Cicadas spend most of their 17 year life below ground as small white insects with no wings. They are called nymphs at that point in their lives. They are emerging from the ground right now. When they do, they will crawl up a tree or bush, break through their exoskeleton, which they have now outgrown, and crawl out as winged adults. They leave the exoskeleton on a tree, leaf or bush. It is called a molt or exuvium. It is not alive, it will not hurt you and it is not dangerous to touch. The adult cicada will stay on the tree for a while and then will fly to the top of the tree where it begins to sing in order to look for a mate. You can hear it outside, now. Once they find a mate, they lay eggs and then the adults will die. Adult cicadas will not bite. They are harmless to humans. Humans, however should take care not to harm them. If they land on you, they will feel scratchy because their legs hold fast on to what they grab, but they will not cause harm, in fact, most adult cicadas don't even eat. Their entire life as an adult takes from 2 to 4 weeks and is spent singing and laying eggs. If you see them, enjoy their song and try not to hurt them. The emergence should last for most of June.
Here is a great resource: http://www.washingtonpost.com/wp-dyn/articles/A20434-2004May12.html
Courtesy of Mr. Tanenbaum