356 ELEMENTS OF BIOLOGY 



is in most human races very little protective coloration; the exo- 

 skeleton is almost non-existent and very defective as a protection 

 against carnivorous enemies; the integument does not contain 

 poison glands. Powers of escape and of combat are inferior as com- 

 pared with many other animals. 



Luminescence in Animals. The visible light produced by 

 animals is known to be associated with an oxidative reaction in 

 which at least two substances that resemble proteins or protein 

 products are concerned. One of these, called photogenin, is the 

 source of light and is not used up as light is emitted; the other 

 substance, photophelein, is destroyed in the reaction and conse- 

 quently is being constantly replaced by the metabolism of the 

 animal. The organs that emit light differ greatly in structure, but 

 all are essentially glands. In the common firefly the organ is single; 

 in some deep sea fishes light-emitting organs occur in numbers and 

 may be associated with condensing lenses that converge the light 

 into beams. The reaction between the substances concerned in light 

 emission appears to be initiated by nerve impulses as in glands in 

 general; hence it is probable that the intermittent flashing of the 

 common firefly indicates a periodic nerve stimulus that sets in action 

 the peculiar oxidative metabolic process. The mechanism is highly 

 efficient, for light is emitted without heat, a characteristic that Man, 

 in constructing lighting devices, has striven for without success. 



Sound-producing Apparatus. The mammalian larynx with 

 its vocal cords and the bird syrinx with its vibratory pessulus have 

 already been described (pp. 171 and 349). AH other sound-produc- 

 ing mechanisms are constructed on similar principles, namely, the 

 vibration of a membrane over a column or container of air, except 

 in the case of the whine produced by the rapid movement of the 

 wings of certain insects. The insects are the only invertebrates that 

 have organs specialized to produce sounds. 



On the wing covers of male katydids are peculiar areas of thin 



