The weather is cool in Havana during January, February and March, so that the stegomyia under ordinary conditions become quite sluggish in their movements and a great many die. Mosquitoes decrease so much during these months that yellow fever becomes more or less rare, and it was very difficult for us to infect our mosquitoes. While for one hundred and forty years there had never been a single month in which there had not been some reported case of yellow fever in Havana, still only a very small proportion of these cases was recognized during the first three days, and it must be remembered that a mosquito has to bite within that period of the disease in man to become infected. We were, therefore, keeping our only infected mosquito with a great deal of care and tenderness, knowing that we had to depend upon her to start our vaccination work. In all Havana there is probably not a single fireplace or other means of artificial heat, so we sent to the United States and imported an oil-stove wherewith to keep her ladyship’s room always at summer temperature. Her home was a large glass jar on a table in the center of a sunshiny room. In this jar was hung a lump of white sugar on which she fed when hungry, and to this was added now and then a small piece of banana. A small vessel containing water was also kept in the jar. In order that she might have a plentiful supply of fresh air, the glass top was not placed upon the jar, but a sleeve of mosquito-netting was tied over its top. Within the last fifteen years a great deal of study has been given to mosquito life. It is now known that there are some seven hundred different species of mosquitoes; that in all species of mosquitoes the natural food is the various vegetable juices. In all species the female has to have a feed of blood before she can lay eggs. Blood does not seem to be necessary for life, but merely stimulates the function of ovulation. In order that the female may get this blood, she is furnished with biting apparatus not possessed by the male. The male never bites, not being physically able to do so. The female stegomyia, therefore, is alone concerned in the transmission of yellow fever. Las Animas Hospital being the yellow-fever hospital for the city of Havana was a center of attraction for most of the doctors of the Health Department. Dr. John W. Ross, Dr. John Guiteras and myself were at this hospital almost every day. Dr. Guiteras had charge of the vaccination work and of the laboratory where the mosquitoes for this work were being bred. Our lady mosquito was therefore directly under his charge. She had given so many people yellow fever and was therefore so valuable for our prospective work that we all when at the hospital would drop in to see how she was coming along and to pay our respects.