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Mosquitos- pretty obvious, but I've found these fuckers can be either at best a small nuisance or at worst enough to completely ruin an outing depending on location. I was hiking up in the Sierras a decade ago with a couple people we camped about a quarter mile from a large lake under the stars. One of the guys had the misfortune of his arm falling out of his bag while sleeping and when we woke up in looked like he was was a leper based on his arm and was itching all week.
Ticks- I'm usually not too scared of running into most animals when outside, but getting bit by one of these fuckers and contracting Lyme is like one of my top 3 fears outdoors. Even worse for the ones that make you allergic to meat. The fact that they actively hunt you once they catch a whiff of you makes it that much worse.
Flies- This is fairly recent, but I never understood people's problems with flies until a recent trip I went on where we were harassed to death by flies an entire fucking week. These little shits are not only big enough to be constantly annoying, but they always seem to bite you just around the time you are getting used to them to remind you what assholes they are. The only reason I dont put them much higher is that I've found that /out/ flies, at least in the Cascades, are much slower and dumber than Midwest flies where I live, and are thus way easier to kill. I feel like a big stupid gorilla trying to hit flies in my apartment, but I was like Mr Miyagi out there.