384 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



the minimal intracellular oxygen pressure necessary for appreciable 

 luminescence. 



4. Decline in the intensity of a flash. — Brown and King (1931) 

 measured the intensity of the light produced by Photuris pennsyl- 



Figuee 1. — Diagram illustrating the mechanism of the control of flashing in the "firefly." 



A. A spontaneous discharge from ganglion cell, g, in the brain, 6, is transmitted to the motor cell, m, in the 

 terminal abdominal ganglion, o, and from thence, along the motor fiber, to photogenic cells, p. This 

 excites these cells to activity, raising their osmotic pressure and thus withdrawing fluid from the tracheoles, 

 t . This admits air, coming through the trachea, tr, into the interior of the cell, e is the end-cell found 

 at the tracheoles of all insects. 



B. Condition during inactivity of the specific cerebral ganglion cells, at which time the lactic acid is oxidized 

 to water and COj, thus lowering the osmotic pressure in the photogenic cells. This allows the passage of 

 fluid, /, into the tracheoles by capillary pressure and thus keeps air from directly penetrating the cells. 



vanica as a result of a single electrical stimulus. A series of responses 

 follows a single adequate stimulus with an increasing time interval 

 between each response, until the responses, declining in intensity, 

 eventually fade away (fig. 2). The decrease in light intensity following 



