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Unformatted text preview: Bull. Br. arachnol. Soc. (1990) 8(5),161-164 Function and structural variability of the stabili- menta of Cyclosa insulana (Costa) (Araneae, Araneidae) Cornelis R. Neet Institut de Zoologie et d'Ecologie Animale, Universit de Lausanne, Btiment de Biologie, CH-1015 Lausanne, Switzerland Summary Field observations on linear and circular stabilimenta were undertaken on coastal populations of Cyclosa insulana (Costa), in Ibiza (Spain). The linear and circular stabilimenta and sticky spiral appear to be independent components of the web. The camouflage function of the linear stabilimentum is confirmed. A comparative analysis of webs built under different degrees of exposure te, wind shows that the circular stabilimentum probably has a mechanical strengthening function and that web size is affected by the degree of exposure to air currents. The tact that different independent web components having differ- ent functions have been grouped under the common and perhaps misleading term of stabilimentum is emphasised. Introduction Several orb-weaving species of the families Araneidae and Uloboridae are known to build a con- spicuous white silk structure near the hub of their webs: the stabilimentum. The well-known example of the zig- zag ladder structure of the stabilimentum ofArgiope bruennichi (Scopoli) is only one of numerous types of stabilimenta that have been described, and which have been classified in two general categories: circular stabilimenta, which include discs spun by someArgiope species and concentric loops spun by several Uloborus and Cyclosa species, and linear stabilimenta, that are found in many species of Argiope, Cyclosa, and other genera (Edmunds, 1986). Although there are many intermediate forms which render such a classification questionable (Lubin, 1986), it is convenient for the purpose of this paper. Three current theories account for possible functions of stabilimenta (Edmunds, 1986 ; Lubin, 1986). These are the mechanical strengthening of the web (McCook, 1889; Simon, 1892-5; Robinson & Robinson, 1970, 1973), defence by camouflage (Hingston, 1927; Marson, 1947a, b; Ewer, 1972; Eberhard, 1973; Lubin, 1975; Tolbert, 1975) and advertisement to prevent web destruction by birds (Horton, 1980 ; Eisner & Nowicki, 1983; Edmunds, 1986). All three theories have received empirical support from various observers (Edmunds, 1986 ; Lubin, 1986). However, the defensive functions of camouflage and advertising have received the best supportive evidence, as in the case ofUloborus diversus Marx (Eberhard, 1973) and Argiope flavipalpis (Lucas), where the linear stabilimentum distorts the spider's apparent shape, especially when the spider vibrates its web (Edmunds, 1986; Edmunds & Edmunds, 1986)....
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This note was uploaded on 01/27/2012 for the course ECOLOGY 300 taught by Professor Zumdahli during the Spring '11 term at St. Mary NE.
- Spring '11