A long exposure with rear sync flash...
All firefly larvae that belong to the family Lampyridae have a photic organ on their eighth abdominal segment, consisting of two spots. This is quite different from adult fireflies, whose organs are generally contained between the fifth and seventh abdominal segments. It is believed, however, that this structure is homologous with that of some adult fireflies, because "they are in the same location in the abdomen, have the same shape... and produce the same type of photic emissions (Branham and Wenzel 2003)."
The microstructure of larval photic organs differs from that of adults as well. In larval fireflies, the photic organ is directly stimulated by the nervous system and the light reaction is triggered slowly. Adults, on the other hand, do not have this direct nervous connection; rather, the nerve terminates on tracheolar end cells which quickly initiate the luminescence reaction. Additionally, the larval photic system is not as specialized; photocytes are arranged randomly in larvae while adults have organized rings of photocytes. The bioluminescence reactions are believed to have evolved independently in adults and larvae, so there is still much left to explain about these structural differences (Oertel et al. 1975).
Since the physiology of larval light organs differs from that of adult fireflies, it is no wonder that their luminescence spectra differ as well. In fact, the luminescence spectrum of firefly larvae is broader than that of adult females, and appears “green-shifted” in color (De Cock 2004).
Additionally, the timing of luminescence may be different in some species of firefly larvae; for example, Photuris larvae produce a slow-changing glow rather than the conventional “quick” flashing patterns seen in adults (Ghiradella and Schmidt 2004); In fact, a larva's glow may last 50 times longer than adults (Oertel et al. 1975)!
It is believed that the emission of green light in larval fireflies serves as a defensive mechanism, because the eyes of potential predators “show their highest spectral sensitivity in the green region of the light spectrum (De Cock 2004).” Interestingly, L. splendidula larvae only glow when disturbed, providing evidence for this theory (De Cock 2004). It is believed that bioluminescence originated in firefly larvae as an aposematic warning display, suggesting to predators that they are unpalatable (Branham and Wenzel 2003).