thirsty bug

Over the hot, dry weekend, I found a hornet-like black bug, less than an inch long, on its back on my window sill. He twitched now and again, but otherwise did not move. I thought he was dying, and did not disturb it. The next day, he was still there. I poured a few drops - maybe half a teaspoon - of water near it, thinking it must be thirsty. Nothing happened. I tried again, this time making sure the water touched the hornet. The bug lept up, and seemed to be embracing the water. He was so light, it looked as though he were climbing it. He worked his front limbs up and down in the water, as though he were piercing a peach. He dipped his head under. He rolled his head back and forth in the water, as though letting it cool his neck. As this happened, the body of the bug beneath the wings seemed to be visibly expanding. 

I watched for a number of minutes. When I left, he was dropping his open lower jaw, once every ten seconds or so. It was like watching an adjustable wrench, the moveable part lowering then rising, lowering then rising. I assumed this was to drink water, but looking closer, the lowered jaw at that point was not touching the liquid. The water he was in was not thin, but like a rather heavy blob on the sill - the other drops as well, reflecting the window screen above in a colorful, three-dimensional way. I left my apartment for a few hours. He was no longer there when I returned.