Why do insects fly in a moving car

The principles of flying, there are 4 forces, 1 lift (push up), 2 gravity (pull down), 3 drag (pull backward) and 4 thrust (push foward).

Lift and gravity oppose each other as well as drag and thrust. Lift and thrust (push) must both be greater than their opposing forces (pull) for flight to happen.

A fly on a good day knows how to deal with all 4. However, in a moving car with a cabin, the air moves with the car along with the fly. This is normal because the car provides a shield (windshield) against drag. So the fly will not experience drag at all. The lack of drag is what allows the fly to fly. In a convertible car, the fly has a smaller space to fly normally, thats in front of the windshield or footwell where it can avoid drag. Thats why the fly cannot do its thing when you are on a motorcycle where there is no windshield. In other words the drag will be much higher than the fly’s thrust.

Secondly, a fly will not be able to fly out of the car because the difference in drag from inside the car and outside seems like a wall that the fly cannot pass so it gets to a point where it knows it cannot fly to and it retreats back into the car where there is no drag.

Thirdly, most times you see a fly in the car, they seem confused, well, i guess they are because during acceleration or deceleration, the fly has to put more thrust to move forward or less to move back. This plays with their flying instincts just the way we feel fast acceleration and deceleration in cars, planes, lifts, escalator etc.