Sticky feet are ideal for clinging to vertical surfaces and ceilings, but not so good when it comes to staying clean. ‘Contamination of the adhesive surfaces by loose material can seriously compromise attachment ability’, says Michael Orchard and colleagues from the University of Hull, UK, and the Australian National University. Yet, the sticky feet of ladybirds and ants self clean routinely, so how do they do it? The Anglo-Australian team suspected that the surface upon which an insect is walking could have something to do with their ability to shuck off dirt. ‘We hypothesised that... cleaning would depend on the free surface energy [FSE – a measurement of a surface’s slipperiness] of both the contaminating particles and the surfaces, with surfaces with high FSE (glass) predicted to be better cleaning surfaces than those with low values of FSE', they say (p. 279).
Coating glass vials with slippery silicon, the team then systematically contaminated the hairy feet of large and small species of ladybird and the smooth sticky pads of ants with large and small PTFE (slippery) and glass microspheres (less slippery). Next, the team monitored the insects' behaviour and recorded how long it took them to regain their footing on the vertical walls of the untreated and siliconised glass vials.
The team report that all of the insects found it easiest to remove glass spheres on a glass surface and it always took them longer to clean feet contaminated with slippery PTFE particles. They also found that small contaminants were the most difficult to dislodge, especially for the smallest insects. Considering the slipperiness (FSE) of the vial surfaces, the team found that the ladybird's hairy adhesive pads shed dirt more easily on the non-slippery (high FSE) glass than on the slippery (low FSE) siliconised vials. However, the slipperiness of the surface did not affect the smooth-padded ants. So the slipperiness of the surface can affect an insect's ability to self-clean, but this depends on which attachment mechanism the insect uses.