You just never know about bugs, when they'll suddenly disappear, when the second (or third, or fourth) hatchout will occur. Black flies go to bed at night, but mosquitoes and no-see-ums don't. If you're bug-sensitive or allergic, though, I would want at least a net set-up of some kind under that tarp. (I'm allergic so I wouldn't try what you describe without a full net tent.) They make fairly lightweight umbrella type nets that cover the upper half of your body. I've used them in shelters during bug season. (I used to swelter under sleeping bags and head nets.) One of the problems with open tarps is that as the evening cools a bit, the bugs will collect inside the tarp, sing to you all night, and eat you for breakfast.