Aspects of pitcher morphology and spectral characteristics of six Bornean Nepenthes pitcher plant species: Implications for prey capture

Abstract

Pitcher plants of the genus Nepenthes attract and trap invertebrate prey using nectar-secreting pitchers. Pitcher morphology and spectral reflectance characteristics were investigated for six Nepenthes species from northwest Borneo (N. albomarginata, N. ampullaria, N. bicalcarata, N. gracilis, N. mirabilis var. echinostoma and N. rafflesiana). Morphological measurements focused on the size of the pitcher rim (or peristome, the site of the major nectaries) in relation to pitcher length. The results show considerable interspecific variation in morphology. Spectral reflectance measurements quantified the degree of colour contrast between the peristome and pitcher body, from ultraviolet (UV) to red wavelengths. The contrast maxima for each species were compared with insect visual sensitivity maxima. The six species showed a wide range of reflectance patterns, with pitchers of N. rafflesiana possessing the greatest degree of 'fit' between contrast maxima and insect sensitivity maxima, in the UV, blue and green regions of the spectrum. Based on the morphological and reflectance analyses, we hypothesized that pitchers of N. rafflesiana would be more attractive to anthophilous (flower-visiting) invertebrates than the sympatric N. gracilis. Analysis of prey contents generally supported the hypothesis, suggesting possible interspecific resource partitioning. Morphological and spectral characteristics of the other species are discussed in relation to published studies on prey capture by those species.