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K, you have to consider the air inside the car to be just like air inside of a sealed vessle. When the vessel/car accellerates, the air inside has no reason to change it's relative position (assuming consistency of mass within the air, throughout the vessel).
Now if you were to compare the density of the fly versus the density of the surrounding air, there is not a considerable difference between these densities. You could almost say that within a certain margin of error that that fly and the surrounding air are equivalent, notice I did say margin of error to cover my ass on this one) Hence the fly would move very little when the vessel begins to accellerate.
Now, the more substantial the accelleration the faster the fly would drift backward toward the rear windshield. As an example, if you have ever used a lab-grade centrifuge, you know that in order to seperate particles suspened in a fluid, longer times and faster spindle speeds are required for fluids that have particles of very simmilar masses. The more similar their masses, the longer the time required to seperate them. If there is a great difference in their masses then a lower speed and shorter time is required.
In conclusion, if the fly's mass were 10x or 100x more than normal we would see a more dramatic effect.
Same thing goes for the rear windshield, if all the windows were open and both windshilds were removed, then the fly would not maintain it's position inside the vehile, and eventually the fly would no longer be inside the car, as it accellerates away.
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