22 COLOUR IN NATURE chap. 



which are placed near the ends of the fine tracheal 

 capillaries. The substance of the luminous organ 

 is made up of parenchyma cells, most easily perceived 

 in the transparent layer. The result of his examina- 

 tion convinced Emery that the luminous organ was 

 a specialised portion of the fatty body, the arrange- 

 ment of the tracheae being one of the most marked 

 indications of specialisation. 



His explanation of the luminosity is as follows: 

 the parenchyma cells (fatty cells) secrete the luminous 

 substance, from them it is taken up by the tracheal 

 end-cells, and, being here exposed to the action of 

 the oxygen of the tracheal system, undergoes a pro- 

 cess of oxidation resulting in the formation of light. 

 This process can only occur in regions where the 

 chitinous lining of the tracheae is very thin, as in the 

 capillaries ; the free branching of the slender tracheal 

 capillaries in the luminous organs especially provides 

 for this. In a later paper Emery is inclined to lay 

 more stress upon the parenchyma cells than upon the 

 tracheal end-cells as the seat of luminosity. The 

 luminosity is then due to the oxidation (or combus- 

 tion) of a probably useless body stored in the cells of 

 the organ (the fatty body). To this description we 

 may perhaps add that recent investigations tend to 

 emphasise the importance of the fatty body of insects 

 as an organ connected with excretion. 



As to the nature of the light emitted by the 

 different phosphorescent organisms, the observations 

 are not sufficiently numerous to draw any conclusions. 

 Moseley found that the blue and violet rays were 

 absent in the light of three deep-sea Alcyonarians, 

 while surface forms displayed light of various colours ; 



