Together with a student, we have a new paper out in the Journal of Arachnology (impressively, it is the second paper arising from Nick Tew’s undergraduate research project – Nick is now a PhD student at Bristol University). The paper looks at the factors that influence web asymmetry in orbs of Metellina mengei in the field (Wytham Woods near Oxford) with a large sample of 430 webs. We found that the main factors influencing vertical web asymmetry was web inclination (the more horizontally orientated the web, the more symmetric it was) and size (the larger the web, the more asymmetric it was was).
Abstract
Vertical asymmetry is a widespread feature of orb webs, with the lower part larger than the upper, although its adaptive value is not fully understood. Gravity is thought to play a major role in the generation of asymmetry through increased running speed downwards from the hub. The relationship between spider orientation and gravity has been relatively well studied. However, webs’ inclination from vertical has been less studied. Here we conducted a field study on the tetragnathid orb spider Metellina mengei Blackwall, 1869, which constructs webs that show a marked variation in inclination. Our findings revealed a significant influence of the degree of web inclination and web area on the level of vertical asymmetry, while environmental variables did not have any effect. Thus, our results support the hypothesis that the asymmetry in upwards and downwards running speeds due to gravity is an important determinant of web asymmetry.
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