222 A NATURALIST IN HIMALAYA 



show its light, until in a short time all were glowing 

 brightly and creeping in every direction over the floor 

 of the box. Also when I placed a box of glow-worms 

 in a shower of rain many of them became phosphor- 

 escent even before sundown. It is not essential to 

 moisten the luminary organs, for a drop of moisture 

 on the head of the insect produces a similar effect. 

 The fluid has therefore a retlex action. The moisture 

 increases the muscular activity of the glow-worm, so 

 that it might be thought to excite luminosity in the 

 same way as gently stroking the insect's back. But 

 this is not so. Irritating the glow-worm by blowing 

 at it or stroking it never produces the same intensity 

 of glow as does the presence of a drop of moisture. 

 Contact with moisture has a special power in develop- 

 ing the luminosity of these insects. It is not alone 

 water that excites the light. A drop of spirit has a 

 similar effect, and none shine more vividly than those 

 which are enveloped in the moist and viscid mucus 

 thrust over them by snails. Moisture of any kind is 

 the chief stimulus to luminous activity. 



At night it is not the influence of darkness that calls 

 forth the light, but rather the profuse dew that covers 

 all the ground. The insects which I kept in absolute 

 darkness by day showed no light. Nor is it the case 

 that the advent of darkness coincides with the appear- 

 ance of the glow-worm's light. Luminosity does not 

 appear until an hour or more after sundown, when the 

 earth has sufficiently cooled to permit the deposit of 

 a layer of dew. In all likelihood the luminous activity 

 of the glow-worm is under a rhythmical sway, appearing 

 by night and disappearing by day through the regular 

 pulsations of an internal nervous mechanism. But 



