ental disturbance. The value of North American fly pollinators is not thoroughly understood, and concerns relating to pollinator declines in North America have not extended to dipteran pollinators. In this paper, methods for monitoring shifts in dipteran pollinator abundance are discussed. The need for further basic research into fly pollination is addressed, and the significance of dipteran conservation is considered.
Dipterans are among the most common insects that visit flowers. At least 71 dipteran families contain anthophilous species (Larson et al. 2001). Flies have been mentioned as pollinators or regular visitors of at least 555 species of flowering plant (Larson et al. 2001) and pollinators of over 100 cultivated plants including cacao, mango, cashew, tea, and onions (B. M. H. Larson, D. W. Inouye, and P. G. Kevan, unpublished manuscript). Many families of North American flies contain pollinators. Some of the larger, more notable pollinating flies belong to the families Bombyliidae, Syrphidae, Anthomyiidae, Tachinidae, Calliphoridae, and Muscidae. Small nematoceran fly pollinators include, but are not restricted to, the Bibionidae, Chironomidae, Culicidae, Empididae, Mycetophilidae, and Cecidomyiidae (Kevan and Baker 1983).
Anthophilous flies are diverse, ranging from opportunistic nectar and pollen consumers to specialized nectarivores. Fly proboscides range from a few millimeters up to 70 mm in length in a South African nectarivorous, nemestrinid fly (Manning and Goldblatt 1995). In a distinct syndrome called sapromyophily, certain flies (e.g., Calliphoridae) are deceived into pollinating flowers that produce odors of decay and mimic the decaying flesh in which these flies normally oviposit. The diversity of forms of flower-visiting flies is great; consequently, flies vary significantly in the amount of pollen that they carry, in the time that they spend in flowers, and in their effectiveness as pollinators (B. M. H. Larson, D. W. Inouye, and P. G. Kevan, unpublished manuscript).
Some of the many anthophilous fly species in North America have been assessed for their value as pollinators (e.g., Nye and Anderson 1974, Burgett 1980, Mesler et al. 1980, Levesque and Burger 1982, Boyle and Philogène 1983, Campbell and Motten 1985, Motten 1986, Kearns and Inouye 1994, Jarlan et al. 1997a, b). A recent review (B. M. H. Larson, D. W. Inouye, and P. G. Kevan, unpublished manuscript) that details the pollen-carrying abilities, constancy, and effectiveness of fly pollinators indicates that flies may contribute significantly to the pollination of North American flowers. Even flies that are generalists in their floral visits can contribute to plant reproductive success, and may equal or rival bees as effective pollinators in some circumstances (Motten 1986, McGuire and Armbruster 1991, Kearns and Inouye 1994). The predominance of flies on certain arctic and alpine flowers makes their contributions particularly important in these environments (Downes 1965, Hocking 1968, Mani 1968, Kevan 1972, Pojar 1974, Arroyo et al. 1982, 1985, Primack 1983, Shaw and Taylor 1986, Inouye and Pyke 1988, Kearns and Inouye 1994, Elberling and Olesen 1999). Much remains to be learned about the importance of fly pollination in other habitats. In addition, our understanding of the contributions of fly pollinators is poor for those flowers that attract multiple insect species whose numbers fluctuate independently.
Most entomophilous flowers receive visits from several different types of insects (Moldenke 1976, Herrera 1988, Ellis and Ellis-Adams 1993, Petanidou and Ellis 1993, Waser et al. 1996). For example, in a study of 2200 plant species in California, 71% of the outcrossing species were visited by two potential pollinators and 49% were visited by three or more potential pollinators (Moldenke 1976); 91% of 666 phrygana plant species in a botanical reserve in Greece were visited by five or more insect species (Petanidou and Ellis 1993). Redundancy in pollination systems is probably the rule, rather than the exception. Many species of flies are