The physical factors of
Variables influencing the web structures of the spider such as climate factors, prey abundance, competition with the same and other species, predation, individual variables, etc. have been studied (Eberhard 1990, Foelix 2010). However, the spider’s physical factors that influence its decision about whether to add a silk decoration on a web or not have been poorly understood (Eberhard 1990, Schoener and Spiller 1992, Watanabe 1999, Théry and Casas 2009). Many orb-web weaving spiders (Araneae: Araneidae, Tetragnathidae, Uloboridae) decorate their webs with species-typical silk structures, so called stabilimentum (Simon 1895, Herberstein et al. 2000, Bruce 2006).
Stabilimentum is a conspicuous white silk structure that reflects much more ultraviolet light than areas without stabilimentum in the web (Craig and Bernard 1990, Bruce et al. 2005). This is surprising, given that “web-building” spiders’ silk generally has low UV reflectance, presumably to reduce the visibility of the web to insect prey (Blackledge and Wenzel 2000).
Recently, the “prey-attraction hypothesis” has suggested that the stabilimentum functions to increase foraging success by attracting more prey to the web (Craig and Bernard 1990, Tso 1998, Watanabe 1999, Kim et al. 2012). The stabilimentum decoration on the web of the wasp spider,
This study examined whether decorating webs with stabilimenta was affected by individual physical variables in
Measurements were conducted on the first web constructed by
To observe the effects of individual variables on the stabilimentum decoration, webs with stabilimenta or without stabilimenta were selected for measurements. For webs with stabilimenta, only webs that had both upper and lower stabilimenta were included.
To estimate the web size, the outermost and innermost diameters of each web from the sticky spirals were measured (Fig. 2). The web area was then calculated as: web area= πx longer radius x shorter radius. As the stabilimentum of
Nonparametric statistics in SigmaPlot (version 12.3; Systat Software Inc., San Jose, CA, USA) were used, but the mean and standard deviation of the data were presented instead of medians and inter-quartile ranges on nonparametric statistics. The individual variables (body mass, cephalothorax width, and abdomen length) were compared using the Mann-Whitney rank sum test (Mann and Whitney 1947). Regression analysis with ANOVA was also used to examine the relationship of individual variables and parameters of webs with and without stabilimenta.
This study investigated the web structures of 82 webs made by female spiders: 49 webs with stabilimenta and 33 webs without. Overall, the body mass of the female spiders (N = 82) was 402.1 ± 174.1 mg (mean ± SD), the cephalothorax width was 4.8 ± 0.7 mm, and the abdomen length was 11.6 ± 1.5 mm. The total web area of the 82 webs was 667.2 ± 285.9 cm2 , the mean radius length was 14.5 ± 3.2 cm, the mesh height was 4.5 ± 1.0 mm, and the number of spirals was 32.5 ± 11.0. The stabilimentum area measured in the 49 webs was 1.76 ± 1.72 cm2 .
There was no significant difference in the web area between webs with or without stabilimenta (Table 1). The mean length of radii and mesh size also did not differ in webs with or without stabilimenta (Table 1). However, the webs with stabilimenta had more spirals on average than the webs without stabilimenta (Table 1). However, the number of spirals showed no positive- or negative relationship with the stabilimentum area (ANOVA test for regression analysis:
Actually, the stabilimentum decorations reflected the individual variables (Fig. 3). The female spiders that made the webs with stabilimenta weighed lighter than the females that made the webs without stabilimenta (342.0 ± 114.8 mg compared with 491.5 ± 207.8 mg; Mann-Whitney rank sum test, U = 439.0,
[Fig. 3.] Box plots to compare body mass, cephalothorax width, and abdomen length between females of Argiope bruennichi building webs with and without stabilimentum. In the box plots, the boundary of the box closest to zero indicates the 25th percentile, a line within the box marks the median, and the boundary of the box farthest from zero indicates the 75th percentile. Whiskers (error bars) above and below the box indicate the 90th and 10th percentiles. Values out of the 90th and 10th percentiles were presented as black dots.
The heavier females decorated webs with wider stabilimentum (Fig. 4A; ANOVA for regression analysis:
The results revealed that the physical conditions of the female
A spider’s decision making on an orb web structure should be associated with its allocation of energy for reproduction or capturing prey (Higgins and Buskirk 1992, Sherman 1994, Herberstein et al. 1998, Watanabe 1999, Théry and Casas 2009). Spiders in very poor physical condition cannot expend much energy for web building. They might not have enough protein for silk production, and thus, it could be difficult to cover the energy costs for web construction. On the other hand, female spiders of a good enough weight to reproduce might not need prey, and therefore may reduce investment of energy in web building to allocate protein and energy to reproduce. But females with average physical conditions could change their investment in web building as a functional response to the changes in the spider’s physical state. Lighter reproductive females should expend more energy in web building to capture more prey, even if this entails the energetically expensive decorating of webs with stabilimentum.
In fact, a previous study on