MODE OF ATTACK 261 



higher temperature of persons stricken with fever might be an additional attrac- 

 tion for the mosquito. 



The French commission found that calopus has a decided predilection for 

 the blood of man. It can, however, subsist upon the blood of any warm-blooded 

 animal, but it shows a more or less pronounced repugnance for any other wanii- 

 blooded animal than man. Durham states that at Para he has observed calopus 

 feeding upon " dogs, on an agouti, and a bat," but he does not indicate whether 

 this was in freedom or with mosquitoes in captivity. JSTeumann, in his experi- 

 ments on the transmission of Plasmodium prcecox to canary birds by means of 

 calopus, found that this mosquito showed a decided preference for mammalian 

 blood. In his experiments, conducted under the most favorable conditions, 

 from a total of 2573 female calopus, he was only able to induce 789 to suck blood 

 of canaries, or 30.7 per cent. With Culex, on the contrary, 64 per cent of the 

 females attacked the birds. When the experiment was made with rats the re- 

 sults were reversed and the calopus attacked at once while the Culex showed 

 repugnance. 



It appears also that calopus discriminates between races and individuals. This 

 was already discussed by Finlay in the following manner : 



" Finally, should be taken into account the preferences which mosquitoes 

 manifest towards certain races and individuals ; one notes that the least tor- 

 mented race appears to be the African and the individuals most persecuted by 

 them are those of northern races who have recently come to the tropical regions 

 of America. It appears probable that this depends upon the thickness of the 

 skin and upon the condition of the capillary circulation of the skin ; it is to be 

 assumed that these circumstances will affect the facility with which the female 

 mosquito can obtain the blood which it needs to complete its life-cycle." 



The French investigators made a number of experiments to show whether 

 calopus bites indifferently people of different races. They concluded that it 

 easily bites both the negro and the red-skinned races, but that it has a marked 

 predilection for the white race. If mosquitoes of the same age and at the same 

 temperature are placed upon individuals of the black-skinned, red-skinned and 

 white-skinned races, the white will be bitten most quickly. The red-skin is also 

 quickly attacked. Towards the negro, calopus shows some repugnance, and 

 almost never in the course of their observations did it decide to bite immediately, 

 and often a contact of ten to fifteen minutes was necessary before the proboscis 

 was inserted. They show also that among the individuals of the white race, this 

 mosquito has preferences, and attacks with the greatest avidity young, vigorous 

 indi\iduals of fine skin and good color rather than anaemic or aged persons. 

 When it is hungry, however, the mosquito will bite the first human being it 

 meets. 



The stealthiness of attack has already been mentioned as a peculiarity of the 

 yellow-fever mosquito. This is brought out in an interesting manner by Parker, 

 Beyer and Pothier in the results of their investigations at Vera Cruz (Eeport of 

 Working Party No. 1, Yellow fever Institute, Bulletin 13, March, 1903) : 



" The approach to attack of Stegomyia is extremely insidious, usually ap- 

 proaching an individual on the shady side and without warning. The perti- 



