MODES OF EMERGING FROM PUPJE. 319 



spent the first stages of its existence. c It leaves, 

 says Swammerdam, c its cast skin on the water 

 where it insensibly decays.'* Reaumur doubts 

 whether Swammerdam ever actually saw this inte- 

 resting transformation. We have seen it twice only. 

 The beautiful pupa formed from the blood worm, 

 as before described, proceeds in its transformation 

 much in the same way as the common gnat. But 

 how, it may be asked, can the insect raise its 

 shoulders above the surface of the water, than which 

 it is specifically heavier, and suspend itself there 

 without motion ? ' By a most singular and beau- 

 tiful contrivance, which,' says Kirby, c I shall ex- 

 plain, the more particularly because it has escaped 

 Reaumur, and, as far as I know, all other entomo- 

 logical observers. The middle of the back of the 

 thorax has the property of repelling water, appa- 

 rently 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 sufficient 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 result 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 bottom,! the thorax, which belongs to the 

 heaviest half, being the lowest; and if the pupa be 



* Part i, p. 156. 



t But, if so, we may ask what has become of the power of 

 the thorax to repel water ? J. R. 



