SCIENCE 



[N. S. Vol. XXXVII. No. 942 



There mountain heights, mountain valleys 

 and level plains make up a diversified topog- 

 raphy and hydrography, and the distribution 

 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- 

 extensive with it. This test can not be veri- 

 fied in Illinois, however, as Simulium is gen- 

 erally 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. 'So species, in 

 fact, has been carefully followed, in its devel- 

 opment, around the year, and on only two of 

 our American black-flies, venustum and pic- 

 tipes, 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 

 sufiicient variation among the difl'erent spe- 

 cies in respect to the times at which the suc- 

 cessive 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 spe- 

 cies, 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 

 difiicult to find an adult Simulium in August 

 or September, even in places made almost un- 

 inhabitable by them 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 m,eridionale, but it is 

 certainly due also, at least in part, to summer 



shrinkage of the streams and a consequent re- 

 duction in the number of suitable places for 

 the breeding of these discriminating 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 sea- 

 sonal correspondence observable between this 

 highly unequal abundance of the insect car- 

 riers 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 it is not 

 of the kind anticipated by this reasoning. 

 My attention was first called to the facts last 

 December by Dr. H. Douglas Singer, director 

 of the State Psychopathic Institute, at Kan- 

 kakee. In the Peoria hospital, where the 

 largest number of our new cases have occurred, 

 statistical data were obtainable from July 1, 

 1909, to September 1, 1911, and the curve 

 showing the frequency of new cases in this 

 hospital presents five notably high points, each 

 the culmination of a wave of increase, in the 

 period of two years and two months which it 

 represents. In the first of these two waves 

 the twenty-one new eases of July are followed 

 by seventy-one in August, and this maximum 

 by thirty-seven, twenty-three, twelve and three 

 for the months of September, October, No- 

 vember and December, respectively. In Jan- 

 uary, 1910, there was but one new case; in 

 February and March there were none; in 

 April there was one ; and with this a new wave 

 started, reaching thirty-four new eases in 

 June, dropping to but four in July, and rising 

 in a second, lower wave of sixteen and fifteen 

 in August and September, respectively, drop- 

 ping thence to one in October and none at all 

 until February of the following year. 



The largest number of new cases occurring 

 in 1911 was only seven, in August, the next 

 largest number coming in May, when there 

 were six, and the two crests of these waves 

 being separated by the low period of June and 

 July, with one and three cases, respectively. 

 In a word, the two annual high points come 

 in either May or June of two of these years, 



