Floral tubes and coevolved pollinators

(a) This zinnia flower is composed of an outer rim of showy flowers and a central disk of narrow tubular flowers that produce nectar. Butterflies, but not other pollinators, are able to reach the nectar by using their narrow tongues. (b) The hibiscus flower produces nectar in a floral tube whose diameter corresponds to the dimensions of a hummingbird bill. (c) The saguaro cactus (Carnegiea gigantea) flower forms a floral tube that is wide enough for nectar-feeding bats to get their heads inside. The cactus flower has been drawn here as if it were transparent, to illustrate bat pollination.