What makes an insect a good pollinator.'
The answer to this seemingly basic question is poorly understood. It is often assumed that bees are superior pollinators (Kevan & Baker 1983), based in part on their removal of large amounts of pollen, rapid visitation rates, efficient flower handling, constancy, and ability to learn. These characteristics allow bees to collect resources efficiently, but are not necessarily ideal for the plant, as pollen removal does not necessarily correlate with pollen deposition (Wilson & Thomson 1991), and multiple visits to flowers on a single plant or to near neighbors may result in a high level of inbreeding. And while learning ability and floral constancy may facilitate intraspecific pollen transfer, these attributes are also shared by many non-hymenopteran insects. In some cases, insects that remove less pollen from a flower, groom less of it off their bodies, visit fewer flowers per plant, and travel further between plants, may be better pollinators (Herrera 1987; Thomson & Thomson 1992; Harder et al., this volume). As some non-hymenopteran insects meet many of these criteria, a critical examination of their importance as generalist pollinators is in order. Components of pollinator "quality" are likely to vary across taxa. Herrera (1987) found, for example, that bees pollinate Lavandula (Labiatae) flowers frequently, but generally promote geitonogamy with short inter-flower flights; butterflies, on the other hand, pollinate flowers less often but tend to do so with cross-pollen, based on long inter-flower flight distances. Quantitative estimates of pollination parameters for diverse insect visitors on a range of plants will help to address the question of what makes a good pollinator.
How broad-based or species-specific are sensory attributes and learning abilities in pollinators.'
In considering the sensory attributes and learning abilities of pollinators as they have influenced floral evolution, it would be interesting to know which are general to all insects, which are variable across and within taxo-nomic levels, and how these patterns came about. If associative learning ability, for example, is found in some taxa and not in others, has it evolved independently as needed, was it ancestral to a lineage and subsequently lost in taxa that did not make use of it, or are both scenarios plausible? Asking ecologically based questions within a phylogenetic framework will shed light on evolutionary patterns. For example, do flower-visiting flies possess a more highly developed sense of color vision than copropha-gous flies? Do fruit- and flower-feeding butterflies differ in their processing of and response to colors and odors? Can anthophilous beetles learn to associate colors or odors with rewards more readily than non-flower-visiting beetles?
How important were early pollinators in shaping flower form?
If beetles indeed "stood at the cradle of the flower" (Faegri & van der Pijl 1979), they (as well as contemporaneous flies and non-social hymenopte-rans) would have had almost 100 million years to influence floral evolution before the arrival of social bees - ample time to establish an insect-pollinated floral Bauplan. Thus, the innate color and odor preferences of these early pollinators, as well as whatever learning abilities they may have possessed, are likely to have been formative in early angiosperm evolution. We really have no idea how important the early pollinators were in establishing the fundamental features of the angiosperm flower, and the extent to which later pollinators have had to modify these early designs or have been able to select on floral features from scratch. A fuller understanding of the sensory and behavioral attributes of non-hymenopteran pollinators will be important in our attempts to reconstruct patterns of floral evolution.
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