carnivore carnivorous devour digest eat flower green insect insects pitcher plant red trap veins botanic garden botanics catch danger digests Dionaea muscipula eating flies flowers fly Flytrap glasgow grip insect eating nutrition plants savage Venus nature pitcher plant purple sarracenia purpurea attract-prey attracted attraction carnivorous-plants clumps clusters coloring colors colouring colours cover digested digesting display distinctive edge edges enzymes flora funnel herbaceous insect-trap nectar-like-secretions nutrient-source nutrients operculum passive-trap perennials pitcher-plants pitchers pitfall-traps prey proteases rims slippery species unusual variations variety vegetation