Simple and Functional game, gave me a good few minutes of fun.
Replayed it, was fun to see how all the different fly's worked. Had fun seeing how many I could push up into the corner to kill at once.
The game would perhaps be more replayable if you added a timer for say 1 minute where you have to kill as many flies as possible. And perhaps killing multiple could lead to multipliers. This would keep people more engaged with the game to try and beat their previous score.