OF SELBOENE 127 



collect every ray of light, and large concave ears to command 

 the smallest degree of sound or noise. 



I am, etc. 



It will be proper to premise here that the sixteenth, eighteenth, twentieth and 

 twenty-first letters have been published already in the Philosophical Transactions ; 

 but as nicer observation has furnished several corrections and additions, it is hoped 

 that the republication of them will not give offence ; especially as these sheets would 

 be very imperfect without them, and as they will be new to many readers who had 

 no opportunity of seeing them when they made their first appearance. 



The hirundines are a most inoffensive, harmless, entertaining, 

 social, and useful tribe of birds : they touch no fruit in our gardens ; 

 delight, all except one species, in attaching themselves to our 

 houses ; amuse us with their migrations, songs, and marvellous 

 agility ; and clear our outlets from the annoyances of gnats and 

 other troublesome insects. Some districts in the south seas, near 

 Guiaquil,^ are desolated, it seems, by the infinite swarms of 

 venomous mosquitoes, which fill the air, and render those coasts 

 insupportable. It would be worth inquiring whether any species 

 of hirundines is 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 choaked 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 amioyed 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 hippoboscae 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-Jly ; ^ and to some of side-jly, 

 from it's 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 sen- 

 sation ; while our own breed little regards them. 



1 See Ullaa's Travels. 



2 [Dipterous insects infest various other birds, each as a rule frequenting one 

 particular host ; others are parasitic upon quadrupeds (sheep, horse, deer, bats) ; 

 or upon bees,] 



2 [Hiffobosca equina, L.] 



