it has been suggested that all trap types are modifications of a similar basic structure—the hairy leaf. hairy (or more specifically, stalked-glandular) leaves can catch and retain drops of rainwater, especially if shield-shaped or peltate, thus promoting bacteria growth. insects land on the leaf, become mired by the surface tension of the water, and suffocate. bacteria jumpstart decay, releasing from the corpse nutrients that the plant can absorb through its leaves. this foliar feeding can be observed in most non-carnivorous plants. plants that were better at retaining insects or water therefore had a selective advantage. rainwater can be retained by cupping the leaf, and pitfall traps may have evolved simply by selection pressure for the production of more deeply cupped leaves, followed by "zipping up" of the margins and subsequent loss of most of the hairs. alternatively, insects can be retained by making the leaf stickier by the production of mucilage, leading to flypaper traps.