Chap. 26 



ROUNDWORMS — THE TUBULAR PLAN 



529 



Fig. 26.7. The microscopic filaria worms. Wiichereria hancrofti, swarming in 

 human blood at night. They are parasites in human lymph glands and in certain 

 species of mosquitoes which are essential to their complete life cycle and which 

 transmit them to their human hosts. They are the cause of filariasis (elephan- 

 tiasis). (Courtesy, Craig and Faust: Clinical Parasitology, ed. 5. Philadelphia, 

 Lea and Febiger, 1951.) 



several hundred of them per drop. Many mosquitoes must be killed by large 

 meals of them. Thus, millions of microfilariae are swallowed into death traps 

 as surely as human muscles are death traps for trichina larvae. 



Within the mosquito, the microfilariae immediately bore through the stom- 

 ach wall and enter the muscles of the thorax. There they develop into larvae; 

 their form changes from slenderness to sausage shape, and back again to 

 slenderness and lengthening. This takes about 10 days at the end of which 

 they are physiologically set for a change. They wriggle out of the thoracic 

 muscles of the mosquito and make their way into its mouth parts (Fig. 26.7). 

 The mosquito is now loaded with infective larvae. Mosquitoes that carry 

 microfilariae live near human dwellings, not far to go for a blood meal. 



Everybody must have seen mosquitoes feel the skin for an easy place to 

 bite. The filaria-loaded mosquito does this like any other mosquito, and the 

 larvae in its mouthparts stimulated by the warmth and pressure of the flesh at 

 once bore their way through the mosquito's labium (lower lip) and into the 

 skin. The next chapter of filaria life history is almost a blank. Into what part 

 of the human body the larvae go and how long before they are full grown 

 inhabitants of the lymph passages is mostly unknown. Their arrival there is a 

 certainty. 



