242 TROPICAL MEDICINE AND HYGIENE 



SUBDIVISION OF THE MORE IMPORTANT GROUPS 



OF THE DlPTERA. 



The diptera include carriers of disease and are charac- 

 terized by having a single pair of membranous wings, 

 the posterior pair being reduced to two knob-like 

 processes, the balancers or halteres. They may be 

 blood-sucking or not according to the structure of the 

 mouth parts. 



Of the groups into which the Diptera are divided the 

 Nematocera vera and the Cyclorrhapha schizophora are of 

 the most importance to us, as some of them are proved 

 to be carriers of important diseases. Nematocera 

 anomala has among its groups Simulidce or sand-flies, 

 and Sambon considers that the topographical evidence 

 is in favour of a Simulium being the vector of pellagra. 

 The wing venation of the group is simple ; they are 

 virulent blood-suckers, though the popular name "sand- 

 flies " includes many species belonging to other groups 

 than Simulium. 



Many of the others are blood-suckers, others are 

 important as mechanical carriers of bacillary diseases, 

 and others, both of the dipterous and other insects, 

 are important as destroyers of both larvae and adults of 

 dangerous species and in many cases do much injury to 

 growing crops, to fruit, grain, &c. 



Nematocera vera are subdivided mainly according to 

 their wing venation. This venation varies greatly in the 

 members of the group, and the family known as the 

 Culicidae are characterized by having scales on the veins 

 of the wings and by the forking of the 2nd, 4th and 

 5th longitudinal veins. These characters of the wings 

 separate the Culicidae from all other Nematocera vera. 

 The subdivision of the Culicidae is made on variations of 

 the mouth-parts into subfamilies, and at this point various 

 difficulties arise and other schemes have been proposed, 

 some based on larval characters and others on those of 

 the eggs. Each of these methods would lead in some 

 cases to quite a different grouping and in others would 



