450 MOSQUITOES 



in the blood, marked the beginning of a new era in medical 

 science; it was the first evidence of the development of germs 

 of human disease in the bodies of insects. An account of the 

 life cycle of filarial worms, including the development in the 

 bodies of mosquitoes, the means by which the worms are re- 

 turned to their primary hosts, and the effect of filarial infection 

 on man, will be found in Chap. XVII, pp. 299-306. 



Not all species of filarial worms utilize mosquitoes as inter- 

 mediate hosts, a notable exception being the loa worm of Africa. 

 Four human species, Filaria bancrofti, F. philippinensis, F. per- 

 stans and F. juncea (demarquayi) are known, or thought, to be 

 nursed by mosquitoes. The last two named are not known to 

 have any pathogenic effects, but F. bancrofti is connected either 

 directly or indirectly with a number of human ailments (see 

 p. 306). F. philippinensis is closely allied to F. bancrofti, but 

 differs in that it appears in the peripheral blood diurnally as well 

 as nocturnally, a habit which is supposed to be associated with 

 the diurnal habits of its usual intermediate host, Aedes pseudo- 

 scutellaris. 



Although the successful development of filarial worms is not 

 limited to one particular species of mosquito or even to any 

 particular group of species, the development is not completed 

 equally well in all species. The tropical house mosquito, Culex 

 quinquefasciatus, is the species in which the worms apparently 

 develop most frequently with least fatality to either worms or 

 mosquitoes. In Fiji the development of the worms is more 

 regular and more rapid in Aedes pseudoscutellaris than in any other 

 mosquito. In Aedes calopus, however, the development of the 

 worms is very slow, and they eventually degenerate in the tho- 

 racic muscles without reaching maturity. Many species of 

 Anopheles serve as suitable hosts, as do also some species of 

 Mansonia and other genera. In many species of mosquitoes 

 the filarial larvae are digested, or else die in the course of their 

 development. On the other hand, there are some mosquitoes 

 which are very susceptible to the injury done by the worms, 

 especially in case of heavy infestation, and the mortaUty may 

 amount to a large per cent. Apparently the most critical time 

 for the mosquitoes is when the larvse have penetrated into the 

 proboscis. This is a striking example of the almost universal 

 truth in parasitology, that the host in which the asexual cycle of a 



