490 THE WONDER OF LIFE 



reports of luminous birds are due to luminescent fungi 

 on the plumage. 



In a case like the luminescence of Bacteria, no one even 

 looks for a utilitarian explanation. The luminescence is a 

 by-play of vitality ; it is one of the residual powers of the 

 organism ; and it is probably exhibited in many cases in 

 which we do not and cannot see it. What we need to know 

 is (1) the internal physiology of luminescence, and (2) 

 what its use may be in certain cases where it bears the marks 

 of specialization. 



The luminescence is in some cases indissolubly connected 

 with the cellular metabohsm — as in Noctiluca, Brittle-stars, 

 and some fishes. When they die their hght goes out. 

 In other cases the luminescent material is not luminescent 

 until it is exuded from its producer into the water, as in 

 many Copepods. The light does not require contact with 

 hfe to keep it shining. The trail of the luminous Myriopod 

 is luminous, and in some cases (Copepods, Lampyrids, 

 and Pholads) the luminous secretion can be dried and 

 yet retain its capacity of giving forth hght when it is put 

 into water after several days, weeks, or months. 



In the American Lampyrid beetles, popularly called fire- 

 flies, the hght-producing organ, as described by McDermott 

 and Crane, consists of two layers. The inner one, white 

 and opaque, seems to serve as a reflector, and perhaps 

 protects the insect from its own brightness. The outer 

 one, yellowish and translucent, is the seat of the actual 

 photogenic process. It is interesting to know that innu- 

 merable air-tubes or tracheae penetrate the organ, for 

 this bears out the conclusion otherwise arrived at, that 

 the luminescence is due to an oxidation. 



In the American fire-fly both sexes are luminescent, the 



