Comparison to Adult Light Organs
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).
Why the Difference in Color?