2 6 
sity or rarity of the included air, and consequently the descent 
or ascent of the Larva in those fluids. 
These Bots, as is also the case with two or three other species, 
pass the autumn, winter, and spring months in the stomach, and 
arrive about the commencement or middle of the summer at their 
full growth, requiring a twelvemonth fully to complete their struc¬ 
ture. The slowness of their growth and the purity of their food 
must occasion what they receive in a given time to be proportion- 
ably small; from whence probably arises the extreme difficulty 
there is found in destroying them by any medicine or poison thrown 
into the stomach. After opium had been administered to a 
horse labouring under a case of locked jaw for a week, in doses 
of one ounce eA^ery day, on the death of the animal I have found 
the Bots in the stomach perfectly alive. Tobacco has been em¬ 
ployed in, much larger quantities in the same complaint, and has 
been also longer continued without destroying them. They are 
also but rarely affected by the drastic purgatives which bring 
away in abundance the Tenice and A scar ides. 
It is to be remarked, these Larvae probably never change the 
skin, which is very dense and tough, and becomes at last the 
shell of the Chrysalis ; and we may therefore discover upon the 
Chrysalis all the points and lineaments of the Larva dried up and 
diminished, in most other insects a cuticle separates from the skin 
at the time of this change. The small end of the Chrysalis, in all 
the species of this genus, contains the head of the Fly, the 
contrary being the case with almost all other insects. 
This species, though common with us in England in the Fly 
state, in the Chrysalis state is rare and difficult to procure ; and 
for this reason, that the Larva when full fed and grown to its 
size, escapes from the stomach and through the intestines, with¬ 
out making any stay or lodgment at the fundament, as the next 
species does, which affords us the more easy opportunity of pro- 
