Sometimes, it’s really easy for scientists to tell species of animals apart—they’ll be obviously different shapes or colors. Other times, different species will look nearly identical to the naked eye. In those cases, scientists need to turn to techniques like DNA analysis to tell them apart. Or, like researchers at the Field Museum discovered when studying some near-identical millipedes, you can sometimes just shine a blacklight on them, and under the ultraviolet light, parts of the different species’ genitals will glow different colors.