The male's flash lasts under a second and rises as he flies, producing the characteristic J. A receptive female of the same species responds from vegetation after a delay tuned to the species, near two seconds at typical summer temperatures, and the male turns towards the answer. Both the flash pattern and the response delay change with temperature, so the code shifts across a warm and a cool evening.
The light itself comes from a chemical reaction in the abdominal lantern, in which the enzyme luciferase acts on luciferin with oxygen and ATP. Almost none of the energy is lost as heat, which is why the emission is described as cold light.
The exchange is not always safe. Females of the predatory genus Photuris mimic the answering flashes of other genera, including Photinus, luring males in and eating them, partly to acquire defensive chemicals the males carry and they do not.