56 



holes in them ; to count in the morning all mosquitoes 

 which have entered during the night ; and to keep a 

 careful record of the observations made. If a marked 

 decrease occurs and is maintained, it will be safe to argue 

 that the insects have decreased at the spot where the experi- 

 ment is made. Perhaps, however, the most reliable evidence 

 upon this point will lie in a general consensus of opinion 

 on the subject. We must always, however, guard against 

 the sudden appearance of large numbers of mosquitoes in 

 single houses. People should be warned to report such 

 cases at once. Of course it will generally be found that 

 the insects come from some vessel or pool of water lying in 

 proximity to the affected house, and overlooked by the 

 mosquito gangs. 



It may now be asked, what percentage of diminution 

 in mosquito-borne disease may be expected to follow a 

 given percentage of reduction in the number of mosquitoes ? 

 I regret that I cannot as yet give any actual statistics on 

 the point, but we may perhaps attempt an estimate on 

 a priori grounds. We ask, are we to expect a decrease in 

 disease in the same ratio as the decrease in the number of 

 mosquitoes ; or in a duplicate ratio ? The disease will 

 probably diminish in a duplicate ratio. 



Suppose that in a given locality one in every thousand 

 mosquitoes is infective ; then, roughly, one in every thou- 

 sand bites suffered by the people will be infected bites. 

 Now, if we reduce the number of mosquitoes in the locality 

 by one-half, the mosquito bites also will be reduced by one- 

 half; and, consequently, only half as many people will now 

 become infected as was formerly the case. But, since the 

 mosquitoes themselves are infected by biting previously 



