THE WONDER OF LIFE 491 



flightless female less so than her active partner. The 

 luminescent organs of the male consist of a pair of plates, 

 lying beneath the skin on the ventral surface of the fourth 

 and fifth abdominal segments. Each plate has two layers, 

 and the lower is built up of polygonal cells filled with 

 coarse granules. In this lower layer there is probably a 

 rapid oxidation of some unstable substance, perhaps of a 

 fatty nature. It is possible that the decomposition may 

 be accelerated by some ferment. Mrs. A. B. Howard has 

 called attention to the fact that the light is unaccompanied 

 by perceptible heat. It is therefore produced at the least 

 possible expenditure of energy, as Professor Langley long 

 ago pointed out. 



In fire-flies of the genus Luciola the light given off 

 has a beautiful green fluorescence, and is able, like X-rays, 

 to affect a photographic plate through opaque media, such 

 as layers of wood or leather. The light cannot be taken 

 as phosphorescent, but includes rays which are, at least, 

 ' similar to X-rays and ultra-violet light in so far as they 

 render certain opaque media transparent, and are inter- 

 cepted by glass '. 



In an interesting study on the luminous organs of cuttle- 

 fishes, Dr. W. E. Hoyle calls attention to their occurrence 

 in so many and such scattered families, that repeated and 

 independent origination seems probable. They are almost 

 always on the ventral surface of the Cephalopod, but they 

 occur there in nine different situations. Sometimes they are 

 concealed beneath the mantle or beneath the skin, but 

 they may be effective even then, since the living tissues 

 of cuttlefishes are very transparent. It is plausible to 

 suppose that they serve as recognition marks, and that 

 they act as searchlights, playing over the floor of the sea. 



