The anopheles, the malarial mosquito, is not a mosquito of strong flight; two hundred yards is, in general, a good long flight for her. If there were trees and shrubs and bushes every few yards, the anopheles mosquito might travel very long distances and not be much exposed to either the sun or the wind. But if an area of two hundred yards around each building is kept clear, she will not often be able to cross such a zone without destruction, either by sun or wind. Clearing this zone exposes the ground to both sun and wind, and by these forces alone many of the smaller pools will be dried up and made unfit for mosquito breeding. I have often seen this measure alone, that is, clearing the zone, cause a swampy place which I had expected to have to drain, to become dry and cease to breed mosquitoes. [Illustration: Oilers at Work In Marsh.] [Illustration: Burning Out Ditch With Oil Spray.] It was also the inspector’s duty to see to the drainage. For this purpose he had a properly trained body of men under his control. The drainage was, of course, very much more extensive in area than the grass and bush cutting, for not only had the cleared zone itself to be drained, but all the area within that zone, and also the water-courses leading off from the drained areas had to receive attention. Many times we found that anopheles were breeding very much beyond the two-hundred-yard limit and still coming into the village. In one case we had a very large flight of anopheles which lasted for two or three weeks, and they were found to be breeding more than a mile from the village of Gatun. So, as a matter of fact, our work was very many times carried beyond the two-hundred-yard zone. The inspector used several methods of draining, being governed in his choice by local conditions. One of these methods was the open ditch, such as the farmer used for the ordinary draining in agricultural work. At Panama, there were several objections to this method. While the ditch drained the surrounding ground and stopped mosquito-breeding at this point, it became a most excellent breeding-place itself, as the grass grew in the bed of the ditch. At Panama, rain fell daily for eight months of the year, and the temperature was always high enough from one year’s end to another to encourage vegetable growth. The grass, therefore, grew so rapidly that the ditch had to be cleaned out every two or three weeks. The cost of keeping open a surface drain of this kind we found to be very high.