[Drosophila] taxonomic/identification keys for drosophilids?

[Chris Jones]

It looks like this question has been asked here a couple of times 
before, but no answers were posted. I've collected some fruit flies at 
my local grocery store and would like to identify them. They certainly 
aren't melanogaster, but I'm fairly confident that they are drosophilids 
of some stripe. Does anyone know of an online identification key for 
this genus? Or is there a recommended hard copy resource that I might be 
able to track down? Many thanks.

Chris Jones