132 



ENTOMOLOGY 



FIG. 167. 



undergoes rapid combustion in the rich supply of oxygen fur- 

 nished by the tracheae. 



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

 being almost entirely light rays, with almost no thermal or 



actinic 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 illuminating pur- 

 poses; while the light-giving 

 efficiency of the electric arc is 

 only ten per cent, and that of 

 sunlight only thirty-five per 

 cent. The light of the fire- 

 fly, however, may be rated at 

 one hundred per cent. ; this 

 light, then, is perfect, and as 

 yet unapproached by artificial 

 means. 



As to the use of this lumi- 

 nosity, there is a general 

 opinion that the light exists 

 for the purpose of sexual 

 attraction a belief held by 

 the author in regard to Pho- 

 tinus, at least. Another view 



Tracheal system of an insect, a, an- . . 1 



tenna; b, brain; /, leg; n, nerve cord; IS that the light IS a Warning 



P. palpus; s, spiracle; st, spiracular, or g - a j t() nocturna l birds, batS 

 stigmatal, branch; t, main tracheal trunk; 



v, ventral branch; vs, visceral branch. or Other InSCCtivOrOUS animals \ 



After KOLBE. 1 1 r 



this is supported by the fact 



that lampyrids are refused by birds in general, after ex- 

 perience; young birds readily snap at a fire-fly for the first 

 time, but at once reject it and thereafter pay no attention to 

 these insects. 



