In caves around the world, animals and other creatures have adapted to endless night. Cavefish, for example, have lost their eyes and pigment, evolving a greater power in other senses.

In 1954, Syuichi Mori, a biologist at Kyoto University, put flies into a cave of their own. He took eggs from ordinary flies of the species Drosophila melanogaster put them in milk bottles, which he placed in pots and covered in dark cloth. There they lived in utter darkness. He tended to the flies, generation after generation, dividing them into three separate lines. Meanwhile, he reared three lines of flies in normal light for comparison.

Raising flies is not an easy business. They can pick up infections and die in droves.