FISHES IN RELATION TO MOSQUITO CONTROL. 11 
leaf then often partly sinks, forming a depression over the center 
while the edges remain at the surface. The cup thus formed holds 
enough water to support mosquito larve, and with respect to fish 
the larve contained therein are perfectly safe. When the dead leaves 
get inshore, they of course frequently make places inaccessible to 
sh. 
Grasses and rushes and other plants, having straight stocks and 
no slightly submerged leaves, furnish no protection. Areas over- 
crown with such plants have been carefully examined for mosquito 
larve, but wherever Gambusia was present no immature mosquitoes 
were found. 
The aquatic plant, Naias flexilis, which was common in several 
ponds, forming a dense growth over the bottom, normally does not 
rovide protection, as it does not reach the surface of the water. 
uring the severe fall drought the water, however, became so low 
that it was near the surface or partly exposed in many places, making 
such a dense mass that fish could not penetrate it. erever this 
occurred it furnished excellent protection and Anopheles larve and 
Bape were common. 
he duck weed, Spirodela polyrrhiza, was present in only one 
ond, over which it formed an almost continuous cover. No mosquito 
arvee were seen in this pond, indicating that this plant does not 
furnish protection for the mosquito from fish. It, in fact, is likely 
that mosquitoes can not breed under such conditions, but as this 
pend was Well supplied with Gambusia no data supporting this proba- 
ility were obtainable therefrom. 
The smart weed (Polygonum) is another plant that not only does 
not appear to provide protection but which may actually be repellent. 
Many places overgrown with this weed were repeatedly examined, 
but mosquito larve were not found even in apparently favorable 
hiding places. 
Nearly all marginal plants, by projecting partly into the water, by 
falling into it after maturing, or by becoming partly submerged after 
freshets, furnish protection for mosquito larve. These plants should 
be removed when possible. 
It is evident from the study of plants in relation to mosquito control 
by means of fish that it is highly desirable to remove from the ponds 
those plants having leaves just below the surface of the water and to 
treat alge in such a way as to make them useless as protectors of 
mosquitoes. The presence of these plants was by far the most impor- 
tant obstacle to be overcome in securing mosquito control in the 
many ponds in the extra-cantonment zone of Camp Hancock. A 
rather constant vigilance was necessary in order to keep a large series 
of ponds free of such plants, but it is not very difficult work or 
usually very expensive, for two laborers provided with hoes, rakes, 
a knapsack spray can, and some oil could take care of quite a number 
of ponds during the course of a season. In badly infested ponds it 
is occasionally advantageous to cut the vegetation with a patented 
device known on the market as a submarine saw. 
It is very interesting to observe how quickly the top minnows learn 
to follow the workmen engaged in cutting and raking vegetation from 
ponds. They soon become quite tame and schools of them work 
almost under the tools of the laborers, catching mosquito larvee and 
other insects as quickly as their hiding places are destroyed. 
