THE FIREFLY'S LIGHT 



A Remarkable Structure that Seems in Some Cases to be of Little Value to Its 



Possessor — Possibly a By-product of Evolution — Serves in Many 



Cases as a Signal between the Sexes 



FEW insects possess a more remark- 

 able mechanism than the lij^jhtin^' 

 apparatus of the firefly. It miji;ht 

 be sui)posed that the evolution of 

 such a mechanism wovild be of life-and- 

 death value to its possessor. But it is 

 difficult to see that it has such a value 

 to the firefly. Its existence is possibly 

 more or less an accident of evolution; 

 but now that it is here, the firefly makes 

 use of it, at least in some species, to 

 find its mate. 



The fireflies, which are in reality not 

 flies but beetles, are found throu.^h the 

 eastern and southern part of the United 

 States almost any warm summer even- 

 ing. There are two principal forms, 

 which nearly always occur together; 

 Photinus pyralis and Photuris pen- 

 sylvannica, both members of the family 

 Lampyridae. There are several other 

 more insignificant species, which are 

 so like these two as to recjuire no sepa- 

 rate description. 



Photuris is strong and hard\'. car- 

 nivorous, eating dead comrades or 

 enemies with no discrimination, and 

 generally flies at some altitude, espe- 

 cially among the tops of tall trees, 

 flashing only occasionally, and some- 

 times moving several rods before again 

 emitting its light. For some reason, 

 males are much more infrequently 

 taken than females, and captures which 

 have been made have been found to 

 average but one male for every fifteen 

 females. The light-emitting organs are 

 almost identical in the sexes. 



Photinus is the more plentiful, and 

 flies nearer the ground, flashing fre- 

 quently. It is easily injured, and 

 being so delicate is hard to kec]) in 

 cai)tivity, while Photuris will live well 

 indefinitely if provided with a little 



sod. The male Photinus has much 

 larger photogenic organs than the 

 female, and hence the sexes are easily 

 distinguished. The most characteristic 

 flash of Photinus consists of a long 

 fuhnination of yellow hght. 



The photogenic organs of Photinus 

 are the more comjjlicated of the two 

 species. Two small roundish sjjots ap- 

 pear on the next to the last abdominal 

 segment in both sexes, these spots giving 

 off a greenish light. In the male, the 

 second and third ]josterior segments are 

 also composed of i)hotogenic cells, 

 while in the female, an oval si:)lotch 

 on the third segment emits light, the 

 rest of the segment not having this 

 power. 



The light emitted from the two 

 small round spots in each sex is dis- 

 tinctly greener than that from other 

 segments of the abdomen, but the cause 

 of this is not clearly understood. It 

 seems possible that the color depends 

 upon the intensity of the light, but 

 experiments have shown that often the 

 green light is the brighter of the two, 

 still retaining its greenish color. The 

 color of the light seems to be useful as a 

 distinguishing mark of the different 

 s]K>cies, no two emitting an identical 

 shade. 



It must be remembered that the light 

 of such a beetle might and probably 

 does have an entirely different effect 

 on the compound eye of insects than it 

 does on the simple eye of man. In all 

 study of the light of the Lampyridae, 

 this must be taken into consideration. 



USE OF THE LIGHT 



Manv suggestions have been made as 

 to ])ossil)le or i)robable uses to which 

 this light-emitting i)ower is jnit by the 



' The tropical fireflies fielong to another family, the Elateridae or cHck-beetlis. Pyrophnrus 

 noctilucns is the principal species. Its glow is continuous, not intermittent. 



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