317 
every month of the season, but not in numbers corresponding to their 
area. The large amount of organic matter which they contain seems to 
be detrimental to Anopheles larvae. At one time during a summer drouth 
the common duckweed (Lemna minor) completely covered the surface 
of a part of the swamp where cattails were absent, and I could find no 
mosquito larvae around the edges of this area. 
The marshes (Class II) seem to produce more Anopheles than the 
swamps, probably because of a more favorable vegetation, as already 
noted. 
Ponds (Class III) are dangerous if they contain sufficient vegeta- 
tion to give shelter and protection to mosquito larvae. Grass, reeds, 
rushes, and cattails fayor their escape from small fishes and predaceous 
insects. I was never able to find any mosquitoes breeding in Pond No. 
58, in the Tie Plant yard, or in the “creosote lakes,” the latter probably be- 
cause of the cresote waste emptied into them. No breeding was found in 
No. 52 and very little in No. 45. The three Tie Plant ponds and No. 52 
have no vegetation along the edges to furnish protection. No. 45 showed 
a growth of water-lilies which did furnish some protection, although 
not as effective as grass. The pond on the Normal campus (No. 179; 
Fig. 8 and 9) has a good deal of plant growth along the edges, and I 
found some Anopheles breeding there practically every time I looked for 
them. As some of this vegetation was cut down in the fall of 1918, it was 
not quite so abundant in 1919 as the year before, and Anopheles were also 
less abundant. Several physicians have spoken to me of the number of 
cases of malaria in the Normal dormitory, and I found Anopheles breed- 
ing within twenty feet of it (Fig. 8). 
Roadside ditches (Class IV) generally contain less water than the 
other types, but they are likely to be mosquito breeders unless they dry up. 
Some of them, however, have no vegetation whatever and usually con- 
tain few or no wigglers. Numbers 62 and 63, by the Illinois Central yard 
office, lined with overhanging grass and filled with other ‘vegetation, hold 
their water well through the season and furnish quite ideal conditions 
for Anopheles. \ 
In creeks and streams (Class V) mosquito breeding is limited not 
only by vegetation and water supply, but also by strength of current and 
purity of water. In the main stream of Crab Orchard and Sycamore 
creeks and of Pyles Fork I found very little or no breeding except when, 
as in the case of Pyles Fork, dry weather reduced the stream to a series 
of pools. In the backwaters and in small tributaries with little current, 
Anopheles were often quite abundant. Small streams with little current, 
such as No. 58, running into the swamps, especially those with grassy 
banks,* may be prolific breeders. When waste water of any kind pollutes 
the stream it is likely to reduce mosquito production. 
Practically the same may be said of Class VI (open drains) as of 
creeks and streams. They are, perhaps, more likely to become polluted 
* See Fig. 12—a stream at Murphysboro. 
