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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Vol. 46 • No. 2