XIl] LIFE-CYCLE IN MOSQUITO 2O9 



supposition that during the night the dilatation of the cutaneous 

 capillaries facilitated the entrance of the filariae into the cir- 

 culation. 



Against all these theories can be brought the objection that 

 a closely related species, Filaria diurna, is normally diurnal, 

 and yet presents a somewhat similar life-cycle to that of 

 F. hancrofti. A still more serious difficulty in the way of any 

 mechanical explanation is the occurrence, in some parts of the 

 world, of varieties of F. hancrofti that are diurnal in habit, 

 and yet others which exhibit no periodicity. The researches 

 of Bahr have shewn that in Fiji and Oceania generally, the 

 filariae occur in the blood of patients in no regular manner, and 

 yet the parasites are morphologically identical with F. hancrofti. 

 Similarly, Ashburn and Craig, because it exhibited no perio- 

 dicity, considered the human filaria of the Philippines to be a 

 distinct species, and gave it the name of F. philippinensis , but 

 Low has shewn that it is identical with F. hancrofti. 



Bahr's observations are of great interest, for they suggest 

 that in Fiji the absence of periodicity in the filaria is a partial 

 adaptation to the habits of its usual invertebrate host in this 

 locality, Stegomyia pseudoscutellaris, a mosquito which only 

 feeds during the day. 



At present, however, there is no hypothesis that will satis- 

 factorily account for the phenomenon of filarial periodicity. 



Life-cycle in the mosquito — the intermediate host. As men- 

 tioned above, the filarial embryos are incapable of further 

 development within the body of their host, blit require to be 

 ingested by a mosquito belonging to one of the species that 

 serve as the invertebrate hosts for F. hancrofti. Then the 

 parasites undergo further evolution in the body of the mosquito. 

 The various stages in the metamorphosis of this species in 

 the body of Culex pipiens have been worked out by Manson. 

 The development may be conveniently divided into seven stages. 



First Stage. Shortly after being ingested by the mosquito 

 the transverse striation of the embryo becomes well marked, 

 as if from longitudinal shrinking. Within about one hour the 

 embryo breaks out of its sheath and then shews active move- 

 ments. The parasite bores its way through the walls of the 



H. B. F. 14 



