Vol. Xxix] ENTOMOLOGICAL NEWS. 261 



In the rainy season nearly the whole of this area usually be- 

 came inundated and formed a large swamp with considerable 

 jungle growth. 



In 1 910. when the construction of the dam and spill wav had 

 progressed sufficiently to cause an appreciable interruption in 

 the flow of the Chagres River, great changes began to take 

 place in the topography of the lower Chagres V^alley. Dur- 

 ing that year the Chagres changed from a swift rolling river 

 with sea level at Gatun during high tide to a small lake, some 

 current being still retained through the portion occupying the 

 former river channel. This lake slowly widened and formed 

 estuaries both to the east and west as the low valleys permit- 

 ted. It also gradually extended southward with a quieting 

 effect on the former swift current. During the wet season 

 of 1910 this lake reached a depth of i8 feet. During 191 1 

 sufificient water was retained to increase the depth to 20 feet. 

 In 1912 a depth of 56 feet was present. In 1913 enough water 

 was released to reduce the depth to 48 feet, and the water was 

 at this level when the gates of the spillway were closed on July 

 I, 191 3. \\ ith the closing of all the gates the depth increased 

 from month to month until January i. 1914. when the surface 

 of the lake reached its required elevation, approximately 85 

 feet above sea level. At this elevation the lake has a surface 

 area of more than 170 square miles and a depth of more than 

 87 feet in its deepest parts. At some points it extends south- 

 ward to a distance of 9 or 10 miles outside the Canal Zone. 

 The irregularities of its contour causes it to have a shore line 

 of more than 1000 miles. 



Prior to the formation of this great artiiicial lake, Mcuisonia 

 titillans, the "lake mosquito" — as we are terming it on account 

 of its abundance in the lake regions — and its host plant, the 

 wild w^ter lettuce, Pistia stratiotcs, were present in compara- 

 tively small numbers in the Canal Zone. 



Before the waters of the lake flooded the "Black Swamp," 

 a few scattered masses of the water lettuce occurred in the 

 various small bodies of w^ater found within the area of this 

 marshy region. A few more small isolated colonies were also 



