180 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 further 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 mosquito. 
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 
quinqnefasciatus (= 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 larvas, 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 antennae, 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. 
