CHOICE OF HOST 263 



Twelve hours after hatching ... Ovaries in first stage, salivary glands 



immature. 

 Thirty-six hours after hatching... First appearance of yelk granules, 



ovary in second stage. 

 Fourth day after hatching . . . Ovary in third stage, the nucleus 



obscured by yelk. 



Fifth day after hatching . . . Commencing elongation of follicle. 



Sixth day ... Ovary in fifth stage, appearance of 



floats. 



Observations made on caught anophelines indicated that the rate 

 of development of the second and succeeding batches of eggs is much 

 more rapid than this, and that the second follicle, already at the second 

 stage of development when the first batch of eggs is laid, is able to com- 

 plete its growth in about two days. 



The nature of the stimulus which induces mosquitoes to bite is not 

 clearly understood. It is well known that mosquitoes, and particularly 



anophelines, are attracted to dark obiects, and seem 



, Choice of Host 



to prefer to bite through a dark skin rather than a 



white one. In the Aden Hinterland, in the camp referred to above, 

 the senior writer always found many more Anopheles in the tents of 

 Indian soldiers than in those occupied by Europeans; other observers 

 have also noted the same peculiarity in the case of Anopheles in 

 Africa. The reason for this choice of host is not very obvious, but it 

 may be noted that the Indians were in the habit of keeping their tents 

 almost completely closed, and the atmosphere in them was much more 

 humid than in those occupied by the Europeans, who kept theirs 

 partly open. 



Howletts' experiments tend to throw some light on this subject. 

 This observer carried out a series of preliminary experiments and found 

 that shed blood or human sweat did not attract the females of CuJex 

 fatigans or Stegowyia sciitellaris any more than water. On experi- 

 menting with the effect of heat, however, it was at once found that the 

 females of Stegomyia scutellaris were readily attracted by the hot air 

 radiating from a test tube containing hot water ; the male mosquitoes on 

 the contrary paid no attention to it. As a result of a number of similar 

 experiments he concludes that, ' (a) the bite of a mosquito is a reaction to 

 ' the stimulus provided by a hot surface, (b) that the mosquito is attracted 

 ' to the hot surface mainly by the warm air rising from it, and (c) that the 

 ' strength of the reaction is, within certain limits, proportional to the 

 ' differential temperature " of the surface, i.e. the difference between its 



