In the late spring of 1634, pilgrims in Massachusetts witnessed an incredible sight: Millions and millions of winged, red-eyed insects sprung from the earth. Not knowing better, the Puritan immigrants likened them to pestilential swarms from the Old Testament and called them “locusts.”

But they were wrong. This was the first written record of periodical cicadas, seven species of which emerge every 13 or 17 years in the U.S. Midwest and East Coast. This year, trillions of the insects will once again burst from the ground and take to the trees, making loud mating songs as they harmlessly sip tree sap.

The 2021 cicadas, known as Brood X, are the biggest of the 15 known periodical cicada broods. These