188 NATURAL HISTORY 
must be extremely irksome and injurious to them. 
These are the hippobosce hirundines, with narrow 
subulated wings, abounding in every nest, and are 
hatched by the warmth of the bird’s own body 
during incubation, and crawl about under its feath- 
ers. 
A species of them is familiar to horsemen in the 
south of England, under the name of forest-fly, 
and, to some, of sidefly, from its running: sideways 
like a crab. It creeps under the tails and about 
the groins of horses, which, at their first coming 
out of the north, are rendered half frantic by the 
tickling sensation, while our own breed little re- 
gards them. 
The curious Reaumur discovered the large eggs, 
or rather pupae, of these flies, as big as the flies 
themselves, which he hatched in his own bosom. 
Any person that will take the trouble to examine 
the old nests of either species of swallows, may 
find in them the black shining cases or skins of the 
pupe@ of these insects; but for other particulars, 
too long for this place, we refer the reader to |’His- 
toire d’Insectes of that admirable entomologist, 
tom, iv., pl. 11. 
LETTER XVI. 
Selborne, Nov. 20, 1773. 
Dear Sir,—In obedience to your injunctions, I 
sit down to give you some account of the house. 
martin or martin; and, if my monography of this 
little domestic and familiar bird should happen to 
