If we look at flowering plants of the world, those pollinated by beetles are frequently dark purple and green. Bees don't pay attention to those colors. Some flowers that are pollinated by beetles have trap-like shapes--and the flowers of Darlingtonia fit that description. The reason why beetle-pollinated flowers have a trap-like shape is that beetles don't fly in and out of flowers very efficiently--beetles tend to stay in flowers and feed on flower parts. By bumbling around inside a flower, beetles can pick up a lot of pollen or deposit it on the pollen-collecting surfaces, the stigmas (the forklike structures in this picture). The longer a beetle stays inside a flower, the more likely it is to pollinate the flower.