i8o Arthropods as Essential Hosts of Pathogenic Organisms 



One of the most surprising features of the habits of these larvae 

 is the periodicity which they exhibit in their occurrence in the peri- 

 pheral blood. If a preparation be made during the day time there 

 may be no evidence whatever of filarial infestation, whereas a prep- 

 aration from the same patient taken late in the evening or during 

 the night may be literally swarming with the parasites. Manson 

 quotes Mackenzie as having brought out the ftirther interesting 

 fact that should a "filarial subject be made to sleep during the day 

 and remain awake at night, the periodicity is reversed; that is to say, 

 the parasites come into the blood during the day and disappear from 

 it during the night." There have been numerous attempts to explain 

 this peculiar phenomenon of periodicity but in spite of objections 

 which have been raised, the most plausible remains that of Manson, 

 who believes that it is an adaptation correlated with the life-habits 

 of the liberating agent of the parasite, the mosqmto. 



The next stages in the development of Filaria nocturna occur in 

 mosquitoes, a fact suggested almost simultaneously by Bancroft 

 and Manson in 1877, and first demonstrated by the latter very soon 

 thereafter. The experiments were first carried out with Culex 

 quinquefasciatus {— fatigans) as a host, but it is now known that a 

 number of species of mosquitoes, both anopheline and culicine, may 

 serve equally well. 



When the blood of an infested individual is sucked up and reaches 

 the stomach of such a mosquito, the larvs, by very active movements, 

 escape from their sheaths and within a very few hours actively mi- 

 grate to the body cavity of their new host and settle down primarily 

 in the thoracic muscles. There in the course of sixteen to twenty 

 days they undergo a metamorphosis of which the more conspicuous 

 features are the formation of a mouth, an alimentary canal and a 

 trilobed tail. At the same time there is an enormous increase in 

 size, the larvae which measured .3 mm. in the blood becoming 1.5 mm. 

 in length. This developmental period may be somewhat shortened 

 in some cases and on the other hand may be considerably extended. 

 The controlling factor seems to be the one of temperature. 



The transformed larvae then reenter the body cavity and finally 

 the majority of them reach the interior of the labium (fig. 120). A 

 few enter the legs and antennas, and the abdomen, but these are 

 wanderers which, it is possible, may likewise ultimately reach the 

 labium, where they await the opportunity to enter their human host. 



