METABOLISM 81 



this direction. The most familiar example is that 

 of the firefly, which has light-giving organs at the 

 base of the abdomen. In the depths of the sea 

 many of the fishes that inhabit those abysses have 

 similar light-producing spots distributed over the 

 body in characteristic ways. One of the species 

 even develops such an organ at the end of a fila- 

 ment like a pendent incandescent lamp. 



FIG. 30. Lantern fish (Linophryne lucifer). The phosphorescent 

 bulb doubtless functions as a lure to entice other fishes within reach of 

 the jaws. The body is distended with a large fish that has been 

 swallowed. (After Collet.) 



It has been found that the efficiency of the firefly's 

 light is practically 100 per cent, that is, none of the 

 energy is lost as heat; whereas in an ordinary in- 

 candescent lamp but 3J per cent is utilized as light, 

 and in an arc but 15 per cent, the rest of the energy 

 being wasted as heat. In the insects mentioned 

 above, the glow is produced by the oxidation of 

 some specially secreted substance, probably fatty in 

 nature, and the flashes of the firefly correspond 

 with the intervals of taking in air through the res- 

 piratory tubes. 



