Suppose that you place P generation wild type males and mutant virgin females in a vial and allow them to mate. You leave this vial undisturbed in the incubator for 12 days. Could you collect F1 flies for your inter se cross out of this vial? Why or why not?
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The answer depends which species (melanogaster, simulans, yakuba etc.), strain/population (e.g. Canton S, DGRP), mutant line (e.g. eb, CiD), food substance, larval density, and incubator settings (temperature, humidity etc.).
The rough answer is no, but if you clear the already hatched flies from the vial in the morning (~1 hour before lights on) and collect flies within a few of hours they should be virgin. Day 12 will likely be too late to collect many as most will have hatched at 25 degrees, at 18 degrees this may slow development but you should be certain that it will not affect your experiment, and report it properly in the methods. You could also collect and store them for a few days in single sex vials (all males or all females), then check the food that you store them in for larvae before using them, this helps to ensure virgins are actually virgins. If you find lots of eggs or even a single little larvae then throw the vial away. This also allows the females to mature properly, allowing them to eat some yeast and they will be more willing to mate when needed.
An experienced fly handler should be able to sort consistently enough to have just a few back up vials. If it is a beginner then make many vials with few flies in, this reduces the damage if a male slips through, it's much nicer to throw away a vial of 10 females than a vial of 100!
(Answer based on >7 years practical experience with D. simulans and D. melanogaster, collecting tens to hundreds of thousands of virgin flies in that time).