The photogenic organ of the firefly is made up of two kinds of cells, a dorsal mass of small cells several layers deep, the reflector layer, and a ventral mass of large cells with indistinct boundaries, the photogenic layer (Fig. 25). The photogenic cells contain a mass of granules, spherical in the male and short rods in the female. The photogenic cells are divided into groups by large tracheal trunks which pass into the light organ and branch to form tracheoles connected with tracheal end cells. The exact distribution varies in different species, but in all the arrangement is such as to give a very abundant oxygen supply. Each group of photogenic cells is surrounded by a clear ectoplasm containing no granules. The tracheoles pass through this and either end openly within the photogenic cells or anastomose with tracheoles from neighboring tracheæ. Nerves, but no blood-vessels--which are absent in insects--enter the organ. It is difficult to determine if the nerves supply the tracheal end cells or the photogenic cells. [Illustration: FIG. 25.--Sectional view of photogenic organ of the firefly (_after Williams_), showing reflector or crystal layer (_U_) above and photogenic cells (_P_) below. _C_, cuticula; _T_, trachea; _c_, capillaries of tracheal end cells; _H_, hypodermis; _EC_, tracheal end cells; _N_, nerve.] The dorsal reflecting layer is made up of cells containing numerous minute crystals of some purin base, either xanthin or urates, or both. They have a white milky appearance and while they are certainly not good reflectors in the optical sense, they do act as a white background, scatter incident light, and partially prevent its penetration to the internal organs of the firefly. Although a few crystals similar to those of the reflector layer are found in the photogenic cells and in other cells of the body, it is known that the photogenic cells are not transformed into the reflector cells. The two layers are distinct and permanent from an early stage in development. Curiously enough, the light organ of the larva of the firefly (glowworm) is quite distinct from that of the adult. Like so many other structures in insects, the adult organ is developed anew from potential photogenic cells during the pupal period. Even the egg of the firefly is luminous and glows with a steady light, and during the pupal period light may sometimes be seen coming from the thoracic region.