iS6 'INFINITE TORMENT OF FLIES' 



ophthalmia, and of the ' sore-eye,' so common in 

 Florida, from one human being to another. 



The diseases already mentioned are caused by 

 bacteria. But flies also play a part in the conveyance 

 of a large number of organisms which are not bacteria, 

 but which, nevertheless, cause disease, and cause it 

 on the largest scale. 



Of all the twenty-two orders into which the modern 

 entomologist divides the class Insecta, that of the Dip- 

 tera, or true flies, is, perhaps, the easiest to recognize, 

 for it is characterized by one very obvious feature, the 

 presence of the fore-wings only. The hind-wings are 

 replaced by a pair of small - stalked, club - shaped 

 ' balancers,' which are readily visible in some kinds 

 of fly — e.g., the daddy-long-legs — but in others are by 

 no means conspicuous. Thus it is an easy matter to 

 determine whether an insect be a fly or not. To 

 determine what particular kind of fly it be is, however, 

 a very different affair. At present some forty thousand 

 species of Diptera are known, and have been more or 

 less completely described or figured ; and Mr. D. 

 Sharp estimates that this number is ' only a tithe of 

 what are still unknown to science.' Further, the group 

 has been rather neglected. Flies, speaking generally, 

 are neither attractive in their appearance nor engaging 

 in their habits, and it is a cause for no astonishment 

 that entomologists have preferred to work at other 

 groups. 



In considering the part played by flies in dissem- 

 inating diseases not caused by bacteria, we can neglect 

 all but a very few families, those flies which suck 

 blood having alone any interest in this connexion. 



From the point of view of the physician, by far the 

 most important of these families is the Culicidae, with 

 over three hundred described species and five sub- 

 families, of which two, the Culicina and the Anophelina, 

 interest us in relation to disease. The gnats or 



