Sometimes a picture can tell you a lot about evolution. This particular picture has a story to tell about how two species–in this case a fly and an orchid–can influence each other’s evolution. But the story it tells may not be the one you think.

Coevolution, as this process is now called, was one of Darwin’s most important insights. Today scientists document coevolution in all sorts of species, from mushroom-farming ants to the microbes in our own gut. But Darwin found inspiration from the insects and flowers he could observe around his own farm in England.

Darwin’s thoughts about coevolution began with a simple question: how do flowers have sex? A typical flower grows both male and female sexual organs,