False-colour ultraviolet image of a dandelion flower showing the 'honey guides' (dark areas) invisible to the human eye, but seen by insects. The dark lines and patches here guide the insect to the collection of nectar stored in the centre and to the pollen on the anthers. The image was taken using black and white ultraviolet film and then false-coloured in a realistic manner. If colour ultraviolet film had been used the flower would have appeared blue.