2l6 FILARIA BANCROFTI [CH. 



Effect of filarice on the mosquito. The developing filarise 

 have a very deleterious effect on the health of the mosquito, 

 and heavily infected individuals can easily be recognized by 

 their sluggish 'appearance. When a very large number of 

 embryos are ingested by a favourable intermediate host, the 

 resulting development usually causes the death of the mos- 

 quito. In the case of S. pseudoscutellaris, Bahr noticed that 

 when mosquitoes were fed on a slightly infected patient nearly 

 all of them were alive 21 days later, but out of a batch of 

 200 insects that fed on a patient shewing very numerous 

 filarial in his blood, only 17 managed to survive an equal length 

 of time. The period at which the infected mosquitoes died 

 was dependent apparently upon the degree of development of 

 the lilariffi. Thus, in warm weather most of the insects died on 

 the sixth day after feeding, and in the cooler season on the tenth. 

 The last stages of development, when the filaria is entering the 

 proboscis, seem to be the most critical time for the mosquito, 

 as in one batch 23 Stegomyia were alive on the 15th day after 

 infection, but only three of them lived four days longer. 



Possibility of another "indirect" mode of transmission by 

 the mosquito. The pathogenic effect of F. bancrofti upon its 

 intermediate host is of considerable interest, for it suggests 

 that the worm has only recently become adapted to its present 

 mode of transmission. Moreover, it seems possible that 

 Manson's theory that the filariae first escape into water and 

 subsequently enter man, has been discarded rather too hastily. 

 When the mature larvae migrate from the thoracic muscles, 

 although most of them come to rest in the proboscis, it is no 

 uncommon occurrence to find them in the legs and other regions 

 of the mosquito. These " mistakes " on the part of the larvae 

 also afford support to the theory that this filaria has only re- 

 cently adopted its present mode of entry into the body. When 

 the young filaria escapes from the proboscis of the mosquito on 

 to the skin, it is only capable of penetrating the surface in the 

 presence of moisture. On the other hand the young filariae 

 will live in water for several hours, and if a drop of water con- 

 taining them is placed on the skin the parasites at once disappear 

 down the openings of the gland-ducts. 



