Trillions of cicadas are starting to emerge in 15 US states after 17 years underground. Gene Kritsky has been studying them for decades and has created a citizen science app, Cicada Safari, to help track the brood. More than 3000 cicada species have been described worldwide. Most have a yearly life cycle, but seven species in the US belonging to the Magicicada genus remain underground as nymphs for 13 or 17 years before emerging, a process called periodical brooding. The only other two species of cicada to do this are found in Fiji and India.
Watch brood X cicadas emerge in once in every 17-year event
