FISHES FOR CONTROL OF MOSQUITOES. 25 



tide gates often fail to function properly; the ditches are grown 

 ^ith vegetation; and inasmuch as most of these protected areas lie 

 below the level of high water natural drainage is sluggish and nu- 

 merous more or less permanent pools are formed. Third, bordering 

 the tidal marshes and often the meadows and extending as a fringe 

 up the creeks at and just above the limit of tide action is a zone of 

 typical fresh-water swamp v.arjdng in width with the topography. 

 Islands of this character also occur within the tidal area. 



The mosquito pests in this region are produced chiefly under the 

 second and third conditions. Prolific breeding is almost universal 

 where these conditions occur uncorrected. Under the first condition 

 it is exceedingly rare to find a mosquito larva in any place open to 

 free circulation of water. It is only on the margins of the tidal area 

 or at points where obstructions occur that breeding is found. This 

 corresponds exactly with the natural distribution of the killifishes. 

 They range freely with the tides, and mosquito larvae do not thrive in 

 l^laces to which they have easy access. While physical conditions in 

 the actual tidal currents are unfavorable to mosquito development, 

 there are large areas perfectly suited to it where larvse are rarely 

 or never found. Furthermore, wherever on such areas obstructions 

 occur which bar the entrance of the killies, such as dense growths of 

 vegetation at the heads of ditches and alterations due to construc- 

 tion work, which are frequent in this region, there breeding is likely 

 to occur. On the other hand, under the second and third conditions 

 instances were found where the killies were admitted to ditches or 

 creeks or had been planted in gravel pits and ditches filled with 

 stagnant water in which there was no mosquito breeding. Through- 

 out this whole region the contrast between mosquito producing and 

 nonproducing areas in their relation to tidal action and killifishes 

 is most striking. Almost invariably areas deprived of killifishes 

 from whatever cause become sources of prolific breeding of Culex 

 pipiens, while similar areas to which the killies have access are free 

 from larvae. There need be added only brief accounts of a few 

 specific examples selected from a large number. 



In the midst of an undyked and tide-swept area on Darby Creek, 

 not far from Corbindale, is an old farm now abandoned for agricul- 

 tural purposes. The drainage ditches on a dyke-inclosed field of 

 perhaps 20 acres have become so clogged and filled that they are 

 altogether ineffective. Surface water has filled what remains of the 

 ditches and has spread in a shallow sheet over the surface generally, 

 its depth varying with the rainfall. The whole field is heavily 

 grown with reeds, with patches of cat-tails and marshmallows in the 

 deeper pools, and a border of grasses, giant ragweed, and Cepha- 

 lanthus. It has reverted to the condition of the tidal marshes with- 

 out the tides and killifishes. Throughout the summers of 1918 and 

 1919 Culex 'pijnens bred heavily throughout this area. They were 

 found by the author on visits made on July 26, August 16, and Sep- 

 tember 13, 1918, and on June 2.5, 1919, and they were reported fre- 

 quently by Vernon Lock wood. On none of these occasions were any 

 Funduli found except on the last date a few in one of the closed 

 ditches. Immediately across the dyke bounding this field on one 

 side is a section of the natural marsn, and a wide open ditch which 

 carries the tide to it. This ditch and its laterals always swarmed 



