Function of epidermal surfaces in the trapping efficiency of Nepenthes alata pitchers

Several epidermal microstructures characterize surfaces of pitcher plants and are presumably involved in their trapping function. Here we report the effects of Nepenthes alata surfaces on insect locomotion and trapping efficiency. The architectural designs of pitcher surfaces were characterized using scanning electron microscopy. Two insect species - fruitfly (Drosophila melanogaster) and ant (Iridomyrmex humilis) - were tested for their ability to remain and walk on them. The relative contributions of various epidermal structures to trapping ability were quantified. Pitchers were very effective traps for both insect species. They were slightly more efficient in capturing the ants, but slightly more effective in retaining captured flies. Trapping efficiency was attributed to the combined effects of several surfaces displaying different functions. The waxy zone played a key role in the slippery syndrome: in addition to the wax itself, the subjacent layer of convex lunate cells interfered considerably with insect locomotion. The unsubmersed glandular zone displayed an important retentive effect and secretions of the digestive glands are suspected to be adhesive. Pad performances of the hairy and smooth system of attachment are discussed to explain the differences between the two insect species. This study aims to encourage biomechanical studies of plant-insect surface mechanisms.