196 BRITISH APHIDES. 



not tried. I am told that these crystals answer to the 

 ordinary tests for salicine. 



The elaboration, or even the concentration of vege- 

 table alkaloids in the organs of insects is very interest- 

 ing. Chrysomela populi is known to leave a track of 

 salicylic acid behind it when irritated. 



The botanical genera of Salix and Popnlus contain 

 this bitter principle in their barks, and they are the 

 chief sources for its commercial preparation, valuable 

 as a febrifuge. 



