ox GAD FLY. 291> 
It has also been supposed, that by the 
buoyancy of these little bladders, which 
the fly can dilate or contract at pleasure, 
it is enabled to rise, sink, and move freely 
through the air ; somewhat in the same 
way as the air vessels of birds and fishes 
facilitate their movements. 
Lucy, Here is a Harry-Longlegs in 
the window ; I see the halteres plainly ; 
they are quite close to the head of the 
wings. 
Mother Yes, they are very distinct in 
that kind of fly; but in some species 
they are but just perceptible. 
The first genus in diptera is the oestrus, 
or gudjiy, 
Lucy, Is not that the fly which some- 
times sets horses mad ? 
Mother, It is, however, often falsely ac- 
cused, for there are many other flies,, 
which torment them terribly. One spe-- 
cies, the oestrus hovis*^ attacks our cows. 
This fly is as large as a drone bee, and 
* Bos, Boiisy an ox. Plate XXL . 
