Similar to the sticky dew of carnivorous plants, daddy longlegs produce a death-trap made of glue right near their mouths. The gluey fluid behaves like cornstarch in water: The more the prey struggles, the stronger the glue holds. The work was published in the Journal of Experimental Biology last week.
These spindly-limbed, soil-dwelling arachnids -- also known by another rad nickname, harvestmen -- have drumstick-shaped hairs on their pedipalps, short leg-like appendages attached to their heads used to maneuver food to their mouths. The little glandular pedipalp hairs, called clavate setae, appear to be coated with tiny droplets at their tips (see picture above). Researchers speculate that these structures might be sticky and used to capture prey. “It reminded me of a carnivorous plant, the sundew, which uses sticky tentacles to capture insects,” says Jonas Wolff from the University of Kiel in Germany.
Using high-speed video cameras, Wolff’s team observed 38 instances of a harvestman species called Mitostoma chrysomelas capturing fast, jumpy bugs called springtails with just a slight touch of their pedipalps. With the help of the sticky secretions, the harvestmen attacked springtails much bigger than themselves -- and they were successful at least half of the time. You can watch some of the videos here.
Springtails have a special jumping organ called the furca that works like a spring. When they’re trying to escape, these insects generate high forces, but daddy longlegs are able to absorb the collisions. “On high-speed video, you can see that the harvestman works like a wobbly wire ball buffering the strong impacts,” Wolff tells Inside JEB.
Then the team tested the adhesive properties of the secretions. Using a tiny setup consisting of glass pipette tip with a microscopic glass bead at the end (as a force sensor), the researchers were able to pull on a single pedipalp hair. The adhesion of single hair, they found, increased proportionally with pull-off velocity, and the adhesive force could hold the body weight of a typical springtail. In other words, the faster a struggling springtail moves, the stronger the droplet’s attachment force.
"This shows that the secretion is a non-Newtonian fluid,” Wolff says. Unlike most fluids, Science explains, the glue becomes more viscous when subjected to a sudden force. “It behaves like a solid under high impact, like cornstarch solutions.” The more the prey tries to escape, the more stuck it gets.
When stretched, the glue droplets exhibit viscoelastic characteristics, taking the form of beads-on-a-string over time, like spider capture threads. But springtails do have counter-adaptations against gluey traps: Not only do their carapaces repel moisture (since glue needs time to set), Wired reports, they’re also covered in tiny, detachable scales. However, microscopy revealed that the gluey secretions completely wets the springtail’s protective cuticle, allowing the glue to get its grip fast.