Control of Drag-Line Spinning in Certain Spiders

Synopsis. Araneomorph spiders from many different families show some regional differentiation of the duct which carries the drag-line silk, but only in the orb-web spiders is there a well-defined control valve. This valve, and its associated muscles, is described for Araneus diadematus (Argiopidae), an ecribellate spider, and is compared with that found in Uloborus octonarius (Uloboridae), a cribellate spider. It is suggested that the remarkable similarity between the valves in these two groups implies evolutionary convergence. Some evidence is presented which suggests that variations in body pressure are used to control the drag-line spinning, at least in the more primitive Araneomorphs. The drag-line plays a very important part in the life of a spider, ranging from a simple lifeline in the Salticids to a fundamental part of the complex web in the Argiopids. Its function is sometimes less clear in the more primitive spiders, for instance in the Gnaphosids and Clubionids which are typically nocturnal hunters, but even with these it may be a safety device. Spiders are a very varied group, and one might, therefore, expect to find equally varied uses for the drag-line in the different families, and even between the species