Fireflies likely control their luminous flashing by suppressing oxygen consumption in mitochondria. This new insight comes from a team led by Chia-Wei Li of National Tsing Hua University and Yeu-Kuang Hwu of Academia Sinica, both in Taiwan, in collaboration with researchers at ETH Lausanne (Phys. Rev. Lett. 2014, DOI: 10.1103/physrevlett.113.258103). Fireflies glow through the action of the enzyme luciferase, which combines luciferin and oxygen to make oxyluciferin in an electronically excited state that emits light as it relaxes. Researchers believe fireflies control the effect through oxygen flux in their lanterns, but the specific mechanism has been debated. Li, Hwu, and colleagues mapped firefly lantern trachea in submicrometer detail using X-ray microscopy and microtomography. They then used the maps to derive oxygen diffusion capacity, calculating that the amount of oxygen that can diffuse through the lanterns is close to what is consumed when they light up. Competition for oxygen between bioluminescence and respiration must therefore be well controlled, the researchers conclude. The results suggest a mechanism in which fireflies temporarily turn mitochondria off so that luciferase may receive sufficient oxygen to cause a flash.