The hub as a launching platform: rapid movements of the spider Leucauge mariana (Araneae: Tetragnathidae) as it turns to attack prey

Spiders are effectively blind with respect to the lines in their webs, and they commonly use exploratory leg movements to find lines, just as a blind man finds objects using a cane. Nevertheless, a mature female Leucauge mariana (Keyserling 1881), which spins a relatively open, sparsely-meshed hub and whose legs I and II hold widely-spaced radii rather than dense hub lines, turns precisely and rapidly when prey strike her orb. She can turn > 90 degrees, finding and grasping new lines with all her legs, in as little as 0.1 s and can reach a prey several body lengths away in as little as 0.23 s after impact. The hub design and resting postures of the spider's legs allow her to sense where the prey strikes the web, generate the force necessary to turn her body rapidly, and find lines to grasp. The spider may move most (if not all) of her legs, without obtaining further guidance information once the leg has begun to move until it nears the site where it will grasp a line. The order in which legs are moved is relatively consistent, and each tarsus moves to a site where lines are relatively abundant; some then make small, quick searching movements to find and grasp lines there. When radial lines were experimentally cut near the hub in a sector in which a prey was subsequently introduced, legs I and II first made small searching movements, and then executed much larger searching movements. The rapid leg movements directed toward specific areas where lines are abundant, and the small searching movements employed at these sites suggest that the spider modifies her behavior when she is at the hub of an orb.