I AQUATIC BEETLES 71 



possible that they can have any use at all, seeing that 

 they are possessed for a few days only by a pupa 

 buried in the earth, and that they are left behind when 

 the beetle emerges.' Such reasoning only carries 



FIG. 17. Pupa of Hydrophilus piceus, ventral view. X 2 



weight when it is employed by one who has a minute 

 and intimate knowledge of the structure of the animal, 

 and of all its external circumstances. Since neither 

 the questioner nor myself possesses this knowledge, 



