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 oi sexual 

 attraction a belief held by 

 the author in regard to Pho- 

 tinus, at least. Another view 



. - * , . 



IS that the light IS a warning 



^ j tQ nocturna l birds, bafS 



or Other inSCCtivOrOUS animals ; 

 _,. . i i_ ,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. 



Tracheal system of an insect, a, an- 

 tenna; b, brain; /, leg; n, nerve cord; 

 p palpus; ,, spiracle; st, spiracular, or 

 stigmatal, branch; t, main tracheal trunk; 

 v, ventral branch; vs, visceral branch. 

 After KOLBE. 



