AIR-CELL UNDER WATER. 245 



In this inner cocoon there is not so much as a 

 single drop of water ! The pupa could not exist 

 in it if it were not 

 full of air; and the 

 difficulty is how to 

 fill a little cell with 

 air which is already 

 buried many inches 

 under, and on every Pu pa and perfect insect. 



side surrounded by, water. De Geer states that he 

 did not find a particle of water in all the cocoons 

 which lie opened. In order to ascertain whether or 

 not it was really necessary for the pupa to be thus 

 carefully housed in a silken ball full of air beneath 

 the water, he took out several pupae and put them 

 into some water in a saucer. At first they swam, 

 but presently they sank to the bottom. They 

 lived for several days, but ultimately every one 

 died, showing plainly that the air-cell was abso- 

 lutely necessary to their existence. 



Yet, although these pupae were air-breathing 

 creatures, strange to say, De Geer found that if 

 taken from their little cells out of the water, they 

 shrivelled up and died. He put several in a dry 

 box, and invariably found that in a short time they 



