THE GLOWWORM 47 



the light is not really quenched ; the female merely turns 

 the luminous surface towards the ground. Even when the 

 abdomen is cut off, the light continues to be emitted for 

 a long time under favourable conditions for some days. 

 The luminous matter becomes brighter in oxygen, but is 

 quenched by carbonic acid, hydrogen, sulphurous acid or 

 a vacuum. The light is not accompanied by dark heat- 

 rays ; its photographic action is weak ; all the rays 

 belong to the part of the spectrum which is visible to the 

 human eye ; besides the light-rays Rontgen rays are 

 given off. Not only the abdomen of the adult female, 

 but the larva, the pupa, the adult male, and the eggs are 

 luminous at times. 



Microscopic examination shows that the luminous organ 

 of a glowworm consists of innumerable cells, arranged in 

 a dorsal layer, which is rendered opaque b}^ organic con- 

 cretions, and a ventral layer, which is clear. The cells 

 are supplied both with air-tubes, branching repeatedly so 

 as to reach every cell, and with nerves. 



There is reason to believe that the light is not directly 

 emitted by the cells of the luminous organ, but by a sub- 

 stance which these cells secrete. Bongardt found that 

 this substance, after being dried and kept in a vacuum 

 for twelve months, gave out light again when moistened 

 and placed in air. Fresh luminous organs can be made 

 to glow more intensely by irritation, e.g. with the point of 

 a needle, but the effect does not instantly follow the 

 stimulus. A glowworm when crushed emits no light. 

 Water, as well as oxygen or air, is necessary to the emis- 

 sion of light. It is probable that the light is due to an 

 oxidation-process, like that which makes phosphorus glow 

 in the dark, but the luminous matter of the glowworm 

 contains no phosphorus. 



We have only a single species of glowworm in Britain ; 

 two others occur in central Europe. The fire-fly of southern 

 Europe is a beetle belonging to the same family as the 

 glowworms, and several other luminous insects are known 



