18 



INSECTS AND HUMAN WELFARE 



person. In the late stages of the disease the microscopic 

 larval worms occur abundantly in the blood. For some unex- 

 plained reason they remain in the deep-seated blood vessels 

 during the day, but usually appear more abundantly in the 

 peripheral circulation during the night. Here they are readily 

 obtained by mosquitoes with their meal of blood. In the 



FIG. 7. Culex qidnquefascialus, female. 



alimentary canal of the mosquito the larval Filaria discards 

 a sheath-like envelope which has previously invested it, and 

 works its way through the wall of the stomach into the tho- 

 racic muscles where it increases greatly in size and finally 

 migrates to the base of the proboscis. From two to three 

 weeks are necessary for this metamorphosis, and for some 

 time longer the Filaria may remain in the proboscis awaiting 

 its opportunity to enter another person through the wound 

 occasioned by the mosquito's beak. Once they have been 

 transferred to their human host, the parasites enter the 

 lymphatics where they attain sexual maturity and give rise 

 to the abundant microscopic larval Filarias that reenter the 

 circulation to await ingestion by another mosquito. 



