Lightning bugs, aka Fireflies, are harmless. Their little butts just emit flashes of light from internal chemical reaction, like a short lived glow stick. If you encounter a field with a bunch of them, it's real pretty.

Great Smoky Mountains National Park has a "Lightning Bug Lottery" every year, a certain number of passes are randomly given out to applicants to see the park at night during peak breeding season for fireflies. Supposedly they will all sync up their lights and converge in a huge group on one tree.

I've seen a smaller event once in my hometown. Just a whole tree was sparkling for a few minutes. I think the most amazing thing about it is the light doesn't really show up well on a camera, so you kinda have to just put your phone down and enjoy it with your eyes. The only place you can keep that moment is in your mind.