This is a nice little game. If you hadn't told me so through text I would have only assumed that I was a frog. There might be some way to convey that to the player like having a tongue appear to catch the fly or by having some frog like sounds while you play.
The gameplay is relatively simple but I think it works overall. I think the biggest mechanical change I would make is just how the flies appear. Right now they seem to come on a strict clock and the they come 2 at a time. Switching this up would make the game feel a lot more interesting in my opinion. Additionally, I'm unsure of how you're spawning the flies and making them move but right now it feels very basic and sometimes the flies are too far up or down to really be able to see them. I think giving them just a little AI so they can fly a little more erratically would be really interesting.