OF AQUATIC LAEV^. 
85 
means of these trachea that the animal breathes, by taking the 
oxygen from tbe surrounding water and giving off the carbonic 
acid gas which has been produced by circulation. 
These plates are connected with the tracheal system of the 
body as follows. As in most other larvae there is one leading 
tracheal canal running down either side of the body ; this, on 
arriving at the last segment of the abdomen, splits in two, the 
lower half passing into the leaf-like plate on that side, and the 
upper half, together with the upper half of the other side, 
passing into the upper plate ; the third plate being supplied 
from the lower branch of that side. 
'J hus there is no real circulation of the air in the trachea ; 
diffusion probably being sufficient to renew the air as fast as it 
becomes vitiated. 
So here we have Agrion with three external breathing plates 
situated on the eleventh segment of the abdomen, and on the 
ninth there are five small pointed plates, three on the belly and 
one on either side, which, according to Burmeister, are the seat 
of the male organs of generation in the perfect insect. I shall 
call attention to these after speaking of some of the other allied 
larvee. 
In the case of Calo'pteryx the sac which I mentioned above 
also occurs ; and, on dissecting it, there will be found three 
plates, similar to those of Agrion, within it, affixed to the 
innermost extremity and surrounding the anal orifice. 
The water for aeration is admitted through the valve in the 
extremity of the abdomen, and is expelled by the flattening of 
the rings containing the sac. 
Thus we have Calopteryx with three plates situated internally, 
but in all other respects similar to those of Agrion; that is, 
they are fixed on the circle surrounding the anus, but the anus 
is internal. 
lAbellula has the breathing apparatus still more developed ; 
