These observations become especially interesting when they are applied to the social bees. Our honey-bee, for example, is incapable of getting the nectar from narrow flower tubes whose depth exceeds six or seven millimeters, for its beak does not measure more; also, it can utilize certain clovers and not others. The workers of the common bumblebee (Bombus terrestris) are hardly better fixed, but the garden bumblebee (B. hortorum) and especially Gerstaecker's Bombus (B. gerstaeckeri) are well equipped, for the beak of the workers measures fourteen to sixteen millimeters in the first and eighteen to twenty-one millimeters in the second, according to Knuth. We know that the beak of the male bumblebee is a little shorter than that of the workers, while that of the female is the longest. We know, also, that these insects have an especial liking for the aconites, but the nectar of our yellow-flowered aconite is much more deeply situated than that of the blue-flowered aconite, and so the observations of Dalla Torre on Gerstaecker's bumblebee are explained. With their elongated beaks, the females of this species suck the yellow-flowered aconites, while the males and workers content themselves with the blue flowers.
But very often the bumblebees do not allow themselves to be baffled by the depth of the corollas. With their mandibles they gnaw a hole in the wall of the tube which is in the way, and thus easily reach the nectar. Having the habit of gnawing the cells in their nests, these insects employ the same method when they find themselves in the presence of coveted flowers which are inaccessible without being cut open. The solitary bees of the genus Xylocopa (carpenter bees), which gnaw into wood in order to place their cells there, are still more prompt than the bumblebees to perforate floral organs, although they are of greater size and furnished with a long beak. I have seen (1903) our Xylocopa violacea make long slits in the nectar spur of the garden balsam and the monkshood without ever trying to introduce the beak directly into the orifice of the corolla. In a clump of cultivated verbena, the insect "darted upon the flower, turned the especial stem under its venter and, turning the corolla tube, made a slit or long wound in its lower part/' This was a brutal bee, which made play of mutilation. According to H. von Ihering (1903), a Brazilian social bee, Trigona ruficrus, does the same thing with orange flowers.
Our domestic honey-bees do not ordinarily make holes, but they know how to profit by those made by other honey-seekers, as I have seen with the monkshood flowers visited by our carpenter bee. Like the carpenter bee, they dart upon the flower spur.
But when this was intact they appeared to disdain the flower and made only a short stop ; the conditions were not favorable to the honey harvest. The visit to the already mutilated corollas was of an entirely different character. The bee felt of the surface of the spur with the point of its jaws; having recognized the existence of an opening, it seized the spur with its legs, plunged its beak into the tube, and sucked up the nectar. . . . The beak of the bee being shorter than that of the Xylocopa, it became evident in many flowers that the busy gatherer did not reach the liquid, but many times it overcame this difficulty by prolonging the cut to a proper point. How did it do this work? It is not easy for me to describe the precise manner. ... As to the act itself, it was undoubtedly performed. When the bee abandoned a flower I immediately examined the spur, and the presence of a fresh wound almost bleeding (if one may so express oneself) gave me manifest proof of a recent mutilation.
Bees often profit in this way by the slits made by the Xylocopa in the spurs of balsams, but they do it very seldom with the Verbenas the tube of which is too long or has been destroyed by the carpenter bees. In all the cases observed the work was done by workers in search of nectar. Those which collect pollen have another method, and force themselves directly into the corollas. Thus, in paying her visit to the flower spurs when searching for nectar, the bee gave proof of observation and of memory, and acted not as an automaton, as Bethe pretends, but a>s an intelligent being. According to Herman Miiller, she always makes use of a hole in the corollas.
How are insects guided toward flowers? This is one of the most disputed questions in entomological biology,-at least one of those which were so, for to-day it seems rather clear. Anthophi-lous insects visit flowers to seek their nourishment but what is the guide which leads them there? Is it sight? Is it odor? Or both senses together? It is the vision of colors, said Conrad Sprengel (1793) after numerous observations. It is above all the vision of colors and of forms, reply Lubbock and Forel, who place little importance in the sense of smell of the Mellifera. Again, Herman Miiller, Knuth, Perez, Buttel-Eee-pen, etc., equally give predominance to the visual sensations, but recognize the important role of the olfactory sensations. "Aside from color," says Paul Knuth, "the perfume is the most powerful means of attraction ; in many cases, even, it is not easy to distinguish which of the two means is the more efficacious. " The conclusions of Plateau (1907) are diametrically opposed. "The sense of smell, so highly developed with most insects," says he, "far from being an 'accessory factor, is really the principal sense which leads them to discover flowers enclosing pollen and nectar." Without being as explicit on the subject of the role of the olfactory senses, Gaston Bonnier approaches Plateau in that he regards the part played by colors as very subordinate. Without doubt, he observes, bees can recognize colors, but that is not the question; "it is necessary to know whether or not brilliant colors attract the insects in preference to less visible colors, all other conditions being equal." To this he replies in the negative after researches to which we will refer later.
Bonnier has had the merit of proposing the problem and, alone with Plateau, of braving all criticisms, sometimes hardly kind, of the numerous partisans of the opposite thesis. Before taking part in this controversy, it will be well to examine the facts which serve it as food.