Exodus 8:14

They gathered them together upon heaps - The killing of the frogs was a mitigation of the punishment; but the leaving them to rot in the land was a continual proof that such a plague had taken place, and that the displeasure of the Lord still continued. The conjecture of Calmet is at least rational: he supposes that the plague of flies originated from the plague of frogs; that the former deposited their ova in the putrid masses, and that from these the innumerable swarms afterwards mentioned were hatched. In vindication of this supposition it may be observed, that God never works a miracle when the end can be accomplished by merely natural means; and in the operations of Divine providence we always find that the greatest number of effects possible are accomplished by the fewest causes. As therefore the natural means for this fourth plague had been miraculously provided by the second, the Divine Being had a right to use the instruments which he had already prepared.