Although orb web construction behaviour is relatively well studied, there are few studies of the mechanisms with which behavioural decisions are executed, in terms of where the spider grasps lines and attaches them to each other. Video analyses were used here to show that the distance from the previous sticky loop at which the araneid spider Micrathena duodecimspinosa gripped the radius with her leg oIV during sticky spiral construction varied according to the spider's position in her web with respect to both gravity and the edge of the web. This grasping site, in turn, was correlated with the site on the radius where the spider attached the sticky line. The spacing between loops of sticky spiral, a functionally important aspect of orb design, was thus determined in part, though not completely, by the site at which leg oIV grasped the radius at the moment of attachment.