I watched as a giant wolf spider walked across the bottom of my sleeping bag last year deer hunting in the LBO. I've never had a bug screen in a floorless shelter but that had me considering it. I've woken up to some level of vermin (shrew, mouse, big ass spider, never found out) snuggled up to me on cold or wet nights in September. That's just part of the floorless...I had an issue with some ladybugs in my tarp early last season. (like 3 million lady bugs).
Never again will I use a floorless shelter. And never again will I have a shelter w/o a bug net.
Live and learn