IO6 ENTOMOLOGY 



former layer is composed of tracheal structures and intervening paren- 

 chyma cells. The tracheae branch profusely in the photogenic layer, 

 where the larger air-tubes are each surrounded by a more or less cylin- 

 drical mass of cells ; tracheal branches penetrate between the cells of each 

 cylinder, at the edge of which they pass into tracheoles which penetrate 

 the photogenic tissue and anastomose with those of adjacent cylinders; 

 in the meshes of the tracheolar network is a granular substance of fatty 

 nature ("differentiated fat-body"), the oxidation of which is the source 

 of the luminosity, it is inferred. The photogenic tissues of Photinus, 

 after being dried and kept in sealed tubes, have retained their photogenic 

 power for more than eighteen months, glowing after this interval upon 

 the " application of water in the presence of air or oxygen" (McDermott). 

 Three factors are involved in the production of the light: a substance to 

 be oxidized, oxygen and water. 



The rays emitted by the common fire-flies are remarkable in being 

 almost entirely light rays. According to Young and Langley, the radia- 

 tions of an ordinary gas-flame contain less than three per cent, of visible 

 rays, the remainder being heat or chemical rays, of no value for illumina- 

 ting purposes; while the light-giving efficiency of the electric arc is only 

 ten per cent, and that of sunlight only thirty-five per cent. The luminous 

 efficiency of the fire-fly, however, is not much under one hundred per 

 cent. ; in Photuris pennsylvanica it is about ninety- two per cent., according 

 to Coblentz an efficiency as yet unapproached by artificial means. The 

 actinic power of the light is so slight that it affects a photographic plate 

 only after a long exposure. Coblentz, who has recently applied most re- 

 fined methods of measurement to the radiation of fire-flies, found that 

 exposures of one to five hours were necessary with the spectograph. He 

 was unable to detect any infra-red radiation; the thermal radiation, if 

 present, being immeasurably small as yet. The intensity of the glow 

 averages ^-g-.-J-jnr candle power in our common fire-flies, according to 

 Coblentz. 



This luminosity serves to bring the sexes together. "The male flies 

 over the tops of the grasses, weeds, etc., dropping down between them 

 and flashing; any females that come within the range of his flash, answer 

 by their slower flash; if the male sees this answering flash from one, he 

 approaches her, flashes again, to which she answers, and he then finally 

 locates her definitely by means of subsequent flashes," as McDermott 

 says. He found that he could get responses from the females by imi- 

 tating the flash of the male with a small electric bulb or even with a com- 

 mon safety match, and that he could deceive the males also by flashing the 

 tiny electric light after the manner of the female. 



