LIGHT AND ELECTRON MICROSCOPE STUDIES ON THE LIGHT ORGAN OF THE FIREFLY (PHOTINUS PYRALIS)

Abstract

1. The following structures were observed in the light microscope preparations of the firefly light organ: photogenic layer, reflector layer, photogenic granules, urate granules, transverse tracheae, tracheae, cylinders, tracheal twigs, tracheal end cells, tracheal end bulbs, tracheoles which extend between the photogenic cells, cuticle, differentiated outer layer of the photogenic cells, mitochondria in tracheal end cells and in differentiated outer layer of photogenic cells. 2. Urate granules were revealed in the electron micrographs to be of about the same density as the surrounding cytoplasm. Vacuoles which may be due to fixation artifact, or to a dissolution of material from within the granules, were observed. Filamentous mitochondria with pronounced transverse lamellae were observed lying between the granules of the urate cells. 3. Electron micrographs demonstrate, radiating from the centrally located region of the tracheal twig within the end cell, numerous cytoplasmic processes, or villi, that contain filamentous mitochondrial material. In some end cells the mitochondria within the villi show longitudinally oriented lamellae. In other end cells, the internal lamellae are arranged transversely to the long axes of the mitochondria. Thus, within the same type of cell the intra-mitochondrial structure is seen to vary. 4. The distal "rounded body" of Dahlgren, here referred to as the end bulb, is located a the distal part of the end cell. Within this body which surrounds the tracheal twig at the position where it branches into two tracheoles are four structures: First is the centrally located intima of the tracheal tube; radiating from near the outside surface of the tracheal intima are numerous microvilli about 2,000 Å in length; centrifugally to the radiating microvilli is a layer of relatively small yet numerous mitochondria; limiting the end bulb is the well developed end bulb membrane. 5. The membrane and cytoplasm of the end bulb are continuous with those of the tracheoles. Mitochondria, on the other hand, are mainly limited to the body of the end bulb. 6. The differentiated peripheral layer of the photogenic cells placed at the surfaces of the light organ and around the tracheal cylinders was found to be composed of a limiting membrane overlying a differentiated cortical cytoplasmic layer containing a large number of mitochondria. 7. The tracheoles extend between, and never into, the photogenic cells. 8. All elements of the tracheal system, including the smallest tracheoles, were observed to possess well developed taenidia.