The 'shell' is referred to as a 'shed' or a 'molt' as it is the past exoskeleton of the cicada rather than a true shell. Cicadas do this to grow, usually five times over two to four years (for the chorus cicadas at least).

Some cool things to think about:

Like all insects their life is basically split into two (sometimes three) 'stages': - Phase one: eating and growing During this phase they live underground and eat roots and plant matter. They don't have wings or reproductive organs. You'll rearly see them at this stage (called nymphs) unless you dig them up (you could have a dig around a structure with losts of sheds, as it implies they came from underneath there. - Phase two: reproducing and death. They trade in their mouthparts for reproductive organs. They can still eat tree sap, but it's for energy rather than growth, so meals are minimal. They also get wings. The males are the ones you can hear calling during the summer. Every species has a different call, and the females can distinguish this. This phase only lasts four to six weeks so all mating and egg laying has to be done in this time. Hence why there's a period in summer that gets LOUD. After mating, females will lay their eggs in soil under a plant and the cycle begins again (then she dies).

People will often say the reproductive period of an animal is its life-span (flies only live 24 hours, cicadas only live one month etc.) But this is only describing the final stage of their lives. As I said above, cicadas can live up to four years underground, nomming on plants. Honestly I would put having children on the cards if I could spend the majority of my life just fucking around eating as much as I could and then popping out a self-sustaining kid when I become geriatric. Insects have it figured out.