INFLUENCE OF ENVIRONMENT 235 



Rainfall has an effect on salt marsh breeding similar to that which 

 has just been described for the upland breeding species, but the domi- 

 nant water factor is the tide. The average elevation of the salt marshes 

 when in contact with salt marsh creeks is about 2.3 feet above mean tide 

 at Sandy Hook. Of course, from this elevation the marshes slope gradu- 

 ally downward as they approach the highland, reaching their lowest 

 point at the line of junction. Tides higher than normal are required to 

 cause water to flow over them. Tides sufficiently high for this purpose 

 come once each month. High tides also occur as the result of strong 

 favoring winds. 



Tides high enough to cover the grasses initiate mosquitoes but bring 

 killifish in to destroy the wrigglers. Tides not high enough to cover the 

 grass will water soak the marsh, but will not bring in the killifish be- 

 cause the grass stems keep them back. If the water from the latter type, 

 which is sometimes called a "trickle tide," remains long enough to per- 

 mit the brood of mosquitoes to complete development and emerge, it is 

 the greatest contributor to mosquito prevalence. Dry weather will often 

 dry up the pools left by such a tide before wrigglers can produce mos- 

 quitoes, or wet weaather by preventing evaporation may favor emer- 

 gence of large broods. 



FOOD SUPPLY 



Since accurate data on the food of mosquito larvae is rather scarce 

 and since this question is highly important in the practical work of 

 mosquito control, contributions made by Rudolfs (21, 22) in 1925 and 

 1926 are reproduced: 



"Last summer a number of samples were collected from places where 

 permanent and temporary mosquito-breeding occurred. Duplicate sam- 

 ples of water were taken, one with, and another without mosquito larvae. 

 The water was analyzed and microscopically examined for plant and 

 animal life (10). The mosquito larvae were dissected and their intes- 

 tines examined. Of the approximately fifty samples a few analyses and 

 results of examination are here given as typical cases. These examples 

 are presented as a preliminary work. 



"A short description of the places where the samples were collected 

 will give some idea of the environment of the breeding places. 



"Number 38. A ditch, about five feet wide and three to four feet deep, 

 stagnant water, with some scum on top. Ditch highly polluted by drain- 

 age from workingmen's houses in neighborhood. Prolific breeding of 

 Culex pipiens. 



"Number 39. A small pool in several acres of woodland. Many similar 



