South Africans don't use bug zappers or commercial flypaper to ward off pesky flies, but instead hang up a bunch of Roridula gorgonias leaves.

Attracted to the shiny adhesive droplets on the leaf's hairs, the flies are soon trapped by this 'natural flypaper.' But R. gorgonias plant is also home to a population of Pameridea roridulae (mirid bugs), which dine on the trapped insects and the mirid bugs never get stuck.

Curious to find out how that works, Dagmar Voigt and Stanislav Gorb from the Max-Planck Institute for Metals Research, Germany, decided to take a look at the non-stick bugs to see how they elude R. gorgonias' grasp and they published their results in The Journal of Experimental Biology on August 8 2008.

They were able to call on R. gorgonias enthusiasts in Germany, according to Voigt, and Klaus Keller in Augsburg agreed to supply the team with the hairy plants and their residents.

Back in their Stuttgart lab, Voigt and Gorb decided to test how non-stick mirid bugs really are. Wrapping a bug in a leaf the team were amazed when they unrolled it and 'the bug jumped up and ran away!' says Voigt. The bug was completely non-stick. Next the team checked the mirid bug's surface by pressing a bug against a glass slide and looking at the slide under a microscope to see if they were covered in a special glue-proof coating. The bugs seemed to be coated in a greasy fluid. Voigt explains that all bugs are covered in a greasy layer, so what made the mirid bug's surface more non-stick than other insect coatings?

Flash freezing the bugs, Voigt and Gorb took a high-resolution look at the insect's coating with a cryo-scanning electron microscope (cryo-SEM). The mirid bug's coating was 30 times thicker than the blowfly they compared it with. But how was this extra thick coating protecting the mirid bugs? Did it come loose when contacted by adhesive? Or was the greasy coating somehow disrupting the glue's adhesive powers?

Touching a sticky hair against a piece of mirid bug cuticle and looking at it with cryo-SEM, the team could see that the glue seemed to run like a fluid over the thick greasy surface. However when they looked at a R. gorgonias hair in contact with a section of blowfly cuticle, the glue formed a discrete blob that looked like a gel with well-defined edges. The mirid bug's greasy coating seems to disrupt the glue in some way, preventing it from adhering to the insect's surface.

Finally, the duo measured how strongly the glue became attached to various insects' surfaces. Having removed the mirid bug's protective layer by washing in cold chloroform, the team found that the glue stuck as strongly to the mirid bugs as to other insects, with the glue stretching to produce filaments as long as 5·cm. But when they successfully attached glue droplets to unwashed mirid bug cuticles, the cuticles easily broke free from the glue, rarely forming filaments more than 1.5·cm long. Voigt suspects that insect victims eventually exhaust themselves, fighting against the adhesive filaments.

Voigt and Gorb are keen to understand more about the mechanism that keeps P. roridulae roaming free, while other insects succumb to the glue that mirid bugs simply shrug off.

REFERENCE: Voigt, D. and Gorb, S. (2008). An insect trap functioning as a habitat: a cohesion-failure mechanism prevents adhesion of Pameridea roridulae bugs to the sticky surface of the plant Roridula gorgonias. J. Exp. Biol. 211, 2647-2657.