48o 



as Simulium is generally distributed. Pellagra, on the other 

 hand, is intensely local, so far as is now known ; but to this 

 interesting point I shall presently return. 



The life histories of the American species of Simulium are 

 very imperfectly known, and the same may be said of those of 

 all other parts of the world as well. No species, in fact, has 

 been carefully followed, in its development, around the year, 

 and on only two of our American black-flies, venustum and 

 pidipcs, has any kind of definite life-history work hitherto been 

 done. Probably studies of this sort are now in progress in other 

 places than Illinois, but if so their results have not yet been 

 made known. In our own State we have gone far enough with 

 this phase of our problem to make sure that six of our species, 

 and possibly all of them, produce two or more generations in a 

 season, and that there is a sufficient variation among the different 

 species in respect to the times at which the successive generations 

 emerge to make it certain" that some Simulium species may be 

 producing adults at every time of any average year, from early 

 April to late October. We have, in fact, ourselves collected 

 adults of one or more species, and have bred others, in each of 

 these seven months, but much more frequently in April, May, 

 and June than in any later ones. 



The actual number of individuals on the wing, indeed, 

 diminishes rapidly after the main spring outburst, so that it is 

 usually difficult to find an adult Simulium in August or Sep- 

 tember, even in places made almost uninhabitable by thein 

 in April and May. This may be due in part to unknown features 

 of the life history of two of the most prolific species, pecuarum 

 and méridionale, but it is certainly due also, at least in part, 

 to a summer shrinkage of the streams and a consequent reduction 

 in the number of suitable places for the breeding of these dis- 

 criminating insects. Whatever is the explanation, the fact 

 itself is notorious, and it is of especial interest to our inquiry ; 

 for if Simulium transmits pellagra, there should be, generally 

 speaking, some seasonal correspondence observable between 

 this highly unequal abundance of the insect carriers of the disease 

 and the number of new cases occurring. 



There is, indeed, a very notable seasonal periodicity shown 

 in Illinois in respect to the number of new cases of pellagra, but 



