The word "periodical" means that these insects appear "periodically"...from time to time...as the article title indicates.
As always, I include a shout-out to my DeHaan Services blog that also highlights the primary article in "Featuring Periodical Cicadas".
Writing Tip for Online Articles
My article lists the next 15 scheduled appearances of the different broods of the 13-year and 17-year cicadas. With clever annual marketing and cooperation from search engines, people might return to this article "periodically" through 2025.