found in those regions. Whoever
contemplates the myriads of insects that sport in the sunbeams of a
summer evening in this country, will soon be convinced to what a degree
our atmosphere would be choked with them was it not for the friendly
interposition of the swallow tribe.

"Many species of birds have their peculiar lice; but the hirundines 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 _hippoboscoe
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.

"A species of them is familiar to horsemen in the south of England 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 rather _pupoe_, 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 _pupoe_ of these insects; but for other particulars, too
long for this place, we refer the reader to 'L'Histoire d'Insectes' of
that admirable entomologist. Tom. iv., pl. ii."

LETTER XVI.

SELBORNE, _Nov._ 20_th_, 1773.

Dear Sir,--In obedience to your injunctions I sit down to give you some
account of the house-martin, or martlet; and if my monography of this
little domestic and familiar bird should happen to meet with your
approbation, I may probably soon extend my inquiries to the rest of the
British hirundines--the swallow, the swift, and the bank-martin.

A few house-martins begin to appear about the 16th April; usually some
few days later than the swallow. For some time after they appear the
hirundines in general pay no attention to the business of nidification,
but play and sport about, either to recruit from the fatigue of their
journey, if they do migrate at all, or else that their blood may recover
its true tone and texture after it has been so long benumbed by the
severities of winter. About the middle of May, if the we