STATES OF INSECTS. 289 



here occurs a difficulty. For the extrication of the imago 

 it is necessary that they should remain quietly suspended 

 at the surface ; and moreover that the thorax,*[in which 

 the opening for its exit is to be made, should be at least 

 level with it : and this is precisely what takes place. If 

 you watch one of these pupae when it ascends from the 

 bottom, you will see that as soon as it has reached the top 

 it remains suspended there motionless ; and that its tho- 

 rax is the highest part of the body, and level with the 

 surface. Now the question is, in what way this is accom- 

 plished ? How can a pupa of greater specific gravity than 

 water, remain suspended without motion at its surface? 

 and how can its thorax, which is at its heaviest end, be 

 kept uppermost ? — By a most singular and beautiful con- 

 trivance, which I shall explain ; the more particularly 

 because it has escaped Reaumur, and, as far as I know, 

 all other entomological observers. The middle of the 



back of the thorax has the property 'of repelling water 



apparently from being covered with some oily secretion. 

 Hence, as soon as the pupa has once forced this part of 

 its body above the surface, the water is seen to retreat 

 from it on all sides, leaving an oval space in the disk, 

 which is quite dry. Now though the specific gravity of 

 the pupa is greater than that of water, it is but so very 

 slightly greater, that the mere attraction of the air to the 

 dry part of the thorax, when once exposed to it, is suffi- 

 cient to retain it at the surface ; just as a small dry needle 

 swims under similar circumstances. That this is a true 

 solution of the phenomenon, I am convinced by the re- 

 sult of several experiments. If, when the pupa is sus- 

 pended at the surface, a drop of water be let fall upon 

 the dry portion of the thorax, it instantly sinks to the 

 vol. in. u 



