50 
genus. And this would seem to render it manifest, that through 
these plates they receive the air, and emit it by the spiracula. 
At the lower end of the larva, fig. 3. a small indentation may, 
with attention, be observed, which is the mouth of the larva. It 
is a simple aperture, and altogether unprovided with any of the 
apparatus belonging to the mouths of larvrn in general; and near 
the mouth are seen two black points of horn, which appear to be 
perforated in the centre, and are found by dissection to be the 
termination of two considerable branches of the air tubes, and 
correspond to the two nipples on the first segment of the larva of 
the CE. Equi. An enlarged view of the mouth and inferior part 
of the larva of the CE. Bovis is seen in fig. 4. Bound the orifice 
of the mouth are placed some projecting mamillae, which are 
imperforate, and perhaps serve the purpose of feelers. 
The intestinal canal in this larva is a simple membranous tube, 
which extends from one extremity to the other, and serves the 
double purpose of stomach and intestine. 
In the larva of the Oestrus Bovis there are only two principal 
trunks of air vessels, which are connected near their origin by a 
lateral trunk. From these trunks branches are seen passing in 
every direction through the parenchyma or matter of the body 
of the insect, some going to the intestine, others to the skin, and 
a greater number appear to be forming an anastomosis with each 
other. 
When young the larva is smooth, white, and transparent ; as 
it enlarges it becomes browner ; and about the time it is full 
grown it is totally of a deep brown colour. 
The larva having attained its full growth and size, effects its 
escape from the abscess by pressing against the external opening, 
which occasions its enlargement by the points pressed upon being 
absorbed, and the skin also may give way and extend itself under 
their impression. When the opening has obtained the size of a 
