Dining opportunities are rare for ticks. The notorious pests spend as much as 90% of their lives clambering around in leaf litter and only embark on epic expeditions to the tips of grass and leaves when they need to feed. Wrapping their limbs around slender smooth stalks of grass when they ascend, ticks must also grasp any passing opportunity to latch onto smooth skin or the hairy pelts of animals. ‘We wondered how their feet are designed and how strongly they can hold on to surfaces’, says Dagmar Voigt from the Technische Universtität Dresden, Germany, adding that in addition to clinging on to a wide range of different surfaces, the feet of an adult tick may have to bear up to 135 times their unfed weight after ticks have consumed a blood meal. Undeterred by the tick's fearsome disease-spreading reputation, Voigt headed out into the undergrowth to capture castor bean ticks (Ixodes ricinus) sheltering under leaves and blades of grass to learn more about their feet.
Back in the lab, Voigt and Stanislav Gorb at Christian-Albrechts-Universitaet zu Kiel, Germany, took a series of increasingly closer looks at the tick's feet. Recalling that Hermann Burmeister had studied tick feet in 1859, Voigt describes how she also saw the two lengthy curved tarsal claws nestled above a pad-like structure – similar to the arolium that many insects use for attachment – that he had described first. However, as the duo scrutinised the structures in greater detail they could see that the pad was packed with springy fibres while the surface was heavily pleated and the long transparent tarsal claws were largely made up of the elastic protein resilin. ‘This was a surprise because we have never observed resilin in the claws of other arthropods’, says Voigt.
But how would the ticks use this equipment to cling on to surfaces ranging from smooth plant walls to human skin? Filming the arachnids’ feet as they secured themselves to a plate of glass, Voigt saw the claws separate and the pleated arolium pad unfold like a fan as it pressed against the smooth surface, generating a large footprint. And when the tick lifted its foot away, it left behind a large patch of tarsal fluid. ‘Compared to other arthropods, ticks release a remarkable volume of this fluid’, says Voigt. But how tightly would the animals hold on?
Designing a series of increasingly rough resin surfaces, Voigt also made a silicon cast of her own forearm skin in addition to allowing the blood-sucking pests to roam freely on her while measuring how well they clung to each surface when inverted. Impressively, the ticks were able to hold on best to Voigt's own skin, with 90% of the arachnids remaining attached to her and the smooth glass. In contrast, only 52% of the animals managed to cling to the silicon skin cast while only 46% got a grasp on one of the mid-roughness (3 μm) resins. And when Voigt measured the force generated by the ticks as they marched horizontally across various surfaces, the arachnids managed to hold on tightest to the smooth glass surface, with a force that was more than 500 times their ∼17 μN weight.
So, ticks are remarkably well adapted to clinging onto smooth skin in addition to snagging themselves in hair with their grappling hook claws, and Voigt is optimistic that her discovery could help in the design of tick-proof surfaces to deter the little pests from hitching a ride when we're out for a ramble.