B24 
Class IX, Sink-holes—The last permanent type is composed of breed- 
ing-places caused by the sinking of the ground over abandoned coal mines 
after the props have rotted down. These sink-holes fill with water, and 
grass and cattails spring up around the edges and sometimes in the center 
of the pond. Fig. 14 shows one of them. Fig. 15 was taken during the 
drouth of midsummer, 1919, when the water was everywhere very low. 
In many sink-holes the water-line had receded so far from the cattail 
growth that no protection was left for the mosquito larvae. As a conse- 
quence of this and of the concentration of predaceous insects and ani- 
mals, there were practically no mosquito larvae in such situations. The 
significance of this fact will be brought out later under a discussion of 
control measures. The accompanying map shows the remarkably large 
number of these sink-holes (eighty in all) just north of Murphysboro. A 
considerable number are to be found in the vicinity of the Mobile and 
Ohio Railroad shops and yards, and Dr. O. B. Ormsby, of Murphysboro, 
told me that a rather high percentage of the railroad cases in town were 
malarial. While my work here could not be made continuous enough 
to warrant definite statements, these sink-holes are at least probably partly 
responsible for this condition. 
SuRvEY Work IN ANNA AND JONESBORO 
Two days in August, 1918, were spent in survey work at Anna and - 
Jonesboro. Previous inquiries as to the average number of malaria cases 
per year brought surprisingly different replies, as follows: for Jonesboro, 
300, 2,000, 50; for Anna, 2,000, 1,500, 150, 600. Only one of these physi- 
cians makes the blood tests, and this may account for the widely different 
estimates, as this is the only method of certain diagnosis. He estimated 
600 cases in Anna. One of the oldest doctors there told me. that most 
physicians believe that there is likely to be some malaria in every case 
they are called upon to treat. 
Breeding-places——A majority of the physicians and of other citizens 
interviewed expressed the opinion that most of the malarial mosquitoes in 
the two towns were bred in the bottom-lands of the Mississippi, and I 
think that this is a source of a part of them. It is a matter of common 
knowledge in Anna and Jonesboro that when the wind blows steadily 
from the west and southwest for about two days, swarms of mosquitoes 
are carried in from the lakes and sloughs in the bottoms. These inva- 
sions are always followed, the doctors all told me, by a marked increase 
in the number of malaria cases. The fact that the nearest of these bot- 
tom-land breeding-places is six miles from town will serve to show the 
distance which mosquitoes may be carried by the wind. It also appears 
to be a matter of common knowledge that since the recent drainage of 
many of the sloughs and lakes, the severity of these periodic attacks by 
mosquitoes has been greatly lessened. It should be remembered that many 
residents of Anna and Jonesboro go to the Mississippi bottoms to fish 
