I have several of those plants. They are all low-light plants. especially the pothos and the lipstick plant and, I think, the polkadot plant. The afternoon sun is probably too much for them, even the morning sun might be. Mine don't get ANY sun and almost no light. These are plants that, given the right conditions, will thrive on neglect. At least, mine do.
I'm going to agree with the comments about potting soil and pots, and recommend that you stay away from bug spray. I can't tell from your picture if that's really a gnat, but if it is, you may have some fungus in your soil. If you have fungus gnats, that's what they like to eat and it can be a problem for your plants. Just to reinforce what others have already said, fungus and fungus gnats love soil that doesn't drain well.
The best thing you can do, IMHO, is to either make or buy a gnat trap. Very easy to do and very cheap.
Good luck!
