36 Charles Darwin. 



laborious operation. The forest afforded constant 

 opportunities for observing the wonders of phospho- 

 rescent insects, the brilliant fire-fly (Lampyris occiden- 

 talis) illumining the darkest recesses with its light. 

 Darwin's investigations with these insects showed 

 that the light was most intense when they were irri- 

 tated ; where the skin was injured the light was 

 bright, while other portions gave no evidence of 

 phosphorescence. Decapitating one he found that 

 the light was still uninterrupted though not as bril- 

 liant, and in a specimen which he killed the strange 

 light gleamed for an entire day. 



He collected the various forms of these insects, the 

 larvae and adults, and made a series of experiments 

 that have ever been of value to those who have made 

 them a study. The larvae he kept alive by feeding 

 them upon raw meat, and while watching them, he 

 learned that they used their tails as suckingorgans,and 

 that the latter contained saliva glands which were 

 employed in a singular manner. Thus when the larva 

 felt the pangs of hunger, it reached its tail over and 

 deposited upon its mouth, or the food it was prepar- 

 ing to eat, a drop of saliva. 



The elater, or PyropJiorus luminosus, was the most 

 conspicuous luminous insect here, abounding in vast 

 numbers and dashing through the gloomy recesses of 

 the forest, often followed by an apparent blaze of light. 



Darwin discovered during this expedition the true 

 explanation of the leaping powers of the elater, 

 finding that it was due to the elasticity of the spine, 

 which threw the insect whirling into the air like a 

 veritable spring. 



