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from Havana, they are much more abundant at some seasons of the year. 

 It appears to me that they increase in numbers from April or May till 

 August, and thereafter gradually decrease till February or March. 

 Another point, however, requires to be borne in mind, inasmuch as it 

 affords an explanation of the recurrence, hitherto unaccounted for, of 

 yellow-fever epidemics without new importation, in localities previously 

 considered as immune. I allude to the hibernation of mosquitoes, a 

 phenomenon which is not observed in our climate, at least in all its 

 phases; but which constitutes, according to the best authorities, the 

 regular mode by which the species is propagated in cold climates, during 

 winter. Taschenberg informs us that: "the fertilized females of the last 

 generation hibernate during winter in out-of-the way places such as the 

 cellars of dwellings, and set about propagating their species the following 

 spring. ' ' 



Among the conditions which favor the development of mosquitoes 

 may be mentioned ; heat, moisture, the vicinity of stagnant waters, low, 

 dark localities sheltered from the wind, and the summer-season. It is 

 necessary, however, to bear in mind Humboldt's observation that the 

 abundance of mosquitoes is not always in accordance with recognizable 

 meteorological or topographical conditions. 



I have already referred to the difficulty which our mosquito, by 

 reason of its comparatively small wings, must experience in its upward 

 flight after it has filled itself with blood. It will also be hinderd by the 

 same cause, from going far from the place where it has accomplished its 

 iast bite, and, in general, from traveling any considerable distance through 

 the air without resting. This circumstance will not prevent, however, its 

 being conveyed, hidden among clothes, caught under a hat, inside of a 

 trawling bag, etc., to considerable distances, after a recent bite, perhaps 

 carrying upon its mouth-parts the inoculable germ of the disease. 



The preferences which mosquitoes show for certain races and 

 individuals should also be borne in mind; the African race being, 

 apparently, the one least tormented by them, and the greatest sufferers 

 being the Northern races newly arrived in the tropical regions of America. 

 It is probable that this may be due to the comparative thickness of the 

 skin, and to pecularities in the cutaneous capillary circulation, since those 

 circumstances must influence the facility with which the female mosquito 

 will be able to procure itself the blood which it requires in order to 

 accomplish its life-cycle. 



After this long, but necessary account of the habits of our Cuban 

 mosquitoes, and of the Culex mosquito in particular, let us consider by 

 what means that insect might transmit the yellow fever, if that disease 

 happens to be really transmissible through the inoculation of blood. The 

 first and most natural idea would be that the transmission might be effected 



