FLEAS. 9 



Occasionally fleas, particularly the human flea and dog flea, may 

 breed out of doors. Mr. D. L. Van Dine has recorded an instance in 

 Hawaii where a lawn was infested with the dog flea, and instances are 

 known in the United States of this flea and the human flea breeding 

 in protected situations, as under shrubbery or in the shade of build- 

 ings in sand which contains a considerable amount of animal or vege- 

 table matter. 



In nearly all cases the breeding places are very closely associated 

 with the haunts or resting places of the host. Instances where adult 

 fleas get upon man well away from such haunts must usually be con- 

 sidered as being the result of the adult fleas having become detached 

 from a host rather than by the fleas having been reared in such 

 situations. 



FACTORS INFLUENCING FLEA ABUNDANCE. 



Everyone familiar with the flea knows that there is marked sea- 

 sonal variation in abundance and often distinct variation from year 

 to year. As has been stated, fleas continue to breed throughout the 

 year in California and in parts of our Southern States. This is even 

 more marked in India, Panama, and other tropical countries. The 

 human flea and dog flea are seldom found to worry man during the 

 winter months. This is explained by a falling off in the rapidity of 

 breeding, the comparative inactivity of the adult fleas, and, as Mr. 

 Mitzmain has shown, the tendency for the human flea to remain 

 largely on the lower animals during winter. Throughout the United 

 States the fleas which attack man are most prevalent during the 

 summer months. In India there is a marked decrease in numbers 

 with the oncoming of the hot, dry season. This was particularly 

 noticeable in the case of the European rat flea, which, according to 

 observations of the Indian Plague Commission, began to disappear 

 early in April, and from May 15 to the beginning of November not 

 a single specimen was seen. 



The variation from year to year is no doubt principally due to 

 weather conditions. Dr. Howard states that he believes the years 

 of greatest flea abundance are those in which the summer rainfall is 

 above normal. No doubt humid summers, even though the rainfall 

 were not abundant, would produce the same results. These statements 

 are borne out by the effects of dry conditions on the various stages of 

 fleas as observed by several investigators. 



Although fleas of one kind or another are to be found all over the 

 United States, there are certain regions where one or more species 

 are especially abundant. In general, in those portions of the coun- 

 try where mild winters and comparatively humid summer atmos- 

 pheric conditions are the rule fleas are found most prevalent. The 

 amount of rainfall is also a factor in this regional abundance. While 



92999°— Bull. 248—15 2 



