JOHN BUCK • -327 



Plate I 



Schematized diagrams of the spread of luminescence (stippled areas) in light 

 organs stimulated in various hypothetical ways and the corresponding intensity- 

 duration curves which would be obtained by recording from the whole organ.* 

 The three stages of spread of excitation (A, B, C) are indicated at the corre- 

 sponding points on the graphs which form the last member of each series. The 

 concentric spread of luminescence can be thought of as reflecting either a sort 

 of nerve-net propagation of excitation from the initiating areas through succes- 

 si\e ranks of effector units (as in Renilla) or as the pattern of direct effector 

 innervation, the apparently progressive spread from each focus indicating con- 

 duction delay from the underlying stimulation center (as would be more likely 

 in the firefly). It has been assumed that only a single stimulus is given to each 

 effector unit (repetiti\e stimulation would of course alter the cur\e forms dras- 

 tically) and that a given area of organ lights only once. If the decay of lumi- 

 nescence were long in comparison with the rate of spread of excitation, i.e., if 

 the stimulated regions stayed lit at least until the whole organ was excited, all 

 the patterns of excitation would gi\ e the same general type of intensity-duration 

 curve. 



Fig. 1. Luminescence spread in an organ initially stimulated centrally. A possible 

 instance of this sort of propagation is the circular prothoracic photophore of 

 the elaterid beetle Pyrophorus in which the flash is so slow that the spread 

 of luminescence is easily observed. In this organ, Heinemann ( 1886 ) observed 

 that the light appeared first at the center of the organ and spread peripher- 

 ally, and died out in re\erse order. (If the stimulation began peripherally, 

 peak intensity would be gained early, giNing a cur\e skewed to the left.) 

 Fig. 2. Luminescence spread in an organ in which the excited region is of con- 

 stant area. The same form of intensity-duration curve is obtained if the 

 excitation starts centrally and spreads laterally in both directions, and if there 

 is only one excited region instead of two. An example of such propagation 

 is seen in the long slender rachis of sea pens. 

 Fig. 3. Luminescence spread in an organ with multiple excitation points. This 

 type of propagation, which is the most likely type in lampyrid fireflies, gives 

 a roughly symmetrical intensity-duration curve which is quite nonspecific. 

 Fig. 4. Multifocal luminescence spread giving a bimodal type of intensity-duration 

 curve reminiscent of that of the female of the firefly Photuris pennsylvanica 

 (Brown and King, 1931). The initial phase of this curve is equivalent to 

 Fig. 3A, but the subsequent spread of luminescence differs in that the 

 excitation points are relatively closer together in relation to the total area 

 of the organ so that after the initial peak is reached (at A), the die-away 

 of luminescence at the 7 points of impingement of the 6 centers of spread 

 exceeds, for a time (4B), the increase in luminous area due to peripheral 

 spread of excitation. Later the peripheral area increase becomes dominant 

 ( 4C ) , followed by a decay essentially similar to that in Fig. 3C. 



' Thanks are due Dr. Margaret Keister for making the necessary calcula- 

 tions and executing the figures. 



