A sealed bottle containing flies is placed on a scale. When does the scale register the heaviest weight : when the flies are resting on the bottom of the bottle, or when all the flies are in flight?
The scale registers...
The insects in flight are held up by the air, and the air exerts weight onto the bottom of the bottle. The whole system is self-contained.
But when a fly takes off or lands, there will be a momentary change in weight, because of the force exerted by the fly against the glass: action – reaction.