When they were first sold, aerosol insecticides were highly effective in killing flies and mosquitoes. Now, several decades later, a much smaller proportion of these insects die when sprayed. The reason fewer insects die when they are sprayed is that .

Many flies and mosquitoes today are offspring of mosquitoes and flies with insecticide-resistant characteristic

Explanation:

Aerosol insecticides were initially very effective because the generation of those flies and mosquitoes have not yet adapted to survive in an aerosol insecticides sprayed environment. So, most of them were bound to die in such condition. Very few were able to evolve and survive such condition . The adaptability of those few surviving flies and mosquito were transferred to the next generation of flies and mosquito.

The offspring of those few surviving flies and mosquito's were passed with the insecticide - resistant  characteristics. Therefore, it can  be said that most of the flies and mosquito found today are descendants of flies and mosquito with insecticide resistant characteristic.