Fruit fly larvae emerge from eggs after a few hours or a few days of incubation. The adults may mate more than once and females of some species store the sperm to guarantee fertilization.
The female has less control over which sperm fertilize her eggs, but by storing it she can ensure that she has some on hand if no males are around when an egg is released. In other words, pregnant fruit flies have a lot of things going on with them during their pregnancy. 🙂
And the fact that they also release venom while being born means you should read this next part really closely…