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.