394 



THE ANIMAL PARASITES OF MAN 



to remain in the pulmonary capillaries and do not appear in the 

 capillaries of the systemic circulation. If it is true that the periodicity 

 of Loa loa cannot be reversed by changing the hours of sleep, then 

 the explanation is incomplete. In the case of the larvae of Filar id 

 bancrofti (which have a nocturnal periodicity), in order to apply the 

 same explanation we must further assume that the mikrofilariae have 

 less power of resisting the force of the capillary current (i.e., are less 

 sticky). They are washed out of the pulmonary capillaries by day 

 and by night, but it is only at night, when the blood-stream in systemic 

 capillaries is less rapid, that they are able to rest there. In the day- 

 time they are washed on until they reach the capillaries of the organs 

 (possibly again the lungs). The reversal of the periodicity by sleeping 



during the daytime admits of a similar explanation. If this explana- 

 tion be true, then a prolongation of the day conditions, e.g., by 

 continued exercise, should result in still keeping the larvae out of the 

 circulation, but this does not appear to be the case. 



In certain countries, e.g., Fiji, Samoa, Philippines, West Africa, 

 larvae, apparently those of Filaria bancrofti, show no periodicity. In 

 Fiji the usual intermediate host is Stegomyia psendoscutellaris, a day- 

 biting mosquito, so that possibly, as Bahr suggests, the mikrofilariae 

 have partly adapted themselves to the habits of their intermediate 

 host, as the nocturnal mikrofilariae are adapted for transmission by a 

 nocturnal feeding mosquito, e.g., Culcx fatigans, but how this could 

 come about is a mystery. It is not certain in all cases whether 

 the non-periodic mikrofilariae really belong to Filaria bancrofti; 

 some may be L. loa larvae, or possibly unknown larvae. An exact 

 morphological description of these larvae is therefore always necessary. 



