Most orb webs are vertically asymmetric with the hub above the geometric center, even though the basic structure of orb webs with concentric sticky spiral loops implies a round shape with the hub in its geometric center. Spiders are known to modify the basic, round web structure to achieve asymmetric webs by placing the sticky spiral loops eccentrically around the hub and by inserting partial sticky spiral loops below the hub. In addition, spiders could increase asymmetry with larger spiral spacing below the hub than above. In the present paper, I analyzed these web modifications quantitatively in webs of Araneus diadematus Clerck 1757. In addition, I assessed the influence of gravity on the different web modifications during web building by laying some webs horizontally during auxiliary and/or sticky spiral building, and I also assessed how the web modifications affected each other during web building. I found that web orientation during auxiliary spiral building influenced auxiliary spiral eccentricity, which in turn had an impact on sticky spiral eccentricity and overall web asymmetry. Web orientation, together with web asymmetry and spiral ratio, during sticky spiral building only influenced spiral spacing asymmetry. I conclude that A. diadematus uses the auxiliary spiral as a guiding line during sticky spiral building and that it applies different rules to build the two spirals.
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Vol. 39 • No. 2