I THE COLOURS OF ORGANISMS 21 



We cannot here enter into a description of the 

 phenomena of phosphorescence as they appear in all 

 luminous organisms, but may conveniently add to 

 the descriptions already given, an account of the 

 process in terrestrial forms. For this purpose we 

 may take Prof. C. Emery's account of the phosphor- 

 escence of Luciola italica, the Italian glow-insect. 

 Here in the male the luminous organs are placed on 

 the two last abdominal segments, each one appearing 

 as a continuous surface although originally formed in 

 two halves. In the female there are two luminous 

 spots at the sides of the ventral surface of the fifth 

 abdominal segment, the following two (the last) ab- 

 dominal segments appear in dried specimens of the 

 same pale colour as the fifth but are not luminous. 

 The luminous organs consist each of a double layer, 

 a ventral transparent one and a more dorsal chalk- 

 white one containing masses of urates. Some of 

 the large tracheal tubes run along the inner surface 

 of the dorsal plate, and give ofT perpendicular 

 branches which pass downwards and penetrate into 

 the substance of the transparent layer, where they 

 branch very freely, and come to lie close beneath 

 the skin. 



A surface view of the luminous organ shows a 

 series of round or oval spots corresponding to little 

 masses of transparent cells surrounding the termina- 

 tion of these tracheal branches ; it is these spots 

 which are luminous during life. They turn brown 

 when treated with osmic acid, and are separated by 

 broad darker intervals. More careful examination 

 shows that these spots correspond to little cylinders 

 formed by clusters of cells — the tracheal end-cells — 



