Fireflies are butt gals. So are their male amori. On warm summer nights, both sexes home in on the blinking light of their crush’s derrière. The rest is romance. Which makes it almost heartless for us to ask: What makes their bums blink? It’s mostly a matter of chemistry, triggered by seasons, times of day, and those flashing signals. For Photinus pyralis, the common eastern firefly that populates North America, it is a choreographed dance. Next time you’re sitting on the sidelines, or grass, here’s a guide to the bioluminescent dating game going on in your backyard.