The following BPL articles were very helpful to me in thinking through the tarp and tent issue, UL and SUL bivvies seems like the way to go if you don't want bugs:
Advanced Tarp Camping Techniques for Inclement Conditions at http://www.backpackinglight.com/cgi-bin/backpackinglight/00150.html
The Poncho Tarp: Techniques and Gear Systems for Inclement Conditions at http://www.backpackinglight.com/cgi-bin/backpackinglight/poncho_tarp_techniques_gear_inclement_conditions.html
Haven't gone back to tarp camping, yet. Did it when a kid. Worst experience was in the Desolation Valley, Sierras. Set up camp under some old very big trees. Half way through the night felt "things" moving and a couple brush across my face. Turned on the flash light and the most beautiful, delicate, long legged and fast wood spiders were pouring out of bark of the big beautiful trees. Closed up the top of the sleeping bag and slipped down inside so they couldn't get in, moved the sleeping area away from the trees next morning, solved the problem. We used to just sleep on a ground cloth, fold it over our sleeping bags, which were big puffy jobs and got wet and cold in the damp. Lo tech days and youth. No tents.