Firefly flashing has attracted the attention of both poets and scientists for over a century. Here we study the effect of temperature on the flash duration of the Indian species of the firefly Luciola praeusta Kiesenwetter 1874 (Coleoptera: Lampyridae: Luciolinae). Recording in vivo time-resolved spectra of specimens of this species of firefly over the temperature range 20 °C-40 °C, it is observed that the flash duration changes with the change in temperature, and the change is substantially linear. This finding implies that the speed of the enzyme-catalysed chemiluminescence reaction, which produces the light of the firefly, varies linearly with temperature.