Obviously, there were two points at which the destroyers might strike, the egg and the larvae. It was first found that, while the eggs required no air for their development, the larvae wiggled up to the surface and inhaled it through curious little tubes developed for this purpose, oddly enough from their tail-ends. If some kind of film could be spread over the surface of the water, through which the larvae could not obtain air, they would suffocate. The well-known property of oil in "scumming over" water was recalled, two or three stagnant pools were treated with it, and to the delight of the experimenters, not a single larva was able to develop under the circumstances. Here was insecticide number one. The cheapest of oils, crude petroleum, if applied to the pool or marsh in which mosquitoes breed, will almost completely exterminate them. Scores of regions and areas to-day, which were once almost uninhabitable on account of the plague of mosquitoes, are now nearly completely free from these pests by this simple means. An ounce to each fifteen square feet of water-surface is all that is required, though the oiling needs to be repeated carefully several times during the season. But what of the eggs? They require no air, and it was found impossible to poison them without simply saturating the water with powerful poisons; but an unexpected ally was at our hand. It was early noted that mosquitoes would not breed freely in open rivers or in large ponds or lakes, but why this should be the case was a puzzle. One day an enthusiastic mosquito-student brought home a number of eggs of different species, which he had collected from the neighboring marshes, and put them into his laboratory aquarium for the sake of watching them develop and identifying their species. The next morning, when he went to look at them, they had totally disappeared. Thinking that perhaps the laboratory cat had taken them, and overlooking a most contented twinkle in the corner of the eyes of the minnows that inhabited the aquarium, he went out and collected another series. This time the minnows were ready for him, and before his astonished eyes promptly pounced on the raft of eggs and swallowed them whole. Here was the answer at once: mosquitoes would not develop freely where fish had free access; and this fact is our second most important weapon in the crusade for their extermination. If the pond be large enough, all that is necessary is simply to stock it with any of the local fish, minnows, killies, perch, dace, bass,--and presto! the mosquitoes practically disappear. If it be near some larger lake or river containing fish, then a channel connecting the two, to allow of its stocking, is all that is required.