282 THE WONDERS OF INSTINCT 



tion of the substance whereof this sheet is formed. The 

 first suggestion was phosphorus, in the chemist's sense of 

 the word. The Glow-worm was calcined and treated 

 with the violent reagents that bring the simple substances 

 to light ; but no one, so far as I know, has obtained a sat- 

 isfactory answer along these lines. Phosphorus seems to 

 play no part here, in spite of the name of phosphorescence 

 which is sometimes bestowed upon the Glow-worm's 

 gleam. The answer lies elsewhere, no one knows where. 



We are better-informed as regards another question. 

 Has the Glow-worm a free control of the light which 

 he emits? Can he turn it on or down or put it out as 

 he pleases? Has he an opaque screen which is drawn 

 over the flame at will, or is that flame always left ex- 

 posed? There is no need for any such mechanism: the 

 insect has something better for its revolving light. 



The thick air-tube supplying the light-producing sheet 

 increases the flow of air and the light is intensified; the 

 same tube, swayed by the animal's will, slackens or even 

 suspends the passage of air and the light grows fainter 

 or even goes out. It is, in short, the mechanism of a 

 lamp which is regulated by the access of air to the wick. 



Excitement can set the attendant air-duct in motion. 

 We must here distinguish between two cases: that of 

 the gorgeous scarves, the exclusive ornament of the fe- 

 male ripe for matrimony, and that of the modest fairy- 

 lamp on the last segment, which both sexes kindle at 

 any age. On the second case, the extinction caused 

 by a flurry is sudden and complete, or nearly so. In 

 my nocturnal hunts for young Glow-worms, measuring 



