A few years ago, Jonas Wolff stumbled across a high-definition photograph of a harvestman on the internet and was puzzled by the arachnid's pedipalps; they were covered in hairs carrying tiny droplets of fluid at their tips. ‘It [the harvestman] reminded me of a carnivorous plant, the sundew, which uses sticky tentacles to capture insects’, he recalls. Papers dating back a century suggested that the arachnids might use their sticky organs to capture small soil-dwelling arthropods, like springtails. Wolff teamed up with Clemens Schaber and Stanislav Gorb, at the Christian Albrecht University of Kiel, and Axel Schönhofer, at the Johannes Gutenberg University Mainz, Germany, to show how these secretions can trap prey (p. 3535).
First, they took a close look at harvestmen eating. Schönhofer, a harvestman expert, helped Wolff collect several species and their springtail prey. Placing the animals in plastic dishes, Wolff filmed them using a high-speed video camera to find out how the harvestmen attack. Fortunately, one species (Mitostoma chrysomelas) obliged and Wolff recorded 38 instances where harvestmen used their sticky pedipalp secretions to glue their prey, half of which resulted in a meal for the harvestmen. ‘This species successfully attacked springtails larger than themselves’, Wolff says. And the unlikely predators' spindly legs also help them to capture prey. Wolff explains that springtails can generate high forces using a special jumping organ, the furca, which works like a spring, when trying to escape, and the harvestmen's ungainly build seems to help them absorb collisions. ‘On high-speed video, you can see that the harvestman works like a wobbly wire ball buffering the strong impacts’, he explains.
Next, the team tested the adhesive properties of the secretions, which was tricky, because the pedipalp hairs are tiny. Working with Schaber, Wolff constructed a very fine glass pipette tip with a microscopic glass bead at the end to use as a force sensor. The duo carefully mounted a pedipalp on to a table that could be moved in minute increments in three dimensions, attached a single pedipalp hair to the glass bead and then pulled the pedipalp back at different speeds while measuring the pipette tip's deflection with high-speed video. They found that the adhesive force of just a single hair was sufficient to hold the body weight of an average springtail. Wolff was also excited to find that, the faster they pulled the pedipalp away, the stronger the droplet's attachment force was. ‘This shows that the secretion is a non-Newtonian fluid’, he explains. ‘It behaves like a solid under high impact, like corn starch solutions.’ So, the more the prey struggles, the more stuck it gets: ‘A deadly trap’, Wolff concludes.
Wolff also adds that attaching glue to a springtail is not a trivial feat, as their cuticles are covered in complex microstructures that repel liquids and should prevent glue from taking hold. To investigate whether harvestman secretions can wet a springtail's cuticle, Wolff used liquid nitrogen to snap freeze harvestmen glued to springtails and then used cryo scanning electron microscopy to see how the secretion spread. To his surprise, he saw that the secretion completely wets the springtail cuticle, allowing the glue to get a grip.
The team concludes that harvestman pedipalp secretions are used as a sticky trap for prey, and notes that their viscoelastic properties closely mirror those of spider capture threads and insect-trapping plants. ‘This convergence is exciting, and suggests that viscoelastic solids are an effective, and presumably highly economic, means of prey capture,’ Wolff concludes.