466 
FLY. 
lar conformation, and is in fact the instrument 
through which the creature respires. The surface 
of the water which these insects inhabit appears to 
be in perpetual motion, where there are many of 
them near together; and as they are generally found 
in muddy places, it becomes necessary to remove 
them from hence, in order to observe their movements 
in a satisfactory manner. Their bodies are about half 
an inch long, and their tails an inch. If some of 
them are put into a glass vessel of clear water, filled in 
proportion to their measure, they will soon become 
settled with their heads at the bottom, and the end 
of their tails just above the surface. In this there 
is nothing particular or extraordinary ; but upon 
adding more water a wonderful contrivance is mani- 
fested ; the tail is immediately lengthened in pro- 
portion to the height of the water in the vessels, 
and proves to be a mere tube, containing within it 
another much smaller, which the animal is enabled 
to shoot out whenever it has occasion to seek a 
greater depth. In order to determine how far these 
insects could extend their tails, Reaumur collected 
a few of them, which he placed in a proper vessel, 
and found that upon adding water gradually the 
tails still kept above the surface, till the depth 
amounted to between five and six inches. Here 
their extensive power seemed to cease, and they 
were obliged, upon more water being added, to leave 
the bottom, and either keep higher in the water, or 
attach themselves to the sides of the vessel, so that 
the extremity of their tubular tails might still be 
