During one year the quarantine service, which keeps a strict lookout for yellow fever, bubonic plague, and other epidemic diseases, inspected over 100,000 passengers coming into the Zone. It required about 150,000 gallons of mosquito oil a year to keep down the mosquitoes. There are 50 known breeds of these insects on the Isthmus and perhaps some 20 species more which have not been identified. Of the 50 or more species of mosquitoes 11 belonged to the malaria-producing family--anopheles. Their cousins of the yellow-fever-producing family--the stegomyias--boast of only two species. What the other 40 or more kinds are doing besides annoying suffering humanity has not been determined. The mosquito is comparatively easy to exterminate. Its life habits are such that a terrific mortality may be produced among them during infancy. The average young mosquito, during its "wriggler" state of development, lives under the water and has to make about 8,000 trips to the surface for air before it can spread its wings and fly. If oil is poured upon the water it can get no air and death by asphyxiation follows. Two classes of larvaecide are used on the waters to exterminate the baby mosquitoes: One is an oil used to make a scum over the surface; the other a carbolic solution which poisons the water. At the head of every little rivulet and tiny, trickling stream one sees a barrel out of which comes an endless drip! drip! drip! These drops of oil or poison are carried down the stream and make inhospitable all of the mosquito nurseries of the marshes through which the waters flow. In addition to these barrels, men go about with tanks on their backs, spraying the marshy ground and the small, isolated pools of water with larvaecides. [Illustration: SANITARY DRINKING CUP] [Illustration: MOSQUITO OIL DRIP BARREL] [Illustration: SPRAYING MOSQUITO OIL] [Illustration: TYPICAL QUARTERS OF THE MARRIED LABORER] [Illustration: A NATIVE HUT] This method of treatment has not exterminated all mosquitoes on the Isthmus, but it has so materially reduced their number that one may stay in the Zone for weeks without seeing a single one. This is a freedom, however, that must be paid for by vigilance of the most painstaking and unremitting sort. The moment the work is relaxed the mosquitoes again spread over the territory.