Ultrastructure of granules and immunocytochemical localization of luciferase in photocytes of fireflies

Photocyte granules are round to oval, 1–2 μm, contain a peripheral dense area and are of three structural types. Type one granules consist of an amorphous matrix, a bundle of 2–12 microtubules and a flask-shaped vacuole. The type two granule is characterized by a large crystal or several smaller crystals embedded in an amorphous matrix with microtubules lined up along the face of the crystal. The type three granule is filled with a large number of thick-walled tubules (40–50 nm o.d.), usually found in bundles of two to four and a few microtubles. Luciferase has been shown to be localized in these photocyte granules by the immunoferritin technique. Ferritin is not localized over microtubules or flask-shaped vacuoles in type one granules but is randomly distributed over the matrix. In type two granules, ferritin is more densely distributed over the crystals and in type three granules over filamentous structures. There is no ferritin over the microtubules. Other parts of photocytes and of light organs are negative for luciferase. Buffer and anticalliphorin incubated sections showed no ferritin in granules.