The yellow fever mosquito breeds in cisterns, water barrels, pitchers and in the various water receptacles about the house. In our own Southern States it very commonly breeds in the above-ground cisterns which are in general use. Often the larvæ (fig. 135b) are found in flower vases, or even in the little cups of water which are placed under the legs of tables to prevent their being overrun by ants. They have been repeatedly found breeding in the holy water font in churches. In short, they breed in any collection of water in close proximity to the dwellings or gathering places of man. The life cycle under favorable conditions is completed in from twelve to fifteen days. These figures are of course very dependent upon the temperature. The Army Commission in Cuba found that the cycle might be completed in as brief a period as nine and a half days. Under less favorable conditions it may be greatly lengthened. The adults are long lived. We have seen that during the experimental work in Cuba specimens were kept in captivity for sixty-nine and seventy-one days, respectively, and that they were proved to retain their infectivity for at least fifty-seven days. Dr. Guiteras subsequently kept an infected adult for one hundred and fifty-four days. Low temperatures have a very great effect not only on development, but on the activity and even life of the adults. Long before the method of transmission of yellow fever was discovered it was well known that the epidemics were brought to a close by heavy frosts, and it is now known that this is due to the killing of the mosquitoes which alone could spread the disease. [Illustration: 135_b_. Aëdes calopus; larva. (×7). After Howard.] _Aëdes calopus_ has a very wide distribution since, as Howard says, being a domestic mosquito, having a fairly long life in the adult stage, and having the custom of hiding itself in the most ingenious ways, it is particularly subject to carriage for long distances on board vessels, in railway trains, even packed in baggage. In general, its permanent distribution is from 40 degrees north latitude to 40 degrees south latitude (Brumpt), in a belt extending around the world. In the United States it breeds in most of our Southern States. Thus, as in the case of malaria, there are many places where the insect carrier is abundant but where yellow fever does not occur. Such, for instance, are Hawaii, Australia and Asia. An outbreak may occur at any time that a patient suffering from the disease is allowed to enter and become a source of infection for the mosquitoes. In this connection various writers have called attention to the menace from the Panama Canal. When it is completed, it will allow of direct passage from regions where yellow fever is endemic and this will greatly increase the possibility of its introduction into these places where it is now unknown. The result, with a wholly non-immune population, would be appalling.