Have you ever noticed that insects' feet just never seem to get dirty? No matter that they've been standing on, they still stick to the next surface. The same can't be said for man-made adhesives: sticky tape soon becomes contaminated and fails to bond. And many insects maintain the same adhesive structures throughout the whole of their adult lives, so they have to keep them clean somehow. James Bullock from the University of Cambridge explains that many creatures groom their bodies to maintain them in tip-top condition, but running insects can't groom their feet after every step, so he and his colleagues, Christofer Clemente, Andrew Beale and Walter Federle, wondered whether the insect's attachment surfaces are self-cleaning (p. 635). Could the insects be cleaning their feet simply by taking the next step?
‘Broadly speaking insects use two adhesive systems: smooth and hairy,’ explains Bullock. Some insects' feet are smooth and covered in a thin fluid film that helps them to hold on tight, while other insects' feet are coated with fluid covered microscopic hairs that mould to surfaces as they attach. Knowing that stick insects have smooth feet while dock beetles have hairy feet, the team decided to find out whether both species could clean their dirty feet by walking across a smooth surface.
Coating a beetle's hairy foot with simulated dirt (microscopic polystyrene spheres ranging in size from 1 to 45 μm diameter), the team gently touched the insect's foot against a series of microscope slides and either pulled them off directly or slid the foot slightly before detaching it. Measuring the force that it took to pull the insect's feet free, the team found that during the first foot contact, the attachment forces plummeted by as much as 90%. However, after each additional contact the attachment forces improved, especially when they slid the foot before detaching. And when the team scrutinised the beetle's footprints, they could clearly see microscopic beads left behind in the adhesive fluid.
The beetle's hairy feet were self-cleaning. However, the insects seemed to run into a problem with the 10 μm beads: they could never get their feet entirely clean. And when the team took a closer look at the hairs on the beetle's feet, they could see why. The beetle's hairy feet were clogged with the beads, which fitted perfectly between adjacent hairs.
Having tested the beetles' hairy feet, the team turned their attention to the smooth footed stick insects. Repeating the same tests that they had tried on the beetles, the team could see that the stick insects' attachment forces returned to normal when they slid the insects' feet before detaching. So insects with smooth feet are also capable of cleaning their feet when they include a brief slide. However, when the team measured how the attachment forces changed over successive footfalls when they simply pulled the feet free without sliding them, the insects were never able to get their feet entirely clean. ‘Smooth footed insects need some sort of slide to shake off the spheres,’ says Bullock.
So sticky insect feet are self-cleaning, and Bullock and his colleagues are keen to find out exactly how the insects sticky feet dislodge dirt when they get contaminated.