Spiders avoid sticking to their webs: clever leg movements, branched drip-tip setae, and anti-adhesive surfaces

Orb-weaving spiders construct webs with adhesive silk but are not trapped by it. Previous studies have attributed this defense to an oily coating on their legs that protects against adhesion or, more recently, to behavioral avoidance of sticky lines. The old evidence is very weak, however, and the behavioral avoidance explanation is inadequate because orb-weavers push with their hind legs against sticky lines hundreds or thousands of times during construction of each orb and are not trapped. Video analyses of behavior and experimental observations of isolated legs pulling away from contact with sticky lines showed that the spider uses three anti-adhesion traits: dense arrays of branched setae on the legs that reduce the area of contact with adhesive material; careful engagement and withdrawal movements of its legs that minimize contact with the adhesive and that avoid pulling against the line itself; and a chemical coating or surface layer that reduces adhesion.