366 Description of ihe Bird Fly. {[May, 
the theory be found to hold in all cases, modified perhaps by the 
nature of the ground, and other circumstances, will not some cor- 
rection be necessary in determining the temperature of any place 
from a single observation of spring water, unless when that obser- 
vation has been made about the middle of winter or the middle of 
summer ? 
Rose-Bank, March 9, 1816. 
ArticLe VI. 
Deseription of the Bird Fly. By M. W. Carolan. 
(To Dr. Thomson.) 
SIR, 
Tue history of those insects which infest other animals is a 
curious subject of investigation, and includes no small branch of 
natural history. Amongst the varieties of this description, there 
are few which exhibit a more striking fitness for their situation than 
the fly which lives upon some birds, and may be often observed upon 
the partridge and swallow. ‘This fly, like most others, seems to live 
principally upon animal perspiration, or any thing in a putrescent 
state ; but as it could not get at the cuticular pores of birds without 
going through the feathers, it is exactly formed for that purpose. 
It is rather larger than the common fly, of a clear tea-green 
colour, and is seldom seen but upon birds. It is quite flat, so that 
both its body, head, and Jegs. apply closely to any plain surface on 
which it may rest. It is very hard, and not easily crushed or killed. 
Its legs are very strong, and it can move in all directions; and, 
what is curious, runs with most rapidity sidewise, and does not seem 
to run easily straight forward. Its flatness, strength, and polished 
smoothness, without any hairs upon its body, enable it to move © 
with ease among the feathers, and particularly its capacity of 
running sidewise, which gives it the power of going round the body 
of the bird beneath the feathers ; for the feathers of birds are so 
placed in transverse rings that no insect, except it were almest in- 
visible, could go straight forward under them. It sometimes appears 
‘above the plumage to enter in at a new place, which it performs 
with great ease and quickness, and without discomposing a single 
reed of the feathers, 
These may-seem necessary to preserve the health of some animals, 
by continually removing the perspiration that would otherwise be 
lodged about the feathers; and may perbaps act at the same time as 
a kind of stimulant to the skin. Although there are seldom above 
two or three on a small bird, yet from their size they must remove 
a great deal of what is excreted by the skin, as they appear seldom _ 
° 
