PLANTS AND MOSQUITO CONTROL MATHESON 415 



to solve the riddle of this selective breeding. It may be admitted at 

 once that this riddle is far from solved, though the chemical and 

 physical properties and the fauna and flora of breeding places have 

 been rather intensively studied by various workers. 



The early investigators, noting the absence of mosquito breeding in 

 certain ponds, pools, etc., tried to offer explanations for such condi- 

 tions. These explanations ran the gauntlet of a wide variety of sur- 

 mises, usually observing that the water was unsuitable, that there was 

 lack of the necessary food (though the food requirements were not 

 then known and are not even at the present time), or that the exces- 

 sive plant growth covered the surface of the water so as to prevent 

 the larvae from obtaining air, or that filamentous algae were abun- 

 dant in which the larvae became entangled and died or that numerous 

 natural enemies, as predacious fishe?, insects, etc., were present. 



That aquatic plants may act as destroyers of living organisms was 

 observed quite early. Mrs. Treat (1875) noted that the bladders of a 

 TJtricularla sp. contained many organisms, including insect larvae, 

 and she tried to solve the mystery as to how these animals were caught 

 and if the plant used them as food. Darwin (1875) in his " Insectiv- 

 orous Plants " gives a much more detailed account but failed to solve 

 the mystery of how the organisms were trapped. Brocher (1911) 

 fully solved the process, which was later confirmed by Hegner (1926). 

 Different workers in various parts of the world have called attention 

 to this group of carnivorous plants and their possible utilization as 

 agents in mosquito control. 



PREDACIOUS PLANTS 



The commonest species is probably Utricularla vulgaris (pi. 1, 

 fig. 1). It often occurs in great abundance floating directly below 

 the surface of the water. The numerous small bladders and finely 

 divided leaves distinguish it quite easily. The bladders are most 

 interesting structures and have been fully described by Brocher and 

 Hegner. These bladders, when " set " for the capture of prey, have 

 their side walls rigidly compressed; organisms then enter the outer 

 vestibule and by their movements stimulate the closing valve which 

 suddenly opens, the side walls expand and the inrushing water car- 

 ries the entrapped animal within. The valve then closes and the 

 organism is firmlv held. Witliin the bladders the animals are 

 gradually digested and furnish food for the growth of the plants. 

 The enormous amount of food taken by those plants m^y be illus- 

 trated by some of Hegner's observations. He found that the bladders 

 on parts of a plant 220 cm (nearly 7 feet) long contained approxi- 

 mately 150,000 small crustaceans, besides other organisms. From the 

 standpoint of mosquito control the question may be asked, do these 



