118 INSECTS AND MAN 



thousand Chinamen, and discovered that the number of 

 filarise in the blood increased during the night. Was this 

 development of the blood parasite, during the night, an 

 adaptation to the nocturnal habits of the mosquito ? 

 Manson decided to test the matter, so he dissected 

 mosquitoes that had just fed on the blood of a Chinaman 

 known to be afflicted with the parasites. "I shall not 

 easily forget the first mosquito I dissected so charged," he 

 said. " I tore off its abdomen, and by rolling a pen-holdei 

 from the free end of the abdomen to the severed end, I 

 succeeded in expressing the blood the stomach contained. 

 Placing this under a microscope, I was gratified to find 

 that, so far from killing the filaria, the digestive juices 

 of the mosquito seemed to have stimulated it to fresh 

 activity." This was an epoch-making discovery, but 

 Manson was not content ; further research showed him that 

 the filariae became exceedingly active in the stomach of the 

 mosquito, and that they passed through the wall of this 

 organ into the abdominal cavity, and from thence into 

 the thoracic muscles. He discovered, too, that during this 

 passage the parasite increased in size from a length of 

 T u of an inch to a length of ^ of an inch, and that in 

 doing so it developed an alimentary canal and a mouth. 

 " Manifestly it was on the road to a new human host." 



At the time of these discoveries, Manson concluded 

 that the filarise then reached the blood of man through 

 drinking water, having reached the water owing to the 

 death and decay of the mosquito. Later work, however, 

 led to the discovery that the parasites eventually left the 

 thoracic muscles of their insect host and travelled to its 

 labium or proboscis, there to await an opportunity of 

 passing into the blood of a human host, an opportunity 

 which came on the next occasion that the mosquito took 

 a meal of man's blood. We have mentioned that a near 

 relative of the common gnat is a frequent vector of 

 filariasis, but at least seven species of Anopheles, and many 



