Unfortunate insects that come into contact with a Roridula gorgonias leaf don't stand a chance. Within moments the struggling victim is swathed in sticky secretions exuding from the leaf's hairs, and its fate is sealed. Curious to find out exactly how R. gorgonias leaves ensnare their prey, Dagmar Voigt and Elena and Stanislav Gorb from the Max Planck Institute for Metals Research and Kiel University, Germany, decided to take a closer look at the hierarchy of hairs on R. gorgonias leaves(p. 3184).
Measuring the length of the leaves' hairs, the team could see that they fell into three classes: long slender hairs ranging from 3.3 to 5 mm, medium length thicker hairs from 1 to 2.4 mm and short fat hairs from 0.3–0.7 mm. And when they tested the hairs' stiffness, the long thin hairs were the most flexible, while the medium length hairs were almost 4 times stiffer and the short hairs were almost 50 times stiffer, bending only at the bottom of the shaft.
Next the team sandwiched the adhesive secretions, from all three hair types, between glass coverslips and tried to pull them apart to measure the adhesive's stickiness. The longest hairs produced the weakest of the three adhesives (17.5 kPa), while the medium length hairs were almost 1.5 times stickier (24.5 kPa) and the short hairs' adhesive registered 156.2 kPa; almost 4 times the strength of flypaper glue.
So how do these sticky hairs entrap a victim? Voigt and her colleagues suspect that hapless insects fall foul of the plant's sticky leaves in a cascade of events. First, the insect brushes against, and sticks to, a long hair. As it begins to thrash around, it contacts more of the long hairs,becoming entangled in their sticky secretions. Next, it contacts the stiffer medium length hairs with intermediate strength adhesive and is finally trapped by the rigid short hairs with the strongest glue. Eventually the struggling insect runs out of energy and is immobilised.
Given the effectiveness of R. gorgonias' natural flypaper, Voigt and her colleagues are excited to have discovered the hair hierarchy mechanism that helps R. gorgonias get a grip.