292 MOSQUITOES OF NOETH AMERICA 



from 3 to 32 days. The meteorological factor which most influences the larva 

 out of the water is evaporation ; up to 2 mm. evaporation does little harm but 

 becomes injurious in proportion to its increase. Pupae dried upon filter paper 

 survive up to 9 hours and 30 minutes, i^dvanced pupas produce images after 

 drying of from 5 to 8 hours, and they would issue even while upon the filter 

 paper. Pupae of any age produced images when placed upon the moist ground. 

 To determine the resistance of the larva upon moist earth 55 experiments 

 with 3780 larvffi were made. The practical conclusions are that the larvse, after 

 being so kept for three days, will develop in a normal manner. One larva, kept 

 on moist soil for 14 days, under natural conditions, when returned to the water 

 transformed to imago after three days ; this was the greatest endurance observed 

 by the experimenter. As in the previous drying experiments, the time of trans- 

 formation was affected, the larvae producing images in from 3 to 39 days. 



THE GEOGRAPHICAL DISTRIBUTION OF THE YELLOW-FEVER MOSQUITO. 



As is pointed out incidentally in the consideration of the carriage of mos- 

 quitoes by conveyances, the yellow-fever mosquito, by reason of its habits, 

 being a domestic species, having a fairly long life in the adult stage, and having 

 the custom of hiding itself in the most ingenious ways, is particularly subject to 

 carriage for long distances on board vessels, in railway trains, even packed 

 securely away in baggage. It was in this way that the yellow-fever mosquito 

 was carried from America to Africa, or vice versa, according to the view of the 

 origin of the species which is adopted. It should be stated that in the old days 

 of sailing vessels on very long voyages, it was not only quite possible for the 

 yellow-fever mosquito to breed continuously in the more or less exposed water 

 supply of the vessels, but undoubtedly this was a common occurrence. 



By such means this mosquito is distributed far and wide and during warm 

 summers is taken often far beyond the point where it is capable of maintaining 

 itself permanently. Thus there are two distributions for this insect, one in 

 which it is capable of breeding continuously, and another over which it spreads 

 during warm weather, to be annually exterminated by the cold, after breeding 

 for a certain number of generations. Owing to its domestic habits, the mos- 

 quito does not flourish except in the presence of fairly dense settlements. It can 

 breed anywhere where man is congregated and the temperature is sufficiently 

 high. Thus its permanent distribution is determined by the minimum tem- 

 peratures and its temporary distribution by the maximum temperatures of any 

 given region wherever it is sufficiently populated. 



The permanent distribution is limited in a general way by the frost line. 

 Where frost does not occur, the species generally may breed permanently. We 

 have shown in the discussion of the influence of temperature that the species 

 does not thrive below a temperature of 80° Fahr., so that in a uniform climate 

 with a temperature much below 80° the species could not continue to exist. 

 Such climates, however, are rare, and in general when frost never occurs the 

 general temperature is high enough to permit the species to flourish. Besides 

 this, the species no doubt continues in particularly favored locations north of 



