78 THE NATURE OF ANIMAL LIGHT 



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 trans- 

 formed into the reflector cells. The two layers are distinct 

 and permanent from an early stagiB 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 con- 

 cavo-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 "achro- 

 matic 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 sug- 

 gests that they may serve also as color filters. Behind 

 the photogenic cells is a mass of connective tissues through 



