NATURE OP THE LIGHT EMITTED BY FIREFUES. 43 



XII. NATURE OF THE LIGHT EMITTED BY FIREFLIES. 



It is beyond the scope of the present paper to discuss the various experi- 

 ments which have been made in the endeavor to learn how the firefly pro- 

 duces its light. The older publications usually contain bibliographies which 

 may be consulted,* and on the following pages only such data are quoted 

 as have a very direct bearing on the subject. 



The old idea that the light in animals is due to phosphorus has long been 

 refuted. The idea has also prevailed that the light is a species of phosphor- 

 escence. During the past fifty years various experiments have been made 

 which could have been used to refute this idea ; but, owing no doubt to the 

 lack of a thorough knowledge of the behavior of phosphorescent materials, 

 not until within the past few years has this erroneous notion been losing 

 adherents. The phosphorescence idea no doubt originated from the fact 

 that some fireflies appear to be most active at dusk. This observation of 

 the time of greatest activity in fireflies depends of course upon the extent 

 of the experimenter's knowledge of the habits of the various species, some of 

 which do not appear until after it is dark, and are very active for hours there- 

 after, although it would not appear so to the casual observer, for the reason 

 that most of their activity (e. g., Photuris) is among the tops of tall trees. 



The most important characteristics of phosphorescent material are ( i ) the 

 production of a "phosphorescent light" which continues to be emitted for 

 some time after exposure to white light and especially to violet and ultra- 

 violet light ; and (2 ) the extinction of this ' ' phosphorescent light ' ' on expos- 

 ing the glowing material to infra-red rays. Some substances (e. g., fluorite) 

 emit a phosphorescent light, called thermo-luminescence, when heated to 

 from 50° to 200° C. Although the application of heat to insects has been 

 made from the beginning, it can not be a test for thermo-luminescence in the 

 living insect, because of the question of the heat acting as an irritant. 



One of the first experiments tried by the writer, four summers ago, was 

 the effect of heat and cold (liquid air), of light and of darkness, upon living 

 and upon dead insects. The dry specimens showed no thermo-luminescent 

 light on heating; neither was there a phosphorescence when exposed to light. 

 The luminous organs of freshly killed, undried specimens did not show phos- 

 phorescence when exposed to white light or to the ultra-violet spark. Both 

 last year and this year the effect of X-rays was tried on fireflies, but neither 

 phosphorescence nor fluorescence could be detected. In the same manner 

 the precipitate from the fluorescent material found in the body fluid of fire- 

 flies, dried on filter paper, was not excited to luminescence by the X-rays or 

 violet light. These experiments are of interest in connection with the data 

 showing that photogenic material may be kept in a dry state for a long time 

 when it can be made to glow by treating it chemically. Evidently the 

 photogenic material does not possess phosphorescent properties. 



*I,angley, "Cheapest form of light," Amer. Jour. Sci. (3) 40, No. 236, 1890. 



Seaman, The luminous organs of insects, Proc. Amer. Soc. Micros., 13-14. P- i33. 

 1891-2. 



Watase, A treatise on the luminosity of insects. 



Mangold, "Die Produktion von Licht," Winterstein's Handb. der Vergleich. Physi- 

 ologic, Band 3, 191 1. 



