The insect-trapping rim of Nepenthes pitchers: surface structure and function

Abstract

Carnivorous pitcher plants of the genus Nepenthes capture prey with a pitfall trap that relies on a micro-structured, slippery surface. The upper pitcher rim (peristome) is fully wettable and causes insects to slip by aquaplaning on a thin water film. The high wettability of the peristome is probably achieved by a combination of hydrophilic surface chemistry, surface roughness and the presence of hygroscopic nectar. Insect foot attachment could be prevented by the delayed drainage of the thin water film between the adhesive pad and the surface. Drainage should be faster for insects with a hairy adhesive system; however, they slip equally on the wet peristome. Therefore the stability of the water film against dewetting appears to be the key factor for aquaplaning. New experimental techniques may help to clarify the detailed function of the pitcher plant peristome and to explore its potential for biomimetic applications.

Figure 1

Schematic illustration of a Nepenthes pitcher.

Figure 2

(A) Peristome surface (p) of Nepenthes alata, structured by first (r₁) and second order radial ridges. In between the tooth-like projections at the inner edge of the peristome the pores of large extrafloral nectaries (n) can be seen. Below the peristome is the wax-covered inner wall surface (w). (B) The second order ridges (r₂) are formed by straight rows of overlapping epidermal cells.

Figure 3

(A) The contact angle θ_SWG of a water droplet on a surface provides a measure of the wettability of the surface. (B) Simplified model of the contact of an insect adhesive pad with a surface, mediated by an adhesive fluid.

Figure 4

Two different designs of adhesive pads are found in insects. (A) Smooth pad of an ant (Oecophylla smaragdina). (B) Hairy pad of a beetle (Gastrophysa viridula).