312 MOSQUITOES OF NORTH AMERICA 
infection of the mosquito. Lebredo made extensive experiments with filaria 
and mosquitoes in Havana. He found that from August through October the 
filarial larve developed in 15 days; from December to January from 19 to 23 
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 filarie 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 immitis of the dog differs somewhat from Filaria bancrofti in its 
mode of evolution in the mosquito for it undergoes development within the 
Malpighian tubes instead of in the thoracic muscles. The larve, 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 Nod, and also Fiilleborn, found that after that period the larve 
had reached the labium. 
MODE OF TRANSMISSION TO THE VERTEBRATE HOST. 
Manson followed the evolution of the larval filariz 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 Noé, 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 filarie left the mosquito when it sucked blood, and thus they were the 
first to demonstrate the direct transfer of filarie by mosquitoes. No experi- 
ments with Filaria bancrofti 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 filariz, 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. 
