Vertical asymmetries in orb webs

In almost all vertical orb webs the hub is above the geometric centre and consequently, the extent of the capture area is larger below the hub than above. In addition to this vertical web-extent asymmetry, orb webs show vertical asymmetries in number of spiral loops, mesh widths, and angles between radii. However, it was unknown whether these asymmetries are adaptations to the web-extent asymmetry or whether they are linked to gravity in a different way than through web-extent asymmetry. We reviewed known vertical asymmetries of orb webs, and we analysed the asymmetries of webs built by four different Cyclosa species, which show large intra- and interspecific variation in web-extent asymmetry. We found all analysed structural asymmetries to be linked both to web-extent asymmetry and to gravity: Larger web extents below the hub and gravity both led to more sticky-spiral loops and to smaller angles between radii below the hub, whereas web-extent asymmetry and gravity had opposing effects on mesh width (mean and peripheral). Independent of web-extent asymmetry, almost all analyzed webs had narrower peripheral meshes and smaller angles between radii below the hub than above. We interpret the narrow peripheral meshes along the web's lower edge as an adaptation to prevent tumbling prey from escaping, and the small angles between radii as an adaptation to prevent the sticky spiral lines in these narrow meshes to come into contact with each other.