The role and mechanism of Nepenthes rafflesiana pitchers as insect traps in Brunei
Investigations were made into the trapping biology of the pitchers of Nepenthes rafflesiana (Jack), Family Nepenthaceae, in Brunei, Borneo. The plant produces two distinct pitcher forms, upper and lower, which were found to differ in their prey spectra. Although ants were the most important prey in both forms, the upper form caught more anthophilous insects than the lower. High contrast U/V patterns on the upper pitchers were found to contribute to the attraction of some anthophilous insects, as was the fragrance of upper pitcher fluid. Increased height above ground was also found to increase the numbers of anthophilous insects caught. Upper pitchers possessed features of several pollination syndromes, but may have been mutualistic rather than mimetic, as certain criteria for mimesis were not met. Evidence for mutualism is strongest for ants, in which both pitcher forms were found to be specialising, although the involvement of lower pitcher 'wings' in such a relationship, was discounted. The loss of wings from the upper pitcher form may have occurred to reduce strain on the upper pitcher-tendril attachment. Reduction of fluid volume, and strengthening of the attachment in upper pitchers, support this idea. The possession of upper pitchers was found to allow N.rafflesiana access to populations of anthophilous insects that were not as easily available to the sympatric Nepenthes gracilis (Korth). There was found to be a shift in the prey spectrum of lower N.rafflesiana pitchers, as pitcher size increased. Production of lower pitchers was found to increase when upper pitchers were rendered non-functional. This may allow the plant to regenerate after stem damage. Although covering of pitchers failed to produce a decrease in either male flowering success, or &'37 age of viable seeds produced by female plants, pre-flowering pitcher counts suggested that pitchers were an important factor in reproductive fitness.