479 



the two known to be new having been described under the 

 names of venustoides and johannseni. 



As the State of lUinois extends, from north to south, through 

 five and a half degrees of latitude, there is some difference betw'een 

 its most northern and its most southern districts in respect to the 

 predominant species of Simulium ; but as all have similar habits, 

 and all but one of them are active biters, this fact probably 

 counts for little in the present discussion. 



There is some difference also as to the kinds of waters in 

 which the several species prefer to breed, some of them living 

 mainly in the larger rivers, and others occurring onl\- in the 

 smaller streams ; but as the State is well watered in all its 

 parts, and is virtually a level plain, there is no part of it which 

 is wholly beyond the reach of some species of Simulium. It is 

 true that these insects arc rarely seen in some places, and are 

 an annoying nuisance, and indeed a destructive pest in others, 

 especially along the larger rivers in spring ; but since we have 

 found them in considerable numbers at a distance of more than 

 five English miles from the nearest water in which they could 

 have bred, and since there is scarcely a small stream anywhere 

 in some part of which Simulium larvie cannot be found through- 

 out the spring and summer, even temporary roadside drainage 

 ditches often containing them during the spring season of high 

 water, there must be few people in the State who are not at some 

 time exposed to the attacks of the flies. Simulium is, in fact, 

 more completely and uniformly distributed in Illinois than 

 Anopheles, and as there is no part of the State wholly and per- 

 manently free from malarial disease, there would seem to be no 

 part of it free from danger of pellagra, if this is really trans- 

 mitted by black-flies. 



The contrast is marked between these Illinois conditions 

 and those in Italy, where Sambon and his colleagues studied 

 the problem of pellagra and the distribution of the black-fly. 

 There mountain heights, mountain valleys, and level plains make 

 up a diversified topography and hvdrography, and the distribu- 

 tion of Simulium is similarly diversified. It is one of the main 

 lines of Sambon's argument that the distribution of pellagra is 

 limited by the distribution of Simulium. although not co-exten- 

 sive with it. This test cannot be verified in Illinois, however, 



