Cicada

Why do periodical cicadas lie dormant for 17 years? What are they doing that whole time? And how do they know it's time to come up?

My first children's book: https://neonsquidbooks.com/books/animal-sidekicks/

Bibliography: https://docs.google.com/document/d/1_RGZZryFdAAgF17xDu2Kx-IrrPRgoix4Xx3iO-P_KwQ/edit?usp=sharing