Fly genetics requires more than just lab space, microscopes, and math. The field depended on the free exchange of specialized lines of flies which were sent around the country in the post. By convention, the requesting party would pay for postage. This letter from a scientist in Oklahoma to Muller, then in Texas, is representative of how the Drosophila exchange network operated.

Fernandus Payne, a graduate student who had recently graduated from a masters program at IU, introduced the Morgan lab to fruit flies around 1910. Payne becomes important again much later in this story, as he is the primary force behind IU hiring Muller. 

Early on in his fly research, T. H, Morgan found a fly that had white eyes rather than the standard red. Mutant strains like “white” were invaluable, as they enabled the presumed gene(s) behind the mutation to be tracked through crosses. Muller and the other fly room members became experts at spotting and breeding mutant fly strains. Together with the rise of fly genetics there was a proliferation of hundreds of highly cultivated mutant strains each with their own particular features suited to different experiments, as well as a national and international fly trade that facilitated experimental access to the right strains. Today IU Bloomington has one of the largest Drosophila stock centers in the world.