Culled from "Genetics Laboratory Manual"
Copied as it originally appears in the book. See if you can find the sentence written by Mario.

"Drosophila females store sperm for long periods. So, if the male and female parents come from different stocks, you must use virgin females. Otherwise, they will have mated already with males of their own stock. To obtain virgins, you will rely on a convenient aspect of the fly's life cycle. When an adult emerges from the puparium, it is pale, soft, and wrinkled. Over time, the fly uses hydrostatic pressure to inflate itself and its cuticle slowly hardens and darkens. Until this process has a proceeded for at least 8 hours, the fly will not copulate.

"If you isolate a female fly before the end of this 8 hour period, she will still be a virgin. After 8 hours, if any males are present, she will not be."

So, then. The rumors you heard are true.