262 ENTOMOLOGICAL NEWS. [July,'l8 



to be found in a low-lying marshy area in the vicinity of An- 

 con and Balboa. A few plants were also reported present in 

 a marshy area in the vicinity of the upper Trinidad River. 



The spread of this plant at that time was very slow. The 

 isolation of the colonies in shallow pools, which became almost 

 devoid of water during the dry season, proved rather unfavor- 

 able for its multiplication at the Pacific end of the Zone. The 

 scattered small masses in the Chagres \'alley were prevented 

 from increasing to any great extent by the fact that when 

 heavy rains occurred all colonies that had spread from the 

 ponds and pools, in which they were growing, to the small 

 streams, were invariably swept down by the increased current 

 of the latter to the Chagres. They were then carried along on 

 the surface of this river to the sea. However, with the rising 

 of the lake and the cessation of the currents the rivers were 

 gradually changed to sluggish bodies of water. The plants 

 then left the small pools to which they had been previously 

 confined and floated on the surface of the rising waters, driven 

 about by the action of the winds. They were soon carried to 

 the outer periphery of the inundated lowlands, where the 

 thick forest growth and stagnant waters ottered good protec- 

 tion and opportunity to flourish under these altered condi- 

 tions. Consequently they increased so rapidly that large float- 

 ing islands were soon formed. This rapid expansion con- 

 tinued until today in some parts of the lake these plants may 

 be observed covering the surface of the water in masses sev- 

 eral miles in diameter, reminding one of bright-green, level 

 meadows. 



Before the increase of the Pistia took place in the Canal 

 Zone, but few specimens of its associated mosquito, Mansonia 

 titillans, were to be found here. In writing of this mosquito 

 Knab^ remarks, "In working over the mosquito material from 

 the Canal Zone collected by Messrs. Busck and Jennings from 

 1907 on, the absence of this characteristic species was most 



1 Knab, Frederick. Changes in the Mosquito Fauna of Panama. 

 Proceedings of the Entomological Society of Washington, Vol. XV, 

 No. I, page 41, 1913. 



