222 FILARIA IMMITIS [CH. 



The further development of the embryos takes place in 

 certain species of mosquito and has been carefully investigated 

 by Noe and Fiilleborn. The most efficient intermediate host 

 was found to be Anopheles maculipennis, which invariably 

 became infected by feeding on blood containing the embryos. 

 On the other hand, under similar conditions only about 20 per 

 cent, of Stegomyia fasciata shewed any development of the 

 filariae within their bodies. Noe found that development would 

 also take place in a large number of Anophelines, including 

 A nopheles bifurcatus, Myzorhynchus pseudopictus and Myzomyia 

 superpicta, and also in the following Culicines : Culex penicil- 

 laris, C. malarice and C. pipiens. 



When a mosquito has fed upon an infected dog, three or four 

 times as many filariae are found in the stomach of the insect 

 as in the blood of the dog. The reason for this apparent 

 increase is the concentration of blood that takes place in the 

 gut of the mosquito, most of the serum being excreted within 

 an hour after a meal. 



Within 20 to 40 minutes after being ingested by an 

 Anopheles, the filariae are found in the Malpighian tubules, in 

 which site they undergo their further development. The 

 worms are guided to the tubules by some chemiotactic influence, 

 for they have been shewn to be attracted towards the mouth 

 of an open capillary tube containing an emulsion of a Mal- 

 pighian tubule in normal saline. 



The embryos, during their development in the mosquito, 

 cause a marked alteration in the character of the epithelium 

 of the Malpighian tubules, which may be the cause of the high 

 mortality amongst infected insects. The rate of development 

 of F. immitis, as in the case of hancrofti, depends mainly upon 

 the temperature, for at 26° C. the whole process is complete 

 within ten days, whereas at 20° to 21° C, it is very much 

 prolonged. When the development is complete, the young 

 filariae bore through the walls of the Malpighian tubule into 

 the body-cavity of their host, and migrate towards the 

 head. They usually come to rest within the sheath of the 

 proboscis, but isolated examples may be found in the palps 

 and legs. 



