850 ECOLOGY 



the habitual movements of such insects as the flies. Soon they appear 

 to be attracted by various odors or colors, and after some days they 

 show their accustomed rapid and precise movements. That is, memory 

 appears to replace both odor and color as the directive stimulus of first 

 importance. Probably many discordant results of various observers 

 can be harmonized if the memory factor is taken into account. There 

 are some cases where instinct seems to be the controlling factor, as in 

 the pollinating insects of the figs and the yuccas (pp. 860, 864). 



Many experiments with bees show the importance of the memory factor. When 

 showy flowers are deprived of their corollas, the number of visiting bees at first is 

 small, but after a time the insects become accustomed to the new conditions and 

 visits become numerous. In some such experiments the seed production is less than 

 in flowers with corollas, but this may be due to lessened protection of the ovary or 

 to a less effective dusting of the stigmas with pollen. If flowers are artificially 

 hidden by leaves, bees soon learn the new conditions, and the visits which at first 

 are few soon become frequent. Similarly bees soon learn to visit wind-pollinated 

 flowers if there is placed on them honey and water, or sugar and a fragrant volatile 

 oil. The ability of bumblebees to learn is shown by Bombus terrestris, which has 

 a proboscis too short to get honey from Aquilegia vulgaris; after vain attempts to 

 reach the nectar in the ordinary way, it has been seen to bite a hole in the spur and 

 suck it out, repeating the process thenceforth. Similar holes are bitten in the spurs 

 of Tropaeolum by Bombus hortorum. 



Concluding remarks. — As a directive stimulus, insuring the visitation 

 of flowers by insects, odor seems to be more important than color, be- 

 cause it is distinguished from a much greater distance and by a much 

 larger number of insects; in the higher insects, notably among the bees, 

 which do most of the pollinating, memory seems to be a still more im- 

 portant factor. In the majority of flies and in most lower insects it is 

 doubtful if either color or memory plays a very conspicuous part, the 

 odor sense here being regarded as the most important. Odor is of par- 

 ticular significance where flowers grow in masses. So far as color and 

 form play a part, it is only in the immediate vicinity of the flower and in 

 the most general way. The elaborate theories which assign a distinct 

 role for each floral form and for each shade of color, which regard the 

 lines and spots on the corolla as guides to the nectar,' and which relate 

 the showiness of alpine flowers to the paucity of insects have no support 

 from exact observation and experiment. 



Features favoring the sprinkling of insects with pollen. — In most 

 flowers, especially in those that are open and actinomorphic, the anthers 



• Striking spots or lines may occur in such nectarless flowers as that of the poppy. 



