I was ready to swat some flies, but when I spawned them in, they were stationary? The buttons seemed to resist being pushed occasionally, but I got it working. Nice variety in weapons, but without moving targets, it was sort of moot. It's great you had multiple levels created, did I miss something, do the flies attack/swarm in other levels? I'd suggest looking into "boids" on a small scale as a way to do a fun swarm of flies/insects. Besides that, nice work assembling all the parts together into a game that had clear instructions, multiple levels and the best of intentions!
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Here's some commentary from a second playthrough: I chose the campsite instead and was pleased to see the flies swarming me, nullifying my earlier concerns to an extent. (I still think even in the tutorial they should buzz around, but that's besides the point)
Variety in weapons allowed for some style in the bug swattin', and I like the big boys that required a little extra bonkin'. Since they're flying, perhaps you could vary/randomize their height, and as they approach, they descend? More lightning bolts would be fun, too!