186 



did not show the slightest sign of infection. He maintains that at all events in 

 France, and that most probably means in all more northern countries, the Plasmo- 

 dia are only able to hibernate in man and not in the mosquitoes. Regendanz (1918 

 p. 33) comes to a similar result. He maintains that the temperature needed for the 

 Plasmodia to develop in the mosquitoes in Roumania is not reached before the end 

 of June when the day tp. was usually over 25° C. and the night tp. was only ex- 

 ceptionally below 16° C. 



It will clearly be understood how signiiicant these indications really are; and 

 this especially holds good the nearer we are the northern limits of the area of 

 distribution. For it means that the Anophelin material, every year before 

 the blood sucking period begins, is totally free from plasmodia. Those 

 which had plasmodia before the winter are now free, and the new brood will as 

 imagines be born quite as free from plasmodia as their mothers were. Only when 

 sucking upon malaria patients will the Anophelines get plasmodia; the hibernating 

 females perhaps a second time; the harvest of the summer the first. 



Those Anophelines which begin their blood-sucking period in spring are al- 

 ways free from malaria; it is man himself who year after year must infect the 

 Anophelines again if the disease is to be spread. The longer the winter is, the 

 smaller is also the chance of a serious infection of the Anophelin material; the 

 shorter the time of the summer in which the temperature is above about 16° C, 

 the smaller is the chance of the development of gametocysts, even if the Ano- 

 phelin material has been affected. R will be clearly understood that just at the 

 northern limits of the area of distribution, where the whole period of bloodsucking 

 is restricted at most to five months, it is of the greatest significance that the sum- 

 mer campaign of the mosquitoes begins with uninfected material. Just here a' 

 treatment with quinine will in a comparatively short time restrict the chances of 

 disseminating the plasmodia, especially because the Anophelines are inclined to prefer 

 the blood of domesticated animals, and in this way loosen the ties which conned 

 them with man. 



The question is now whether the view which I maintain with regard to the 

 retreat of malaria from our country also holds good with regard to the vast foreign 

 territory, from which it has also almost entirely disappeared. I suppose that the 

 same theory also holds good with regard to Sweden, but I do not venture to form 

 any opinion with regard to the other, southern countries. I only wish to call atten- 

 tion to the fact that in South Europe where A. maculipennis still attacks man, hor- 

 ses and cattle as far as I know live out of doors most of the year, and the stables 

 are not so large, so closed and so sheltered as in our country; the difference be- 

 tween the temperature out of doors and in the stables is not so great. 



There are still some few points to discuss with regard to the biology of the 

 Anophelines living near the northern borders of the distribution area, and North 



