424 LIFE : OUTLINES OF GENERAL BIOLOGY 



LuMiNESci-NCE IN Fresh Water.— It is usually stated that 

 luminescence never occurs in freshwater animals, but "never" is 

 a dangerous word. Some years ago there was a well-documented 

 account of a luminescent freshwater larva of one of the harlequin 

 flies (chironomi(ls) ; and K. G. Blair has recently published a de- 

 scription of the luminous aquatic larva of a Lampyrid beetle or 

 t'lre- fly, allied to Luciola. The larvje were collected in South Celebes 

 from a mountain stream at an elevation of about 4,000 feet. The 

 larv.-e were seen as luminous points on the stones at a depth of 

 about two feet, and are specially adapted for a life in streams. 

 But the point is that they were luminous, so that's that! 



Nature of the Light.— A body heated to a high temperature 

 may give off light-waves, and we call it incandescent. But bio- 

 luminescence is the very opposite; it is "cold light", without any 

 heat rays. It is altogether visible light, with no infra-red or ultra- 

 violet rays. As Langley and Very pointed out in their well-known 

 paper "On the Cheapest Form of Light" (1890), the luminescence 

 of the fire-fly is all light and no heat — hence cheapest, in the sense 

 that none of the energy is lost in non-visible radiations. The emission 

 of light by bodies after previous illumination or radiation is called 

 phosphorescence, but organic light is not of this nature; it is due 

 to the oxidation of some substance produced by cells; it is a bio- 

 chemical phenomenon. But it is important to recognise that organic 

 light behaves like light from ordinary sources. As Harvey says: 

 "Like ordinary light, animal light will also cause fluorescence and 

 phosphorescence of substances, affect a photographic plate, cause 

 marked heliotropism of plant seedlings, and stimulate the formation 

 of chlorophyll." 



Location of the Light.— In many luminescent animals the 

 oxidisable luminiferous material undergoes its combustion within 

 the living cells where it is produced. This intra- luminescence is illus- 

 trated by fire-flies, many small marine crustaceans, and Noctilucae. 

 It is also characteristic of all the luminous fishes .save one, which 

 will be described later on. 



In perhaps the majority of luminous animals, however, the 

 combustion of the luminiferous material occurs outside the body, 

 it may be on the surface, or in the water, or on the ground. The 

 material is exuded before it becomes luminous. This extra-lumin- 

 escence is well illustrated by Pholas, whose secretion becomes brilliant 

 outside the body; and as Dubois showed (1892) it may become 

 luminescent when re-moistened after being kept in a dry state for 

 several months. But this also holds true for the normally intra- 

 cellular luminiferous material of glow-worms (Lampyris), which 

 has been kept dry for ten months without losing its luminescent 

 property when moistened (Bongardt, 1903). In some of the luminous 

 Cojx'pods the secretion does not become luminous until it is mixed 



