attract-prey attracted attraction carnivore carnivorous carnivorous-plants clumps clusters coloring colors colouring colours cover devour digest digested digesting display distinctive edge edges enzymes flora flowers funnel herbaceous insect-trap insects nectar-like-secretions nutrient-source nutrients operculum passive-trap perennials pitcher-plants pitchers pitfall-traps plants prey proteases rims slippery species trap unusual variations variety vegetation veins