In the book Cicadas, Helen Frost documents the life of the cicada from the beginning to the end.

Cicadas are plant-sucking insects. They have two different types of spit: one which allows them to
harden and form a casing in a plant where they can insert their beaks and move around easily, and the other to breakdown the plants making them easier to eat. Although some humans see cicadas as a problem, some people eat cicadas and others have used cicadas for medicinal purposes.

Information from:
Berenbaum, May R. Bugs in the System. Addison-Wesley Publishing Company, 1995 .