288 STATES OF INSECTS. (Pupa.) 
its surface? and how can its thorax, which is at its 
heaviest end, be kept uppermost ?—By a most singular 
and beautiful contrivance, 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 re- 
pelling 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 ex- 
posed 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 suspended 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 again 
brought to the surface, so that the fluid is repelled from 
its disk, it remains suspended there without effort, as 
before. Just previously to the exclusion of the fly, the 
dry part of the thorax is seen to split in the middle. 
The air enters, and forms a brilliant stratum resembling 
quicksilver, between the body of the insect and its pupa- 
rium; andthe former pushing forth its head and fore- 
legs, like the gnat, rests the latter upon the water, and in 
a few seconds extricates itself wholly from its envelope. 
Before I close this letter, I must state a fact connected 
