420 PHYSIOLOGICAL LIGHT. 



centers, which explains why sensorial or psychic stimuli may affect the 

 production of light. It is proper to add that the protoplasmic cells of 

 insects are directly excitable, like the Noctlluca or the ectoderm of the 

 Goelenterata. In the Lampyridw, the light-producing power is not lim- 

 ited to the organs in question, as the eggs become luminous in the 

 ovary, and at the moment of molting, when the new integument is 

 yet uncolored, in absolute darkness the entire hypoderm shows a feeble 

 phosphorescence. Besides, embryological studies show that both in 

 the larva and in the female nympha the photogenic organs are formed 

 at the expense of the hypoderm. 



The organic mechanism of the photogenic function in the Coleoptera 

 is particularly easy to study in the luminous Materidce, those dazzling 

 beetles of the tropics, and particularly in Pyrophorus noctilucus. 



The egg of Pyrophorus is luminous, like that of Lampyra, and the 

 little larva which comes from it also carries with it at birth the lumi- 

 nous source transmitted to the egg by its ancestors. In the young 

 larva, this is single, bilobate, and situated at the junction of the head 

 and the thorax. It contains numerous rounded granulations and emits 

 a bluish light. After the second molting the cephalothoracic appa- 

 ratus persists, and then others appear on each of the segments and a 

 larger single one upon the last ring. These lumiuous spots may be 

 lighted and extinguished successively, like the gas-burners of a stair- 

 way swept by the wind, and it is a very curious spectacle to see two of 

 these larvae struggling together and twisting about while emitting 

 flashes of light. Imagine what it would be if the combatants were 

 some meters in length. 



In the adult state, Pyrophorus has three lanterns — two dorsal ones 

 upon the cephalothorax and a ventral one at the junction of the 

 thorax and abdomen. The arrangement of these organs is quite 

 similar to that of the organs of the Lampyridw, and their regulating 

 mechanism is also very much the same. For example, by the action of 

 small muscles the ventral organ of Pyrophorus opens and closes like a 

 purse, and owing to its situation and anatomical structure it is easy to 

 show, both by direct observation and by experimentation, that the pro- 

 duction of the light is closely allied to the fluctuations of blood in the 

 organ, and to a great degree independent of the play of the stigmata 

 and the trachea 1 , which are in relation with it. But why should we 

 further discuss the essential function of hypothetical tracheal blast 

 tubes"? When the organ is isolated from the body, dried and pulver- 

 ized, it still gives out light when a drop of water is let fall upon its 

 amorphous dust — a singular combustion truly, but without blast tubes 

 this time. In order to rapidly conclude an examination of photogenic 

 species, I will for the present leave the study of the special mechanism 

 of photogeny, which will be treated in the second part of this article. 



