152 NATURAL HISTORY OF SELBORNE. 
our houses ; amuse us with their migrations, songs, and marvel- 
lous agility ; and clear our outlets from the annoyances of gnats 
and other troublesome insects. Some districts in the South Seas, 
near Guaiaquil,* are desolated, it seems, by the infinite swarms of 
venomous mosquitoes, which fill the air, and render those coasts 
insupportable. It would be worth enquiring whether any species 
of hirundines is found in those regions. Whoever contemplates 
the myriads of insects that sport in the sun-beams of a summer 
evening in this country, will soon be convinced to what a degree 
our atmosphere would be choked with them were it not for the 
friendly interposition of the swallow tribe. 
Many species of birds have their particular lice ; but the hi- 
rundines alone seem to be annoyed with dipterous insects, which 
infest every species, and are so large, in proportion to themselves, 
that they must be extremely irksome and injurious to them. 
These are the hippobosccB hirundinis, 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 
feathers, t 
A species of them is familiar to horsemen in the south of Eng- 
land under the name of forest-fly ; and to some of side-fly, 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 regards them. 
The curious Reaumur discovered the large eggs, or rsithex pupcB^ 
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 of the pupcB of these insects : but for other 
particulars, too long for this place, we refer the reader to VHiS" 
toire d'Insectes of that admirable entomologist. Tom. iv. pi. 11. 
LETTER XVI. To the Hon. DAINES BARRINGTON. 
DEAR SIR, Selhorne, Nov. 20, 1773. 
In obedience to your injunctions I sit down to give you some 
account of the house-martin, or martlet ; and, if my monography 
* See UUoa's Travels. 
t The H. urbica is, 1 think, the most annoyed by them. — En. 
