In Gabon, we investigated the floral morphology and biology of Napoleonaea vogelii (Lecythidaceae), a cauliflorous understory tree of lowland humid rain forest, and made observations on the behavior of insect visitors. The major insects visiting flowers were: a thrips (Ceratothripoides brunneus), a small moth (Glyphipterix sp.), four species of leaf beetles (Gabonia colae, Gabonia sp. undescribed, Macrocoma sp., and Monolepta sp.), three species of weevils (Endaeus sp. 1–3), and two species of ants (Technomyrmex sp. and Monomorium sp.). In N. vogelii, the flower is quite complex, and we propose a floral morphological definition for complexity. Insect visitors are spatially segregated on the flower. The evolution of the morphology and biology of the flower of N. vogelii is influenced by insect visitors, both pollinating and non-pollinating. The thrips are the likely pollinators of the flower.