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Web asymmetry in the tetragnathid orb spider Metellina mengei (Blackwell, 1869) is determined by web inclination and web size.

The majority of webs from both major orb spider families (Araneidae and Tetragnathidae) are asymmetric with the area below the hub being larger than the area above (Masters & Moffat 1983; ap Rhiziart & Vollrath 1994; Kuntner et al. 2010b). This vertical web asymmetry is primarily thought to arise from an asymmetry in running speed caused by gravity, which allows for faster downwards running speeds than upwards against gravity, as has been observed in a number of araneids in the laboratory (Masters & Moffat 1983; ap Rhiziart & Vollrath 1994; Nakata & Zschokke 2010). Given this asymmetry in running speed, it follows from optimal foraging theory that spiders optimise their prey capture rate by investing more time and silk resources in web construction below the hub compared to above (Maciejewski 2010; Gregoric et al. 2013). Another hypothesis relating to gravity that has been proposed to explain web asymmetry is that web-building costs should be higher in the upper part of the web (Herberstein & Heiling 1999), although this has not been supported by empirical data (Coslovsky & Zschokke 2009).

A number of predictions arise from the gravity-determined running speed asymmetry hypothesis: i) Spiders should be orientated downwards to gain full advantage of the faster downward gravity-assisted running speeds. This prediction is supported by a range of studies that demonstrate a link between vertical asymmetry and spider orientation, including studies that show that the few spider species facing upwards in the hub often have reversed asymmetries with the area above the hub being larger than below (Nakata & Zschokke 2010; Zschokke & Nakata 2010), although this is not always the case (Rao et al. 2011). ii) Larger and heavier spiders should build more asymmetric webs as heavier spiders should experience a larger asymmetry in running speeds due to gravity's impact on mass. Studies comparing different sized adults and different ontogenetic stages confirm that larger spiders build more asymmetric webs (Herberstein & Heiling 1999; Hesselberg 2010; Kuntner et al. 2010a). Spiders that build webs with reversed asymmetry build more symmetric webs with increasing size (Nakata 2010), further supporting the notion that upwards running speed is slower in heavier spiders. Alternatively, it has been suggested that web asymmetry is an evolutionary derived trait and that spiders recapitulate this during individual development (the biogenetic law) such that young spiders build symmetric webs and older spiders asymmetric webs even in the absence of any adaptive advantage (Eberhard et al. 2008), but a number of studies specifically tested this hypothesis without finding any support (Hesselberg 2010; Kuntner et al. 2010a; Nakata 2010; Gregoric et al. 2013). iii) Since gravity acts vertically in the direction of the center of the Earth, spiders building webs that are not completely vertical should experience less of a difference in upwards and downwards running speed and therefore should build more symmetric webs. This is supported by experimental studies on horizontal web building in the araneid Araneus diadematus Clerck, 1757, which normally builds vertical webs (Zschokke 2011) and by observational studies of the tetragnathid Leucauge venusta (Walckenaer, 1841) webs classified into three inclination groups (Gregoric et al. 2013). The importance of gravity is further supported by observations of two A. diadematus building symmetric webs in space on board Skylab (Witt et al. 1977). iv) Web vertical asymmetry should only depend on the gravity-determined differences in running speed (predictions i to iii above), suggesting that the majority of other variables known to affect overall orb web geometry, but not expected to affect the asymmetry in running speeds, should not affect web asymmetry. This is the case for spatial constraints (Hesselberg 2013), but has not been specifically studied for other variables such as climatic factors (Vollrath et al. 1997), leg loss (Pasquet et al. 2011) and exposure to neurotoxins (Hesselberg & Vollrath 2004). However, some factors may affect web asymmetry without directly affecting running speed asymmetry, including factors such as experiences of prey capture success in different parts of the web (Heiling & Herberstein 1999), inter- and intra-individual variability in web-building behaviour (Heiling & Herberstein 2000), possibly related to differences in behavioural syndromes (Kralj-Fiser & Schneider 2012) and perceived predation risk (Nakata & Mori 2016).

Most of the above-mentioned studies focus on the effect of only one variable in a highly controlled laboratory study (but see Kuntner et al. 2010a) and use araneids with predominantly vertical webs (but see Gregoric et al. 2013). Here we investigate the asymmetry of webs of the tetragnathid Metellina mengei (Blackwall, 1869) in the field measuring a range of different web and climatic variables with the specific aim of testing the third and fourth prediction of the gravity-determined running speed asymmetry hypothesis (see above). To our knowledge, this is the first study to compare the effect of inclination on web asymmetry with both measured as continuous variables.

Metellina mengei is a medium sized orb spider common in woodland understory in Western and Central Europe in the early spring (in summer and autumn, it is replaced by the very similar M. segmentata (Clerck, 1757)). It builds relatively small webs that show a large variation in inclination. In this study, we observed webs ranging from 5[degrees] to 85[degrees] (but with 85% of 430 measured webs between 408 and 60[degrees]). Metellina spiders always (when present in the hub) face downwards (Tew & Hesselberg, pers. obs.). The data used come from a larger study by Tew and Hesselberg (2017), but here we focus on webs from the edge of the forest, where climatic conditions were more variable than in the forest interior. We recorded 430 webs of adult and subadult M. mengei on three 200-m transects in Wytham Woods, Oxfordshire, UK (51[degrees] 78' N, 01[degrees] 34' W) during 10 days in May and June 2015 and measured the following variables: the inclination of the web to horizontal (0[degrees]--horizontal, 90[degrees]--vertical) measured to the nearest 5[degrees] with a protractor kept level by placing it on a clipboard, web height above ground level, the vertical and horizontal diameters of web, the upper radius of the web, horizontal diameter of web, the vertical diameter of the free zone. From the latter variables, we calculated vertical web asymmetry with the formula: ([R.sub.U]--[R.sub.L])/([R.sub.U] + [R.sub.L]), where [R.sub.U] is the upper and [R.sub.L] the lower radius of the web (Zschokke 1993; Hesselberg 2010), and the area of the capture spiral (web area), with the Ellipse-Hub equation (Herberstein & Tso 2000). In addition, we measured the following climatic variables at the start of each transect (three times each study day): temperature, humidity, pressure and wind speed. See Tew & Hesselberg (2017) for a more detailed description of the methodology.

In order to determine the influence of web and climatic variables on web asymmetry, we used the statistical programming language R (R Core Team 2016) to build a general linear mixed model with web asymmetry as response variable and inclination, web area, web height, wind speed, temperature, pressure and humidity as predictor variables. First order interactions between the first four variables were also included in the model. The study day and transect number were included as random factors. The model was validated following Thomas et al. (2013). Non-significant terms were removed from the full model following the marginal rule until the final model with the lowest AIC score was found. P-values were determined with Type II Wald F tests with Kenward-Roger degrees of freedom. The conditional ([R.sub.c.sup.2] fixed and random effects) and marginal ([R.sub.m.sup.2], fixed effects only) coefficients of determination were estimated based on the method by Nakagawa & Schielzeth (2013).

We found a clear negative relationship between the degree of inclination and vertical asymmetry (F = 17.76, df = 1, n = 430, P < 0.001), which supports prediction iii above. The less inclined (more horizontally orientated) a M. mengei web was, the more symmetric it was (Fig. 1A). We furthermore found a significant effect of web area (F = 8.54, df = 1, n = 430, P = 0.004) in that larger webs were significantly more asymmetric (Fig. 1B). This lends support to prediction ii, since larger webs are usually built by larger and heavier spiders (Heiling & Herberstein 1998). We furthermore found support for prediction iv in that none of the climatic variables tested (wind speed, temperature, humidity and pressure) or height of web above the ground were found to have a significant effect on web asymmetry (Wind speed: F = 1.02, df = 1, n = 430, P = 0.340; Temperature: F = 0.88, df=1, n=430, P=0.371; Humidity: F=1.33, df=1, n=430, P = 0.289; Pressure: F = 0.20, df = 1, n = 430, P = 0.672; Height: F = 0.64, df=1, n=413, P=0.423). However, height did have an indirect effect on web asymmetry in that its interaction with inclination was significant (F = 9.89, df = 1, n = 413, P = 0.002) such that webs build lower in the vegetation did not show a clear relationship between inclination and web asymmetry. Similarly, we found a significant interaction between inclination and web area (F = 6.24, df = 1, n = 413, P = 0.013) such that only larger webs showed a clear relationship between inclination and web asymmetry. None of the first order interactions including wind speed, were significant and neither was the interaction between height and web area (results not shown). The full model with fixed and random factors explained 12% of the variance ([R.sub.c.sup.2] = 0.124), while the fixed factors alone explained 9% ([R.sub.m.sup.2] = 0.089). Thus, the model developed in this study only explained about 12% of the variation in the observed web asymmetry. This suggests that although inclination and web area significantly influence web asymmetry, it is also influenced by a range of other factors not measured in this study such as spider size (although this is partly taken into account by the use of web-size (Heiling & Herberstein 1998), intra- and inter-individual variation in web-building (Heiling & Herberstein 2000), spider age, ontogeny and reproductive status (Hesselberg 2010; Anotaux et al. 2012).

Despite the relatively large number of studies on web asymmetry in orb spiders, there is still much we do not know, especially in relation to webs in the wild. In this study, we focussed on some of the mechanistic factors influencing the asymmetry, but there is also a need to look at the functional aspects. In particular it would be interesting to study prey capture success and the resulting growth rate and reproductive success in spiders that build webs of different inclination (and hence asymmetry) in greater detail as our previous study suggests that inclination is a significant determinant of prey capture success in the webs of this species (Tew & Hesselberg 2017). The webs of M. mengei that were measured in this study varied quite significantly in both inclination and asymmetry (with 3% of webs even having larger upper parts than lower parts--see figure 1). If a given inclination and web asymmetry provides the largest return in terms of prey capture, why do webs in the wild show such a large variation? One intriguing possibility is that predation risk may affect the degree of web asymmetry in that asymmetric webs are posited to be more complex and take longer time to build than symmetric webs causing spiders to build more symmetric webs when they perceive a risk of predation (Nakata & Mori 2016). However, our previous study did not show any differences in the asymmetry of webs between the more exposed forest edge with presumably higher predation risk and the more sheltered interior of the forest (Tew & Hesselberg 2017).


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Nicholas Tew (1,2) and Thomas Hesselberg (1): (1) Department of Zoology, University of Oxford, South Parks Road, Oxford, OX1 3PS, United Kingdom; E-mail:; (2) Department of Life Sciences, Imperial College London, Buckhurst Road, Ascot, SL5 7PY, United Kingdom

Manuscript received 15 August 2017, revised 21 December 2017.
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Author:Tew, Nicholas; Hesselberg, Thomas
Publication:The Journal of Arachnology
Article Type:Report
Date:May 1, 2018
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