Sticky spiral building in Araneus diadematus

This video clip shows in time-lapse the building of half of a loop of a sticky spiral by an adult cross spider Araneus diadematus.

This movie demonstrates indirectly the formation of glue droplets on the sticky spiral. Since the light shines onto the web from above right (from around one o'clock), only those non-sticky (cylindrical) threads are visible, which run from the upper left to the lower right, i.e., the radii in the upper right of the movie, and the auxiliary spiral in the part of the web below and slightly to the left of the hub. In contrast, the sticky threads, or rather the droplets on them, reflect the light in all directions, making them visible independently of the orientation of the thread.

When the spider builds the sticky spiral, it applies the glue in an even layer around the axial thread of the sticky silk. Consequently, the newly laid sticky silk - being still cylindrical - is visible in much the same way as the non-sticky silk (see below). The glue then takes up water from the atmosphere and through surface tension (presumably), glue droplets form (Vollrath & Tillinghast 1991). The formation of glue droplets begins about 20 seconds after the thread has been placed in the middle of each sticky spiral segment and advances towards the egdes of the segment. At the same time, the visibility of the sticky silk changes and becomes similar to that of previously laid sticky spiral thread.

Schematic drawing of the light reflection on cylindrical silk (left) and on silk with glue droplets (right). On cylindrical silk (non-sticky silk and newly laid sticky silk), all incoming light (yellow) is reflected into the same direction (orange arrows). Consequently, cylindrical silk is visible only from a certain angle, but appears especially bright. In contrast, the glue droplets of mature sticky silk reflect the light in various directions, making the silk visible from different angles.