Eastern Tent Caterpillars are the larvae of the Lackey Moth. The moth lays its eggs around a tree branch and the caterpillars hatch from eggs the following Spring when leaves unfold. They go through about a half dozen larval stages of development called instars. They make the silk tents you probably have noticed in the branches of trees in the Spring. During the final stage, they consume most of the food of their whole life cycle, and may nearly defoliate their tree, which likely will recover. They stay in their tent to keep warm, move around the tent to regulate the temperature, and whenever it gets too warm they move to the outside surface of the tent to cool down, as you see in this image.
After about two months they are in their final larval stage and leave their tree to find protected places to spin their cocoons. It was at this stage that I was preparing to photograph an adult caterpillar on a sidewalk beneath its natal tree. I was positioning my tripod when I noticed the caterpillar was on its back and not moving, apparently dead. I felt bad because I assumed I had killed it with my tripod. To record its now visible legs I snapped this picture.
A few seconds later a Carpenter Ant, which must not mind dead prey, grabbed the carcass and started to drag it away. Suddenly the caterpillar came to life, flipped over and violently shook the ant off, after which I took this photo.
I had never heard of a caterpillar playing possum like this. I did a little research and found that some caterpillars and other insects discourage predators by playing dead - a strategy called thanatosis.
To complete the story, a female caterpillar would go on to spin a cocoon and in a couple of weeks emerge as an adult moth full of eggs. Then within about 24 hours it will secrete pheromones to attract a male moth to fertilize the eggs, lay the eggs on a branch, and die, thus completing this life cycle. The male moth may go on to fertilize multiple egg masses over a period of more than a week.