Sub-units in the webs of Dictyna meditata (Arachnida: Araneae: Dictynidae): implications for studies of spider web evolution

Abstract

Studies of web evolution in spiders generally focus on the overall designs of webs in the field. As has been typical for dictynids and several other cribellate families with “irregular” webs, this study detected few discernable patterns in the field regarding the spatial organization of the highly variable, three-dimensional and largely aerial webs of the dictynid Dictyna meditata Gertsch, 1936. Nevertheless, there were three consistent sub-unit designs in the additions that spiders made to their webs in captivity, and in webs that they built from scratch in captivity: “silk ladders”, with a cribellum line that zig-zagged between a pair of approximately parallel non-sticky lines; “twig ladders”, with a cribellum line that zig-zagged between a non-sticky line and the substrate; and long non-sticky lines that each supported a long, slightly looped cribellum line. I suggest, using examples from dictynids and other families with long-lived, geometrically irregular webs, that this pattern of using consistent behavior patterns to add geometrically regular “modules”, is widespread and ancient, but has often been missed due to damage and additions to webs in the field, and to lack of direct behavioral observations. Recent attempts to link web evolution to studies of spider phylogeny could benefit from a change of emphasis, focusing on the additions that spiders make to their webs, rather than on the currently common but necessarily vague characterizations of overall web designs seen in the field.