THE MOTH AND THE CANDLE 233 



phosphorescence) is a process differing greatly from that 

 called " phosphorescence " in minerals and crystals, such 

 as the emission of light by a lump of white loaf-sugar 

 when crushed. You may see that kind of phosphor- 

 escence by standing in front of a looking-glass in a dark 

 room and crushing a lump of loaf-sugar with the teeth, 

 keeping the lips raised. It seems that in many organisms 

 luminescence occurs without any consequent use or ser- 

 vice to the organism. But in higher forms the power of 

 emitting light has been seized upon by natural selection 

 having become of value in attracting the individuals of a 

 species to one another, or in attracting prey, or again in 

 scaring enemies. The luminescent matter is concentrated 

 in certain definite organs, and the access to it of oxygen 

 and even its formation are controlled by the nervous 

 system. 



Among insects far better known than the rare lumin- 

 escent May-flies, are the glow-worms, a family of beetles 

 of which several species are known besides our own 

 familiar one, called Lampyris noctiluca. The fire-flies 

 of Southern Europe — Luciola italica — are small beetles 

 allied to the glow-worm, but both sexes fly and both are 

 luminous, whilst in the common glow-worm the female 

 is wingless, and the flying male, who is guided to the 

 female by her light (which she can vary in intensity), 

 gives but a feeble light. The swarms of Italian fire-flies 

 consist of as many as a hundred males to one female, 

 and the males are far more brilliant than the females. 

 My fellow-student Moseley showed some in oxygen gas 

 at the Royal Society's soiree many years ago. The 

 gas greatly increased their brilliancy. Many valuable 

 experiments in search of an explanation of the brilliance 

 of the male Luciols and their excess in number could be 

 carried out in North Italy. A peculiar grub-like female 



