Harmless nectar source or deadly trap: Nepenthes pitchers are activated by rain, condensation and nectar

Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK.
Proceedings of the Royal Society B: Biological Sciences (Impact Factor: 5.05). 03/2008; 275(1632):259-65. DOI: 10.1098/rspb.2007.1402
Source: PubMed


The leaves of Nepenthes pitcher plants are specialized pitfall traps which capture and digest arthropod prey. In many species, insects become trapped by 'aquaplaning' on the wet pitcher rim (peristome). Here we investigate the ecological implications of this capture mechanism in Nepenthes rafflesiana var. typica. We combine meteorological data and continuous field measurements of peristome wetness using electrical conductance with experimental assessments of the pitchers' capture efficiency. Our results demonstrate that pitchers can be highly effective traps with capture rates as high as 80% but completely ineffective at other times. These dramatic changes are due to the wetting condition of the peristome. Variation of peristome wetness and capture efficiency was perfectly synchronous, and caused by rain, condensation and nectar secreted from peristome nectaries. The presence of nectar on the peristome increased surface wetness mainly indirectly by its hygroscopic properties. Experiments confirmed that pitchers with removed peristome nectaries remained generally drier and captured prey less efficiently than untreated controls. This role of nectar in prey capture represents a novel function of plant nectar. We propose that the intermittent and unpredictable activation of Nepenthes pitcher traps facilitates ant recruitment and constitutes a strategy to maximize prey capture.

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    • "Capillary action between the ridges also renders the peristome highly wettable , and it has been shown that a wetted peristome provides very little traction for insect feet, effectively causing the prey to 'aquaplane' and slip into the pitcher. Conversely, many insect visitors have little trouble safely traversing the peristome when it is dry (Bohn and Federle, 2004; Bauer et al., 2008, 2009, 2011, 2012a, b). The width of the peristome (and thus its presumed effectiveness as a conveying surface) varies considerably among species. "
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    ABSTRACT: Background and Aims Nepenthes (Nepenthaceae, approx. 120 species) are carnivorous pitcher plants with a centre of diversity comprising the Philippines, Borneo, Sumatra and Sulawesi. Nepenthes pitchers use three main mechanisms for capturing prey: epicuticular waxes inside the pitcher; a wettable peristome (a collar-shaped structure around the opening); and viscoelastic fluid. Previous studies have provided evidence suggesting that the first mechanism may be more suited to seasonal climates, whereas the latter two might be more suited to perhumid environments. In this study, this idea was tested using climate envelope modelling.
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    • "Due to its wetness-dependence, the peristome only activates the trap intermittently, and visiting insects can safely harvest nectar during inactive (i.e. dry) times [15]. By this means, the plant may promote the survival of ‘scout’ ants that ultimately recruit larger numbers of worker ants to the trap. "
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    ABSTRACT: Carnivorous pitcher plants capture prey with modified leaves (pitchers), using diverse mechanisms such as 'insect aquaplaning' on the wet pitcher rim, slippery wax crystals on the inner pitcher wall, and viscoelastic retentive fluids. Here we describe a new trapping mechanism for Nepenthes gracilis which has evolved a unique, semi-slippery wax crystal surface on the underside of the pitcher lid and utilises the impact of rain drops to 'flick' insects into the trap. Depending on the experimental conditions (simulated 'rain', wet after 'rain', or dry), insects were captured mainly by the lid, the peristome, or the inner pitcher wall, respectively. The application of an anti-slip coating to the lower lid surface reduced prey capture in the field. Compared to sympatric N. rafflesiana, N. gracilis pitchers secreted more nectar under the lid and less on the peristome, thereby directing prey mainly towards the lid. The direct contribution to prey capture represents a novel function of the pitcher lid.
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    • "Insects have evolved numerous adaptations to enable them to move rapidly across natural surfaces within their ecological niches. Efficient adhesion is crucial for many different aspects of an insect's life, such as mating and oviposition (Bitar et al., 2009; Bitar et al., 2010), foraging and prey capture (Hölldobler and Wilson, 1990; Bauer et al., 2008), defence (Eisner and Aneshansley, 2000; Betz and Kolsch, 2004) and the selection and construction of nesting sites (Federle et al., 1997), especially for arboreal insects (Federle et al., 2002). When surfaces are rough, insects can utilise their tarsal claws to attach to surface asperities (Federle et al., 2002). "
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