Locomotion in a sticky terrain

The mirid bug Pameridea roridulae lives mutalistically on the protocarnivorous plant Roridula gorgonias. The latter resembles an effective, three-dimensional flypaper trap which captures numerous flying insects. We have recently shown that P. roridulae bugs are not trapped by the plant, because they are covered with a layer of epicuticular grease, which is considerably thicker than in other insects. The present study demonstrates that the bugs' morphology and locomotory characteristics also contribute to their specialisation for life on the adhesive plant surface. A structural analysis of the mirid bug's attachment system, and an experimental study on its attachment ability were carried out. In traction force tests, maximum forces of 8.8 mN were measured on adaxial R. gorgonias leaves, corresponding to 126 times the bug's body weight. On smooth surfaces, generated forces were only 47 times the bug's body weight. Compared to closely related mirid bug species avoiding contact with plant adhesive secretion, P. roridulae is distinctly stronger and heavier, and holds its body close to the plant substrate. Two locomotion strategies on the glandular hairy plant surfaces are suggested for mirid bug species from the tribus Dicyphini: (1) avoidance strategy, characterised by the slim body held at a large distance from the plant surface by using long, slender legs, and (2) defense strategy, where trapping of the heavy bugs, situated close to the plant surface, is overcome by generating strong forces during locomotion and by having a thick anti-adhesive epicuticular greasy layer on the bugs' cuticle.