One view of pollination systems is that they tend toward specialization. This view is implicit in many discussions of angiosperm evolution and plant—pollinator coevolution and in the long—standing concept of pollination syndromes. But actual pollination systems often are more generalized and dynamic than these traditions might suggest. To illustrate the range of specialization and generalization in pollinators' use of plants and vice versa, we draw on studies of two floras in the United States, and of members of several plant families and solitary bee genera. We also summarize a recent study of one local flora which suggests that, although the colors of flowers are aggregated in phenotype space, there is no strong association with pollinator types as pollination syndromes would predict. That moderate to substantial generalization often occurs is not surprising on theoretical grounds. Plant generalization is predicted by a simple model as long as temporal and spatial variance in pollinator quality is appreciable, different pollinator species do not fluctuate in unison, and they are similar in their pollination effectiveness. Pollinator generalization is predicted when floral rewards are similar across plant species, travel is costly, constraints of behavior and morphology are minor, and/or pollinator lifespan is long relative to flowering of individual plant species. Recognizing that pollination systems often are generalized has important implications. In ecological predictions of plant reproductive success and population dynamics it is useful to widen the focus beyond flower visitors within the correct" pollination syndrome, and to recognize temporal and spatial fluidity of interactions. Behavioral studies of pollinator foraging choices and information—processing abilities will benefit from understanding the selective advantages of generalization. In studies of floral adaptation, microevolution, and plant speciation one should recognize that selection and gene flow vary in time and space and that the contribution of pollinators to reproductive isolation of plant species may be overstated. In conservation biology, generalized pollination systems imply resilience to linked extinctions, but also the possibility for introduced generalists to displace natives with a net loss of diversity.

We review the physiological, molecular, and neural mechanisms of insect color vision. Phylogenetic and molecular analyses reveal that the basic bauplan, UV-blue-green-trichromacy, appears to date back to the Devonian ancestor of all pterygote insects. There are variations on this theme, however. These concern the number of color receptor types, their differential expression across the retina, and their fine tuning along the wavelength scale. In a few cases (but not in many others), these differences can be linked to visual ecology. Other insects have virtually identical sets of color receptors despite strong differences in lifestyle. Instead of the adaptionism that has dominated visual ecology in the past, we propose that chance evolutionary processes, history, and constraints should be considered. In addition to phylogenetic analyses designed to explore these factors, we suggest quantifying variance between individuals and populations and using fitness measurements to test the adaptive value of traits identified in insect color vision systems.

Summary.A chromaticity diagram which plots the 3 photoreceptor excitations of trichromatic colour vision systems at an angle of 120 ~ is presented. It takes into acount the nonlinear transduction process in the receptors. The resulting diagram has the outline of an equilateral hexagon. It is demonstrated by geometrical means that excitation values for any type of spectrally opponent mechanism can be read from this diagram if the weighting factors of this mechanism add up to zero. Thus, it may also be regarded as a general representation of colour opponent relations, linking graphically the Young-Helmholtz theory of trichromacy and Hering's concept of opponent colours. It is shown on a geometrical basis that chromaticity can be coded unequivocally by any two combined spectrally opponent mechanisms, the main difference between particular mechanisms being the extension and compression of certain spectral areas. This type of graphical representation can qualitatively explain the Bezold-Brficke phenomenon. Furthermore, colour hexagon distances may be taken as standardized perceptual colour distance values for trichromatic insects, as is demonstrated by comparison with behavioural colour discrimination data of 3 hymenopteran species.

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