Control of Drag-Line Spinning in Certain Spiders

Araneomorph spiders from many different families show some regionaldifferentiation of the duct which carries the drag-line silk,but only in the orb-web spiders is there a well-defined controlvalve. This valve, and its associated muscles, is described for Araneusdiadematus (Argiopidae), an ecribellate spider, and is comparedwith that found in Uloborus octonarius (Uloboridae), a cribellatespider. It is suggested that the remarkable similarity betweenthe valves in these two groups implies evolutionary convergence. Some evidence is presented which suggests that variations inbody pressure are used to control the drag-line spinning, atleast in the more primitive Araneomorphs.