FILARIA BANCROFTI IN MOSQUITOES 301 



on some physiologic condition of the host. Still stranger is the 

 fact that in many of the South Sea Islands, Samoa, the Fiji 

 Islands and the Phihppine Islands, the microfilariae show no 

 periodic disappearance, although if a person infected in a place 

 where the parasites do show periodicity be transferred to one of 

 the above-named islands, the periodic phenomena still persist. 

 As stated before, Manson, the great English parasitologist, with 

 characteristic ingenuity, suspected that this parasite, so abundant 

 in the blood, must make use of some blood-sucking insect as a 

 means of transmission, and further concluded that the night 

 swarming of the parasites in the peripheral circulation might be 

 an adaptation to the nocturnal habits of an intermediate host. 

 Working on this hypothesis, he discovered that certain mosquitoes 

 acted as the liberating agents for the parasites. The fact that 

 in those islands where no periodicit}- is shown the usual inter- 

 mediate host is a diurnal mosquito Aedes (or Stegomyia) pseudo- 

 cutellaris, certainly bears out the adaptation hypothesis. On the 

 grounds of the apparently distinct habits and different adaptation, 

 the non-periodic microfilariae have been separated into a distinct 

 species, or at least subspecies, to which the name Filaria philip- 

 pinensis was applied by Ashburn and Craig in 1906. Zoologists 

 are coming more and more to realize the importance of physio- 

 lologic as well as morphologic characteristics as a basis for sepa- 

 rating species and subspecies. The case of these filariae is by no 

 means unique in the organic world. Physiologic and biochemical 

 reactions are the main basis for the classification of the Bacteria, 

 and some Protozoa can be distinguished better by their patho- 

 genic effects and biochemical reactions than by their morphology. 

 To continue their development the larval worms must be 

 sucked up by the females of certain species of mosquitoes. A 

 considerable number of species of mosquitoes of several different 

 genera, including Anopheles, Aedes and Culex, may serve as 

 intermediate hosts for F. bancrofti (see p. 449). The commonest 

 and most widespread transmitting agent is the house mosquito 

 of the tropics, Culex quinquefasciatus (fatigans), a species which 

 also transmits dengue. A few hours after being swallowed by 

 a susceptible mosquito the microfilariae (Fig. 125 A) become rest- 

 less and endeavor to escape from their sheaths. This they 

 eventually accomplish by butting against the anterior end, 

 having gained as much impetus as their close quarters will allow. 



