Which little insect makes light at night?

Fireflies, which are beetles rather than flies, produce light in an organ in the abdomen by combining a molecule called luciferin with oxygen, helped by an enzyme. Almost none of the energy comes out as heat - it is close to a hundred per cent efficient, against about ten per cent for an old filament bulb. This is genuinely cold light.

The flashes are a language. Each species has its own pattern of pulse length and interval, males advertise while flying and females answer from the ground, and getting the timing wrong means no reply. In parts of Southeast Asia thousands of males in the same trees synchronise their flashing into a single pulse across a whole riverbank, and how they agree without a conductor was an open question for a century.

There is a predator exploiting all this. Females of the genus Photuris mimic the answering flash of other species, lure the male in expecting a mate, and eat him - acquiring, in the process, defensive chemicals their own species cannot make. They are known in the literature as femmes fatales, which is the only time entomology has been allowed that phrase.