. The Canadian field-naturalist. Natural history. 1997 Small, Brookes, Lefkovitch, and Fairey: Preferences of the Alfalfa Bee 451 las longer than mm presented no obvious mechanical barriers to the bees to collection of pollen and/or nectar. Presumably it was excessive size per se that did not conform to the bee's innate sense of acceptability, although it is not clear why the bee's overall length and the maximum length of flower visited are approximately equal. In this instance, perhaps complete insertion of the bee's body into a floral throat would render the bee espe- cially susceptible
. The Canadian field-naturalist. Natural history. 1997 Small, Brookes, Lefkovitch, and Fairey: Preferences of the Alfalfa Bee 451 las longer than mm presented no obvious mechanical barriers to the bees to collection of pollen and/or nectar. Presumably it was excessive size per se that did not conform to the bee's innate sense of acceptability, although it is not clear why the bee's overall length and the maximum length of flower visited are approximately equal. In this instance, perhaps complete insertion of the bee's body into a floral throat would render the bee espe- cially susceptible to floral predators, or simply requires too much time or effort to be worthwhile. As noted earlier, the head of the female is about 3 mm wide, so that a corolla narrower than this may limit how far the bee could probe into a narrow floral tube, even by extending its tongue. Indeed, we found that if the width of the throat does not exceed 3 mm, throat lengths less than 5 mm are preferred by the bees. This coincidence with the lengths of the mouth- parts of the bees (mean mm) is hardly surpris- ing, but none the less is interesting. Figure 2 shows a leafcutting bee visiting an alfalfa flower, and dia- grammatically summarizes the apparent limiting maximum dimensions of a sympetalous flower, in relation to the female leafcutting bee's dimensions. Colors distinguishable by humans were not found to differ significantly between attractive and unattractive species. Nevetheless further investiga- tion may well prove that the bees do have at least some color preferences. Goplen (1970) and Goplen and Brandt (1975) found that the purple-flowered Medicago sativa seemed to attract more Alfalfa Leafcutting Bees than the closely related yellow- flowered Medicago falcata, although this may be due to floral features other than color. It should also be remembered that "several authors have recently pointed out the hazards of considering colors, whether visible or ultra-violet, indepen
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