kind, or of the dipterous order of insects, there a$e tvro dis¬ 
tinct plates at this obtuse end, as may be seen in the common 
maggot of the house fly and the blow fly. Keller, however, sup¬ 
poses, and it is generally apprehended, and perhaps truly, that 
the air is not received at this large end, but at the extremities or 
terminations of these branches in the skin, foiming on the sides, 
what are called the spiracuZa. 
The lips at the obtuse end of the Bot seem designed to conceal 
and cover this horny plate, and defend it from the liquors of the 
stomach, or clean and clear it of these gastric fluids or other 
matters when necessary. In the Larva of the CEs. Ovis, or 
Sheep Bot, this respiratory plate is not covered with lips in this 
way, but is surrounded by a prominent margin or extension of 
the skin, which closes over the plate, at the will of the animal, 
and on opening again suddenly, is often attended with a distinct 
snap, that may be heard to some distance. These lips as we 
have stated appear to be mere membranous bags, and filled 
with lymph only, which renders it probable that they are solely 
for the convenience of the Larva, and form no part in the future 
Insect, as are probably nearly or quite all the parts connected 
with the skin. 
Respiration appears to be the office of these air canals, which 
are the lungs of the Larva ; and, considered in this point of view, 
they are much larger than the respiratory organs of any other 
animal: which is the more extraordinary, if the purpose of respi¬ 
ration in animals be the production of animal heat, as the later 
chemists suppose, this being altogether unnecessary to Larvae that 
are supplied so abundantly with it from the high temperature of 
their residence in the living stomach ; nor can these organs be 
formed for the purposes of the future insect, since they cannot 
be detected in either the Chrysalis or Fly. The permeability of 
these respiratory vessels to oxygen gas, is also to be taken into 
the account. They would appear to be in these animals stouter 
