Spiders are famous for gluing prey in their webs, but biologists have discovered that their close relatives, the harvestmen, can create sticky traps with their legs. The spindly limbed harvestmen are often called daddy longlegs. The short, leglike extremities on a harvestman’s face, the pedipalps, are covered in tiny hairs that carry droplets of sticky liquid at the tips. Using high-speed video cameras, German biologists observed that with just a touch of its sticky leg, the harvestman can capture fast-moving, soil-dwelling creatures known as springtails that are larger than itself (as shown in the above video). To test the strength of the adhesive, the researchers used a fine glass pipette tip with a microscopic glass bead at the end to pull a single sticky pedipalp hair. They pulled the pedipalp back at different speeds and recorded the deflection of the pipette tip on video. Just a single hair was sufficient to hold the weight of an average springtail, the scientists report online today in The Journal of Experimental Biology. Even more to the researchers’ surprise, the faster they pulled the pedipalp away, the stickier the droplet became, which shows that the glue is a so-called non-Newtonian fluid that, unlike most fluids, becomes more viscous when subjected to a sudden force. That means that the more a springtail struggles, the tighter the glue will hold it.
(Video credit: Jonas O. Wolff/University of Kiel)