164 F. X. WILLIAMS 



The inner layer (or rather, mass) is surrounded dorsally and 

 laterally by the thinner outer sheet, and is ventrally applied to 

 the hypodermis, its cells are ill-defined, with distinct nuclei, 

 and the cytoplasm contains numerous, very fine black granu- 

 lations, about 0.29 to 0.58 micra, which are thus of the same size 

 as those in the adult light-organ. This photogenic mass is quite 

 distinctly separated from the rather clear, more feebly staining 

 outer sheet, which is one or two cells in thickness, the outer 

 being tall and columnar and radially arranged around the center 

 of the organ. These cells of the outer (reflector) layer are all 

 well-defined and filled with small pale granules. At least one 

 large tracheal branch and one fair-sized nerve (T and N, fig. 13, 

 Photinus consanguineus) enter the outer and penetrate the inner 

 mass by a well-defined pore. The nerve is a branch from the 

 most ventral of the three pairs which proceed from the last 

 (eighth) abdominal ganglion. The latter is situated in the fifth 

 abdominal segment. The trachea involved branched consider- 

 ably within the organ, and terminated as tracheoles (fig. 22) of 

 much slenderer proportions than those in the adult light-organs 

 (fig. 23). No end-cells were found. The capillaries or tracheoles 

 extend throughout the photogenic mass in an intricate and 

 irregular fashion, so that the tracheal cylinders are not evident. 



The larval light-organs are essentially similar in Photinus and 

 Photuris. When freshly dissected the dorsal layer is dead white, 

 and the lower a sort of very pale yellowish green or gray and 

 semi-translucent . 



PHYSIOLOGY OF THE PHOTOGENIC ORGANS 



It is needless to say the study of photogenic organs from a 

 physiological standpoint has proved to be a subject of unusual 

 interest, especially since it suggests, even to the casual observer, 

 the possibility, however remote, of using the luminous powers 

 in the organic world. The fact that "no sensible heat accom- 

 panies the fireflies' light," and that "nature produces this cheapest 

 light at about 1/400 part of the cost of the energy which is ex- 

 pended in the candle flame" (Langley and Very, '90), opens a 

 very inviting field in both pure and applied science. 



