Page:Popular Science Monthly Volume 81.djvu/207

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strict their visits to flowers so abundantly supplied with pollen and nectar, combined with their early and short time of flight, which lasts only about a month, and perhaps also to their nesting near these shrubs. Where bees fly only during the latter part of the season it seems very natural for them to restrict their visits to the Compositæ. These flowers, as in the case of the golden-rods and thistles, are very common, contain ample food supplies and are easy to visit. They are actuated not by the need or desire of avoiding competition, but by the same motives which lead honey-bees to visit the white clover exclusively while it is in bloom.

The oligotropic habit is not beneficial to flowers, it concerns the bees alone. The oligotropic bees are almost without exception solitary forms, to which there are no flowers specially adapted. The social bees, as a rule, visit a great variety of flowers, though in Europe it is stated that there is a bumblebee (Bombus gerstaeckeri) which visits a single species of monkshood (Aconitvm lycoctonum). Here, of course, the adaptations are mutual. This mode of flight, however, has not in general been determined by floral adaptations. Certain species of bees have become satellites of certain flowers because of the advantage thus gained for themselves, and partly also perhaps as the result of habit. Just as there are fly-flowers, butterfly-flowers and bumblebee-flowers, so, on the other hand, there are willow-bees, golden-rod bees, a pickerel-weed bee, a loosestrife bee, a violet bee and a strawberry bee.

Two most important influences are the season of the year and the length of time the bee is on the wing. It is clear that bees which fly only in spring or autumn for about a month have not a great choice of flowers; and, of course, we never look for autumnal bees on spring flowers. Usually the length of time an oligotropic bee flies and the flower it visits is in bloom are about the same. The honey-bee is practically a monotropic bee at certain seasons of the year. While the basswood and white clover are in bloom the honey-bee visits these flowers almost exclusively. Again in the fall in Maine it confines its attention solely to the golden-rods. In California at times it collects nectar exclusively from the sages; in Michigan from the willow-herb, and in other regions from other plants. If from any one of these plants it also obtained its supply of pollen and was on the wing only while it was in bloom, it would be regarded as a monotropic bee in the strict sense of the word. That it exhibits a strong tendency, when collecting pollen, to be constant to one plant species is well known; and the little packets of pollen it brings into the hive seldom consist of two kinds of pollen. But, when a bee flies from spring till fall and requires a large amount of stores, it is evident that it can never become oligotropic.

Another important factor is the small size of many oligotropic bees. This is true of Prosopis nelumbonis, Halictus nelumbonis, and many