambridge, Mass. Light levels were measured with a “Gossen Luna Pro” meter. Strong light did not inhibit stabilimentum construction, and movies (48 f/sec) of this behaviour were obtained and analysed. All significance tests reported below are 2 x 2 two-tailed x2 tests. Stabilimentum patterns Uloborus diversus constructed two types of stabilimentum,* both composed entirely of silk: the “linear” type (Plate I(a)) consisted of one or more mats of white silk laid in lines (“arms”) along radii; the “circular” type (Plate I(b)) contained, in addition to an arm or arms, silk laid in roughly circular loops at the hub (sometimes there was no arm visible in a circular stabilimentum). Occasionally a spider laid stabilimentum silk on lines at the edge of the web or below it (Plate 11). These lines were usually attached to frames or radii in the web, but were sometimes completely free of the web. Linear stabilimenta were most common both in the field and in the laboratory: 65 % of 268 webs in the field had linear stabilimenta, 30 % had circular stabilimenta, and 5 % had none; 42 % of 590 webs constructed in the laboratory had linear stabilimenta, 25 % had circular stabilimenta, and 33 % had none. Pairs of arms were most common in linear stabilimenta, and although the arms tended to be on opposite sides of the hub like those in Plate I(a), there was substantial variation in patterns (Table 11). Ninety-six per cent of TABLEI1 Stabilirnentumpatterns of U. diversus Type of Number of arms in stabilimentum stabilimentum N 0 1 2 3 4 5 Laboratory line 455 - 16% 76% 6% 1% 1% circle 135 1 48 31 14 5 1 Field line 182 - 5 93 2 0 0 circle 86 8 53 32 6 1 0 * The hubs of webs of first instar spiders were usually dotted with white specks which were often so numerous that there was a white “mat” at the hub. These specks were probably by-products of the construction of supple- mentary radii rather than a distinct structure (see Eberhard, in prep.). 3 70 W. G. EBERHARD PLATEI. Linear (a) and circular (b) stabilimenta at the hubs of U. diversus webs. The spider resting at the hub normally just fills the gap between the arms of the linear stabilimenturn. STABILIMENTA ON ULOBORUS DIVERSUS WEBS 371 PLATE11. Web of U. diversus with stabilimentum lines below the web, and the spider at the hub. 167 webs in the field and 96 % of 379 webs in the laboratory which had linear stabilimenta had at least one pair of stabilimentum arms which were on opposite sides of the hub from each other. These trends were reduced to 74 % of 35 webs in the field and 52 % of 61 webs in the laboratory which had circular stabilimenta. As noted by B. J. Maples (1962) in other species of Uloborus, given individuals were capable of making both types of stabilimentum, but most spiders usually spun only one type under constant conditions in the laboratory. Laboratory experiments showed that the light level during the night (when the spiders spun their webs) was correlated with changes in stabilimentum pattern. The amount of night light was changed for 21 consecutive nights, and 18 spiders (instar No. 3 and older) were fed one fruit fly per web, and their webs destroyed each day. The spiders consistently constructed both more circular stabilimenta and more lines under their webs following nights of bright (4.5 Lamberts) illumination; and they constructed fewer, predominantly linear stabilimenta with fewer lines under their webs (both P<0-05)after low light level (less than 1-0 Lamberts) nights (Table 111). Thus spiders resting in spots more exposed to the night sky may construct more elaborate stabilimenta. Webs at such sites would probably be most exposed to large, visually orienting predators, and these trends in stabilimentum size may function to provide maximum camouflage for those spiders most susceptible to attack, and preserve