25 
and less permeable than are the cells of the lungs formed of a 
thinner membrane. 
I have since found that air vessels of a similar structure may 
be detected in the Larvae of most insects, as well in those that are 
not exposed to any extraordinary temperature as those that are ; 
they are, therefore, not constructed with any view to these sin¬ 
gular situations. 
From the superior magnitude of the respiratory organs in most 
of the Larvae of insects, one should be almost led to imagine that 
the respiration in all animals was more intimately connected with 
the reception of food, and the converting it into living matter, 
than any other design. 
In corroboration of this we may observe, that while the respi¬ 
ratory organs are so large in the Larvae, they are remarkably 
small in the perfect insect, which also, in general, has occasion 
for very little food. 
Perhaps the superior size of the air vessels of the Bot, com¬ 
pared with the Larvae of other insects, arises from the greater 
rarefaction and impurity of the air it is exposed to in the sto¬ 
mach, which may render a larger portion of it necessary. 
Whilst upon this subject it may not be improper to notice the 
air vessels of the Larva of the Musca pendula , which are con¬ 
structed in a very different way from any others I have seen. 
The two principal trunks in this Larva are made up of semi¬ 
circular cartilaginous rings or fibres, which are disposed in a spi¬ 
ral direction, so as to form the tube. It is evident by this struc¬ 
ture, that the area of the tube may be entirely obliterated, and 
the sides be brought into contact. 
The convenience attending this structure, to a Larva living in 
putrid fluids of considerable depth, appears to be, that beside 
its use in respiration, it may serve the same office as the air blad¬ 
der in fishes, regulating by its contraction or expansion the den- 
G 
