Figure 4.

Diagrammatic representation of the instrument used to measure the force generated by contraction of the silk ejected during one spitting episode. In this view, the spider is located behind the plane of the image, facing the viewer through the deposited silk (see Video 4). Before expectoration, the two collection wires are vertical; silk contraction causes the movable wire to be pulled toward the fixed wire, in turn deflecting the laser beam to a higher position on the graduated beam splitter and allowing more of the laser light to stimulate the photo diode sensor. Changes in output (mV) from the photo diode are recorded in digital form for later analysis.