46o 
GAD-FLY. 
position of their bodies without any excrementitious 
residue, though on dissection the intestine is found 
to contain a yellow or greenish matter, which is 
derived from the colour of the food, and shows that 
the chyle, as they receive it, is not perfectly pure. 
They attain their full growth about the latter end 
of May, and are coming from the horse from this 
time to the latter end of June, or sometimes later. 
On dropping to the ground they find out some con- 
venient retreat, and change to the chrysalis ; and in 
about six or seven weeks the fly appears. 
The perfect fly but ill sustains the changes of 
weather ; and cold and moisture, in any considera- 
ble degree, would probably be fatal to it. These 
flies never pursue the horse into the water. This 
aversion I imagine arises from the chillness of that 
element, which is probably felt more exquisitely by 
them, from the high temperature they had been 
exposed to during their larva state. The heat of the 
stomach of the horse is much greater than that of 
the warmest climate, being about 102 degrees of 
Fahrenheit, and in their fly state they are only ex- 
posed to 6 0, and from that to about 80 degrees. 
This change, if suddenly applied, would in all pro- 
bability be fatal to them ; but they are prepared 
for it, by suffering its first effects in the quiescent 
and less sensible state of a chrysalis. I have often 
seen this fly during the night-time, and in cold 
weather, fold itself up with the head and tail nearly 
in contact, and lying apparently in a torpid state, 
through the middle of the summer.” 
