Abstract
The slippery zone in pitchers of the carnivorous plant Nepenthes alata bears scattered prominent lunate cells and displays continuous epicuticular crystalline wax coverage. The aim of this study was to examine the influence of the surface anisotropy, caused by the shape of lunate cells, on insect attachment ability. Traction tests with ladybird beetles Coccinella septempunctata were performed in two types of experiments, where surface samples of (1) intact pitchers, (2) chemically de-waxed pitchers, and (3) their polymer replicas were placed horizontally. Beetle traction forces were measured when they walked on test surfaces in either an upward (towards the peristome) or downward (towards the pitcher bottom) direction, corresponding to the upright or inverted positions of the pitcher. On intact pitcher surfaces covered with both lunate cells and wax crystals, experiments showed significantly higher forces in the direction towards the pitcher bottom. To distinguish between the contributions, from claw interlocking and pad adhesion, to insect attachment on the pitcher surfaces, intact versus claw-ablated beetles were used in the second type of experiment. On both de-waxed plant samples and their replicas, intact insects generated much higher forces in the downward direction compared to the upward one, whereas clawless insects did not. These results led to the conclusion that, (i) due to the particular shape of lunate cells, the pitcher surface has anisotropic properties in terms of insect attachment, and (ii) claws were mainly responsible for attachment enhancement in the downward pitcher direction, since, in this direction, they could interlock with overhanging edges of lunate cells.
Highlights
Pitcher-shaped trapping organs produced at the tips of tendrils are characteristic for carnivorous plants from the genus Nepenthes [1,2]
The intact slippery zone of the N. alata pitcher bears relatively large, prominent lunate cells scattered between tabular epidermal cells and microscopic epicuticular wax crystals on top of both cell types (Figure 1A and Figure 1D)
Lunate cells have a special crescent shape with their ends pointed toward the pitcher bottom (Figure 1A, Figure 2A and Figure 2C)
Summary
Pitcher-shaped trapping organs produced at the tips of tendrils are characteristic for carnivorous plants from the genus Nepenthes [1,2]. These traps, using a passive pitfall mechanism for capturing, mainly, invertebrates, consist of several functional zones specialized for prey attraction, capture, retention, digestion, and uptake of nutrients [3,4,5,6]. The digestive fluid collected in the lower part of the pitcher was recently found to be highly viscous preventing trapped insects from escaping [12,13]
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