No. 540] 



STUDIES ON MELANIN 



749 



containing respectively "0 2 40 parts, N 2 80 parts; 2 

 60 parts, N 2 80 parts ; 2 80 parts, N 2 80 parts, and 2 

 pure. In the first two no changes were noted, hut in the 

 third there was a large mortality, showing that the 

 amount of 2 had become toxic and the pigmented areas 

 were small and weak. The same results were attained 

 in a more marked manner with pure Q»." 



Tower's results in this instance are easily accounted 

 for, inasmuch as a very small quantity of oxygen is con- 

 sumed in the process of pigmentation and even in normal 

 air the oxygen is present in enormous excess. The rate 

 of pigmentation is, in all probability, at a maximum 

 even when the oxygen is present in very small concen- 

 tration. Tower further adds (1903, p. 58): "A second 

 set of experiments consisted in diminishing the 2 pres- 

 ent, but even although 2 was absent, pigmentation was 

 not changed. A third set consisted in placing pupae in 

 an atmosphere of C0 2 , N, H, and pigmentation, if about 

 to begin or already begun before the pupae were placed 

 in the gas, was not retarded or changed." Tower does 

 not state what precautions were used to ensure the entire 

 absence of oxygen. I have found that when all oxygen 

 is absent, no pigmentation takes place. 



Newly emerged adults, whose elytra showed no trace 

 of brown pigment, were placed in gas wash bottles which 

 were provided with a tubulated ground glass stopper, 

 arranged in such a manner that when the stopper is 

 turned slightly the stream of gas is cut off and the bottle 

 is tightly closed (see Eimer and Amend 's 1910 Catalogue 

 no. 3658). A current of washed carbon dioxide, from a 

 Kipp apparatus, was then passed through the bottle for 

 twenty-four hours and then the gas was shut off by turn- 

 ing the stopper slightly. In every instance the elytra of 

 the beetles remained colorless as long as they remained 

 in the carbon dioxide and if removed to the air, or else 

 if the carbon dioxide were displaced by a stream of pure 

 oxygen, before decomposition set in, pigmentation pro- 



