The phrase "female flies" refers to female insects called flies. It's like saying "girls" but referring specifically to these small flying bugs that can bother us sometimes. Full definition
Scientists mated female flies with two groups of males, the latter of which were distinguished by green - and red - tagged sperm heads. (sciencedaily.com)
The finding that these neurons are present in the brains of male but not female flies indicates that this sex difference in aggressive behavior is genetically based. (sciencedaily.com)
The team, involving researchers from UCL and Monash University (Australia), compared the lifespans of female flies fed a healthy diet containing 5 % sugar to those given eight times this amount. (sciencedaily.com)