THE ORDER DIPTERA 33 



skin, to set up local inflammation which issues in boils and 

 abscesses. 



Most of these dangers from common flies are, of course, 

 most imminent among an ignorant and apathetic population ; 

 in slums, and particularly in crowded bazaars where food, 

 offal, and a multitude of afflicted mendicants — in fact, every- 

 thing that is most attractive to flies — are to be found side 

 by side : such dangers must also be borne in mind in 

 entrenchments and camps, where close observance of the 

 conveniences of civilised life is not always easy. These 

 matters will be more carefully treated when we come to the 

 species of flies which are known, or believed, to be particularly 

 noxious. 



It is interesting to remember that the fly-danger seems 

 to have been suspected in the days of Chaucer, as appears 

 in our host's chaff of gentil Roger Coke, of London, in the 

 Canterbury Tales: — 



" Of many a pilgrim hastow Cristes curs. 

 For of thy persly yet they fare the wors, 

 That they han eten with thy stubbel-goos ; 

 For in thy shoppe is many a flye loos." 



As regards blood-sucking flies some of these, as is now 

 well known, are also aggressively dangerous as playing a 

 definite part in the maintenance of certain species of 

 pathogenic micro-organisms, among which the most impor- 

 tant, so far as man himself is concerned, are the 

 haemamcebae of malarial fevers, the trypanosome of 

 sleeping-sickness, certain filarial worm-parasites, and the not 

 yet recognised agent of yellow fever. Blood-sucking flies 

 hardly affect man, though sometimes they may seriously 

 afflict animals, by mere abstraction of blood ; but they are 

 responsible for a certain amount of harm by the pain and 

 irritation of their " bites," and by subsequent (septic) aggrava- 

 tion of the bites by scratching. 



External Structure of Diptera. 



The head IS commonly hemispherical, the posterior surface 

 or occiput, being nearly flat. At its sides are the compound 

 eyes, which with few exceptions are large, encroaching on the 

 vertex, and in males sometimes meeting across the crown. 



C 



