BIOLOGY OF LIGHT PRODUCTION — MALUF 



385 



a stimulus implies a decrease in the velocity of a reaction in the 

 light-producing cells which, as Brown and King consider, can depend 

 only on differences in the concentrations of luciferin, luciferase, or 

 (and) oxygen. This consideration issues from the fact that we know 

 a number of things: (1) Molecular oxygen is necessary for the lumi- 

 nescence — i. e., the process is an oxidation; (2) a substrate, luciferin, 

 is oxidized through the agency of an enzyme classified among the 

 aerobic oxidases and termed luciferase; (3) according to the phe- 

 nomena of mass action, the velocity of a chemical reaction varies 

 directly with the concentration (activity) of the reacting components. 

 Owing to the fact that the luciferin-luciferase reaction shows a similar 

 relationship in vitro, where the oxy- 

 gen available is constant, to that de- 

 picted in figure 2, Brown and King 

 concluded that the declining concen- 

 tration of the luciferin-luciferase 

 system was the determining factor 

 in the "die-away" portion of the re- 

 sponse. Because, however, of the 

 much slower decline in luminosity in 

 the in vitro conditions than in the in- 

 tact animal and because of the facts 

 stated in the following subsection, it 

 seems probable that the decline in 

 luminosity in the intact animal is 

 due to a decrease in the concentra- figure 2.-The magnitude of the photogenic 

 tion of oxygen in the vicinity of the 

 photogenic cells. 



5. Significance of luminescence in 

 the economy of an animal. — After re- 

 viewing the literature on the subject, Gazagnaire (1890) came to 

 the conclusion that the intensity of luminescence in geophilid chilo- 

 pods is intimately correlated with the time of sexual activity. Car- 

 penter (1854) had already remarked that the same holds true for 

 insects. The increase in the intensity of luminescence at such a 

 time may be due to a raised rate of metabolism at this time; or, in 

 the words of Carpenter, "the activity of this combustion is stimulated 

 by anything which excites the vital functions of the individual." The 

 notion, however, that luminescence has a secondary sexual function 

 in chilopods cannot be upheld, since all the Geophilidae (the order 

 which includes the luminous forms) are eyeless. In fireflies, on the 

 other hand, the case is quite different. Thus, according to Mast 

 (1912, 1923), "the female [fireflyl in response to a flash of light pro- 

 duced by the male turns the ventral surface of her abdomen toward 



response of Photuris pennsyloanica initiated by 

 a single electrical stimulus. Ordinate: intens- 

 ity of the flash in terms of the initial (=100). 

 Abscissa: time scale (10 units =0.833 seconds;. 

 (After Brown and King.) 



