I like a tarp over a tent too. IF set up so you can still see around you while you're laying down, you can gain the advantage of being able to see someone (at least their legs) while they can't see you. Besides, they are lighter, easier to carry, and more versatile.
I've had a snake climb into my sleeping bag before, and it's not fun. Actually, it's kind of a funny story, so...
I was in the desert near Ft Bliss Texas in my tank on some training thing. It was raining, which is very rare for the area, and my gunner's position leaked. I was getting this drip, drip, drip right on my head, which was driving me nuts, so I broke one of the cardinal rules of armor and decided to sleep on the ground. I grabbed our driver and told him I was gonna sleep under the tank... another big no no.
Anyway, sometime after that I was under there dry and warm in my Army bag and was awakened by something crawling around near my feet. My tired brain processed this as a snake, so I sat up quickly, smacking my skull on the underside of the tank, BONG! This dazed me a bit and kinda knocked me back down. When I had gathered what little wits I have, I tried to sit up again... BONG. After the second time, it finally dawned on me that I was under a tank and couldn't sit up, so I rolled over and set the world speed record for a low crawl. I distinctly remember passing the road runner and Wiley E Coyote as I crawled across the desert at like 300 miles per hour.
I went back later and shook out my bag. The snake slithered away fast so I didn't see what kid it was, but it was probably a rattler. I never slept on the ground in the desert again!
I just ordered a Catoma Adventure Shelters IBNS mesh bivy! No more sleeping with the snakes for me.