^56 Comparative Animal Physiology 



slow. In Porichthys a latent period of 8 to 10 seconds is observed; then the 

 liuht lasts for about 20 seconds.-" Injection of adrenalin will activate the 

 photopenic organs in this llsh and in EchiostDUia.-^ 



Physical Characteristics of Animal Light. 1 he intensity of light produced 

 by photogenic organs is relatively low. The light of a lirefly, Photinns, is 

 the equivalent of 0.0025 to 0.02 candles;" that of the "cucujo" beetle, 

 '0.0006 to 0.006 candles.-'' Such values in candles do not give a good meas- 

 ure of the brightness of the actual luminescing surfaces, since these values 

 vary with the areas of these surfaces. Furthermore, since the surfaces are 

 relatively small, the values are likely to suggest smaller values of surface 

 brightness than are actually present. Also, since organ-sizes vary considerably 

 from one species to another, the order of brightness of luminescing surfaces 

 need not parallel the order of light intensities. Surface brightness is ex 

 pressed in lamberts or millilamberts. Measurements of the surface brightness 

 of a number of luminescing organs have given in general values ranging 

 from 0.3 to 45 millilamberts. 



The color of the light produced by various photogenic organs of animals 

 vary all the wav from blue through red.-'*' "^ There is not uncommonly a dif 

 ference in the colors of light emitted from different organs in the same ani- 

 mal. It is possible that in some cases these color differences are due to the 

 presence of colored Biters covering the luminous areas, but in other cases they 

 appear dehnitely due to differences in the basic light-producing reactants. 



Study of a number of species gives us reason to believe that the light 

 emitted by any single organ shows a continuous spectrum spreading over a 

 restricted region of the visible range of wave lengths. The spectral range for 

 a number of animals is found in the following hst modified from Harvey. -- 



Species Wave Lengths 



Chaetoptenis insionis 550-440 m/x 



Photinns pyralis 670-510 m/x 



Pyrophonis noctiliiciis 640-468 m/x 



Photuris pennsylvanica 590-510 m/x 



Cypridina hilgendorfii 610-415 m/x 



Phengodes laticollis 650-520 m/x 



When one recalls that the range of wave lengths visible to the human 

 eye extends from 760 to 400 m/x it is evident from the foregoing table that in 

 all of the common luminescent species whose spectra have been examined 

 the light produced is entirely within the visible range. Ultraviolet radiation 

 has never been recorded for any photogenic organ. Furthermore, no signifi- 

 cant infrared or heat radiation is produced; hence animal light is said to be a 

 cold light. This fact has led to the popular notion that animal light is nearly 

 100 per cent efficient. To determine the actual luminous efficiency of any 

 light source, however, it is necessary to take into consideration the spectral 

 energy curve for visibility of the particular organism for which the lumi- 

 nous efficiency is being calculated. If the light were 100 per cent efficient for 

 the human eye, all of the radiation would be at the wave length of maxi- 

 mum sensitivity of the eye. For an eye with the spectral sensitivity char- 

 acteristics of the human eye, the light of the firefly, PhotJiris, is about 92 



