Functional changes in web design along the ontogeny of two orb-weavers

Abstract

Orb webs evolved primarily to capture prey, though they also have other functions. Recently, it has been argued that the orb web does not work as a functional unit, but instead some sections or components have presumably been shaped by selection to increase capture success of large prey (relative to the spider size). Changes in these components (e.g., an increase capture area) presumably compromise the design and function of other components (e.g., density of adhesive threads). In this study, we explore the changes in the design of orb webs throughout the ontogeny of two orb-weaving spiders of the genus Leucauge: L. mariana (Taczanowski, 1881) and L. argyra (Walckenaer, 1841). Small nymphs of both species construct webs with a relatively larger capture area and higher density of adhesive spiral loops compared to webs of larger individuals. In addition, small nymphs of L. argyra construct webs with more radii. These features probably increase the probability of capturing large prey. Some web features show different trade-offs in the two species. For instance, the number of adhesive threads increases with capture area in webs of L. mariana, but decreases in L. argyra. The density of adhesive threads in webs of both species decreases as the area of the web increases, but decreases faster in L. argyra. Thus, small nymphs are capable of optimizing different structural components of the web to increase the probability of capturing large prey, but the trade-offs between web features vary between species.