The insects possess the simplest types of intracellular light organs, a mass of photogenic cells, which, in the common firefly (a lampyrid beetle) of Eastern North America, has probably been developed from the fat body, while in the New Zealand glowworm, the larva of a tipulid fly (Bolitophila luminosa), part of the Malpighian tubule cells have acquired photogenic power (Wheeler and Williams, 1915). This is illustrated in [Fig. 24].
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.
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 back
ground, 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.
In the firefly there is no true lens, the light merely shining through the cuticle which is transparent over the light organ, whereas over the rest of the body it is dark and pigmented. In the deep sea shrimp, Acanthephyra debelis, with light organs scattered over the surface of the body, the cuticle covering the light organ forms a concavo-convex lens, behind which are the photogenic cells (Kemp, 1910). As may be seen from [Fig. 26], the lens is made up of three layers which suggests that it may be corrected for chromatic aberration—a veritable "achromatic triplet." In an allied form, Sergestes ([Fig. 27]), the lens is of two layers and double convex. Optical studies of these lanterns have been made by Trojan (1907). The course of the light rays is shown in [Fig. 28]. The lens of these organs is also bluish in color which suggests that they may serve also as color filters. Behind the photogenic cells is a mass of connective tissues through
which enters the nerve, for the light of these organs is under the control of the animal and may be flashed "at will."