Here is a herbage-covered slope where here and there shine out the white flowers of the bindweed of the fields. A butterfly of the genus Pieris flits among these flowers, and with undulating flight moves from one to another, neglecting none. The flowers of the plant have a penetrating perfume, but this perfume cannot act as a guide to the insect, for the distance which separates the flowers is too great. Evidently the color and the form of the flowers are the attractive agents.
In fact, when one destroyed the visible parts of the flowers, without touching the nectaries, once at least the insect was not attracted. This is what Darwin1 observed removing the blue-colored floral parts in a spray of Lobelia erinus assiduously frequented by the workers of the honey-bee. The same observation was made by Lovell (1909) on the flowers of pear, borage, and pumpkin, and by Josephine Wery with bouquets of natural or artificial flowers. Working with the monkshood and with Digitalis, Gaston Bonnier reached an opposite conclusion, but this learned botanist himself notes a similar occurrence. It is necessary "to observe after a certain time has elapsed," because " the bees are accustomed to flowers" and "find themselves for an instant* unsettled before having to take on a new habit.'9 And this observation very justly proves that flowers from which the petals have been removed remain unvisited until the moment when certain bees, hunting for food, discover the nectar. Following numerous experiments with Lobelias, primroses, bindweeds, Digitalis, and so on, Plateau (1896) observes with Bonnier that anthophilous insects (Hymenoptera, butterflies, Eristalis) visit flowers without corollas; but, differing from the French botanist, he does not tell us whether they come immediately or after a longer or shorter period. However, just as in the former case, the experiment lacks the necessary completeness, for the petals were cut off with scissors and there always remained a visible portion.
1 Charles Darwin. The effects of cross and self Fertilization in the Vegetable Kingdom, 1876.
Thus the visual sensations of color and form surely play a role of the first order in directing anthophilous insects toward flowers.