Of the 3,500 species of mosquitoes around the world, only 20 to 25 species carry diseases, like malaria and Zika, that affect humans. But it’s hard to know where those species are at any given time, and current methods–trapping insects and bringing them to a lab for analysis under a microscope–can only happen in a limited way. Researchers at Stanford University are testing another method: using volunteers with cell phones to record a mosquito’s annoying buzz.