312 MOSQUITOES OF NORTH AMEEICA 



infection of the mosquito. Lebredo made extensive experiments with filaria 

 and mosquitoes in Havana. He found that from August through October the 

 filarial larvas developed in 15 days; from December to January from 19 to 2'3 

 days were necessary. These differences were found to co-ordinate very well with 

 the temperatures shown by meteorological records. Fiilleborn, in Germany, 

 found that the filarise failed to develop within the mosquito at ordinary room 

 temperature ; but he obtained satisfactory results by keeping the mosquitoes in 

 an artificially heated room. 



Filaria im7nitis of the dog differs somewhat from Filaria hancrofti in its 

 mode of evolution in the mosquito for it undergoes development within the 

 Malpighian tubes instead of in the thoracic muscles. The larva, when fully 

 developed, pierce the closed distal ends of the Malpighian tubes and in this way 

 get into the general body-cavity and from there into the head and finally the 

 interior of the labium. The development of Filaria immitis requires about ten 

 days ; Grassi and Noe, and also Fiilleborn, found that after that period the larv£e 

 had reached the labium. 



MODE OF TRANSMISSION TO THE VERTEBRATE HOST. 



Manson followed the evolution of the larval filariae within the mosquito only 

 to development within the thoracic muscles. Upon these observations he based 

 a theory to the effect that the mosquitoes falling into the water disintegrated 

 rapidly after death and liberated the parasites, and that the water containing 

 them, when drunk by human beings, carried the parasites once more into the 

 human body. Manson was undoubtedly influenced in the formulation of this 

 theory by the, at that time, general belief that the female mosquito was very 

 short-lived and died immediately or soon after disposing of her eggs. Later 

 Bancroft, finding that the female mosquito remained alive a long time, felt that 

 the filaria must have other means of reaching its ultimate host and the necessity 

 for some other explanation. He suggested two ways in which he thought man 

 might become infected. One of these was by accidentally swallowing the in- 

 fected mosquito ; the other that while the mosquito is biting, the filaria, under 

 some stimulus, re-entering the digestive tract when the mosquito bit, made its 

 way along the proboscis, finally penetrating the human tissues. 



Studies published in 1900, by Low, by James, and by Grassi and Noe, arrived 

 very near the truth. In fact these latter authors succeeded in producing, as they 

 believed, in the case of Filaria immitis of the dog, an infection by means of the 

 bites of filaria-bearing mosquitoes. They further produced convincing evidence 

 that the filarise left the mosquito when it sucked blood, and thus they were the 

 first to demonstrate the direct transfer of filarise by mosquitoes. No experi- 

 ments with Filaria hancrofti and man had ever been attempted on account of 

 the serious consequences of filarial infection. 



The discovery by the above-mentioned investigators, at nearly the same time, 

 that the larval filarise, upon leaving the thoracic muscles of the mosquito, for the 

 greater part made their way to the proboscis, and there remained, head foremost, 

 for an indefinite period, pointed to direct transfer by the mosquito's bite. 



