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Last Non-last

Flower head

Fig. 14.3 Time spent by bumble bees on the last- and non-last-probed heads in each visit to a Cirsiumpurpuratum plant. The five horizontal lines of the plot indicate the 10th, 25 th, 50th, 75th, and 90th percentiles of the data (U = 824.5, p = 0.0056; Mann-Whitney U-test). (Adapted from Ohashi 1998.)

size that would be detected by honeybees from 45-cm distances is about 5 cm in diameter in the best case. Moreover, color contrast (Lehrer & Bischof 1995; Giurfa et al. 1996) and relative motion speed against the background (Lehrer et al. 1990; Srinivasan et al. 1990) also influence the detectability of objects. Thus, this hypothesis may partially explain preference for large displays, especially when individual flowers are small. If large floral displays are infrequent in the population, however, this hypothesis may not hold; pollinators would more frequently choose small, but closer displays.

On the other hand, the flight-cost hypothesis emerges from an economic viewpoint. Harder & Cruzan (1990) and Harder & Barrett (1996) stated that pollinators visit large inflorescences because the proximity of many flowers reduces pollinator flight costs. Their statements have implied that pollinators prefer to visit larger displays on which they can probe more flowers, so that they can reduce the total movement costs required to probe a fixed number of flowers. That is, total movement costs required to probe N flowers is expressed as:

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