LIVING LIGHTS 



to distinguish a light-producing material and a 

 ferment which sets the production agoing. 



In the American fire-fly both sexes are lumines- 

 cent, the 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 under side of the fourth and fifth posterior 

 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 rapid 

 breaking up is quickened by some ferment. Very 

 noteworthy is the fact that the light is unaccom- 

 panied by perceptible heat. It is therefore produced 

 more economically than in any of our lamps, where 

 some of the energy is always lost as heat. 



In the 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 layers of wood or leather. 



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 cuttle-fish, but they occur there in 

 nine different situations. Sometimes they are 

 concealed beneath the skin, but they may be effec- 

 tive even then, since the living tissues of cuttle-fishes 

 are very transparent. It is plausible to suppose 

 that they serve as recognition marks, and that they 



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