Pitcher dimorphism, prey composition and the mechanisms of prey attraction in the pitcher plant Nepenthes rafflesiana in Borneo

1 The pitcher plant Nepenthes rafflesiana Jack produces dimorphic pitcher types. Upper pitchers catch more flying prey than lower pitchers. The most important single prey group in both pitcher forms was Formicidae. 2 Both pitcher forms possess ultraviolet (UV) patterns which were associated with the site of the major nectaries, causing it to stand out in contrast to the adjacent area. Manipulation of these patterns in upper pitchers confirmed their role in the attraction of prey. 3 Upper pitchers produce a fragrance which is attractive to flying prey. The combination of visual and olfactory cues accounted for the greater prey attraction of upper over lower pitchers. This is confirmed by the pitchers of N. rafflesiana var. elongata, which possess neither UV patterns nor fragrance, and catch significantly less prey than the typical form. 4 Lower pitchers resting at ground level, where densities of potential invertebrate prey are highest, are less than optimally attractive. This may reduce the amount of prey caught, thus preventing overloading and consequent putrefaction of the contents, and death of the pitchers.