Jun. 1, 2021 , 12:45 PM The billions of periodical cicadas now crawling, fluttering, and singing from trees in the eastern United States have roused a throng of humans as well, who are mapping the insects and timing their emergence in what may be the country’s longest public science tradition. Using a free app called Cicada Safari, more than 150,000 people so far have uploaded geotagged photos of cicadas, helping scientists track their emergence after 13 to 17 years underground. The insects are an ideal target for science-inclined amateurs—unmissable and mysterious at the same time. “We just don’t know what’s going on in their life” underground, says Douglas Pfeiffer, an entomologist at Virginia Polytechnic Institute and State University. With just a handful of scientists trying to understand a natural event both massive and rare, aid from amateur scientists is invaluable. In recent years, community reports have caught the formation of new populations, helped study the link between emergence and air temperature, and traced how cicada populations respond to stressors.