Spiral and web asymmetry in the orb webs of Araneus diadematus (Araneae, Araneidae)

Most orb webs are vertically asymmetric with larger lower than upper parts, even though the basic structure of orb webs with concentric sticky spiral loops implies a round shape with the hub in its geometric centre. 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 additional sticky spiral threads below the hub. In addition, spiders could increase asymmetry by building larger meshes below than above the hub. 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 influenced each other during web building. I found that web orientation during auxiliary spiral building influenced auxiliary spiral eccentricity, which in turn influenced sticky spiral eccentricity and overall web asymmetry. Web orientation during sticky spiral building only influenced – together with web asymmetry and spiral ratio – mesh size asymmetry. I conclude that A. diadematus uses the auxiliary spiral as a guiding line during sticky spiral building and that it employs different rules to build the two spirals.