OM pretty well nailed it.

I've never had any real problems with snakes, and I hike a lot in timber rattler and copperhead country. I've never seen them at night, which may mean nothing more than I've never seen them; I don't know if they're nocturnal.

I've only had problems with mice in shelters on the AT.

Spiders are an annoyance, no more. (I've never had any encounters with poisonous spiders, just "daddy long legs" types.)

When I started, I used a tent. This was in the days before solo tents; since I usually slept alone, I ended up carrying a two person tent that weighed six or seven pounds. (The space was wonderful, but not wonderful enough to justify the weight. I never had bug trouble, but I often heard animals moving around the vicinity of the tent - and, since I couldn't easily see outside, imagination and ears indicated they were HUGE! I did once have a skunk wander into my vestibule, which was interesting but ended up being non-confrontational since it lost interest and wandered off.

To save weight, I became a dedicated tarp and groundcloth user; the tarp-and-groundcloth (later, tarp-and-bivy) weighed 3 pounds. Under a tarp, I've awoken to a deer grazing ten yards away from my camp, and I've had passing encounters with raccoons. I have even had a skunk come up to the foot of my sleeping bag and sniff; wiggling my toes sent it on its way with no lingering after effects.

With the advent of solo tents that weight less than 3 pounds, a tarp and groundcloth or bivy are simply irrelevant to me. I prefer an all-mesh (or at least top-half mesh) tent; on nice nights, I can leave the rainfly off and have all the pleasures of sleeping under the stars, without the bugs.