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