Fast

The chemical reactions by which fireflies produce bioluminescence are well described (Reactions 1 and 2), the excited-state intermediates involved in light emission have been characterized, many of the genes for their luciferase have been sequenced, and the X-ray crystal structure of the Photinus pyralis luciferase enzyme has been solved (reviewed recently by Wilson and Hastings, 1998). However, the mechanism by which adults produce their bioluminescence as rapid flashes of light, which is vital for its biological courtship function, is not yet understood, despite a long history of scientific investigation. This has been comprehensively reviewed by Case and Strause (Case and Strause, 1978) and recently updated by Ghiradella (Ghiradella, 1998) and Wilson and Hastings (Wilson and Hastings, 1998), also see Movie 1 in the supplementary material for a demonstration. Here we provide experimental evidence supporting the most commonly postulated theory of flash control, namely that the flash results from rapid gating of oxygen (O2) to the light-emitting cells of the lantern (photocytes). There is a great deal of circumstantial evidence supporting such a control mechanism (Case and Strause, 1978; Wilson and Hastings, 1998), including (1) firefly bioluminescence absolutely requires oxygen (Reaction 2); (2) sequential hypoxia and reoxygenation causes a ‘pseudoflash’, simulating many features of a genuine flash (Alexander, 1943; Hastings and Buck, 1956); (3) the tracheolar system of the adult lantern (the structures supplying oxygen to insect cells) has a unique structure compared to the larval form (Ghiradella, 1977; Ghiradella, 1998); and (4) the nerves innervating the flash motor unit do not terminate upon the photocytes themselves but rather the specialised tracheal end (2)Luciferase-luciferin-AMP + O2 f