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Cluster flies (Calliphoridae: Polleniinae: Pollenia) of North America
CJAI 19 February 15, 2012

Adam Jewiss-Gaines*1, Stephen A. Marshall*, Terry L. Whitworth**

*School of Environmental Sciences, University of Guelph, Guelph, ON N1G 2W1; ajewissg@gmail.com, samarsha@uoguelph.ca. **Washington State University, Puyallup, WA 98371-4900; twhitworth@wsu.edu

1Corresponding Author

           Pollenia species, or cluster flies, are conspicuously abundant flies best known to the general public for their habit of clustering on walls, and entering the walls and attics of homes to overwinter. When they become active in early spring or on warm winter days they can become nuisance flies in houses, even though they neither bite nor contaminate food. The appearance of large numbers of these conspicuous flies inside houses is often a matter of concern for homeowners and home builders, and the subsequent accumulation of dead adult flies around windows can lead to allergy problems or later issues with dermestid beetles that breed on the dead flies.
           Some cluster flies lay their eggs on soil inhabited by earthworms, from whence the newly hatched larvae make their way through the soil in search of earthworm hosts to consume as parasitoids (usually) or predators, but the biology of this group is relatively poorly known and a few have been recorded from other hosts including caterpillars and bees. Cluster fly adults, easily distinguished from other blow flies by their dull colouration and yellow, crinkly thoracic hairs, can be among the most abundant insects visiting flowers and are often extremely common on flowers in urban and agricultural settings.
           Despite their importance as nuisance pests and as significant and common pollinators, little is known about the diversity and abundance of Pollenia species in North America. In fact, until a few years ago all North American Pollenia species were treated as one species, Pollenia rudis, and only recently has it been recognized that six members of the genus occur in North America (Rognes, 1991; Whitworth, 2006). The biology of cluster flies in Ontario has been extensively studied (Thompson and Davies 1973a, 1973b; Yahnke and George 1972) under the name Pollenia rudis. It is impossible to know which of the six species in the province were the subjects of this research, but Rognes (1987) indicates that a majority of specimens studied by Yahnke and George were Pollenia pediculata (as P. pseudorudis).
           The eastern Canadian Calliphoridae other than Pollenia were reviewed and keyed by Marshall et. al (2010), but the very distinctive and ecologically distinct Polleniinae were keyed only to genus. We here fill that gap by providing an extensively illustrated key to the North American Pollenia species, and by compiling maps that for the first time detail the North American distributions of the six Pollenia species known in the Nearctic Region. Our key is based largely on Rognes (1991) and Whitworth (2006); distributional and phenological data are from the University of Guelph Insect Collection’s almost 3,000 Pollenia specimens, T. Whitworth’s personal collection of about 1500 Pollenia, and over 5000 specimens from other institutions (Appendix 1).
           Terminology follows the Manual of Central American Diptera (Cumming and Wood, 2009), differing from the terminology of Rognes (1991) and Whitworth (2006) as follows:

Manual of Central American Diptera (Cumming and Wood, 2009)
Roges (1991)/Whitworth (2006)
Postpronotal lobe
Humeral callus
Anterior postpronotal seta(e)
Extra/anterior humeral seta(e)
Presutural anterior intra-alar seta(e)
Inner posthumeral seta(e)
Presutural anterior supra-alar seta(e)
Outer posthumeral seta(e)