This is a link to a pdf file of Dr. Terry Whitworth's paper on Calliphoridae that appeared in the Proceedings of the Entomological Society of Washington in June of 2006. The key has many beautifully detailed illustrations and there is also a simple key that will allow one to discriminate calliphorids from other similar Diptera. I emailed him in January 2008 and he generously agreed to share this paper from his web site link (www.birdblowfly.com).