152 MOSQUITOES OF NORTH AMERICA 



campa appear to be restricted in habitat, each to a particular plant, where they 

 prey upon the other mosquito inhabitants. 



Certain mosquito larvje may be said to be partly cannibalistic, feeding upon 

 their own kind and upon other species when pressed for food. This has been 

 observed with Anopheles, and in Brazil Aedes calopus and Limatus durhamii 

 have been shown to behave in the same way by Peryassu and his associates. 



It would seem, on the whole, that the presence of mosquito larvae in water must 

 be beneficial rather than the reverse, and this is probably the only good thing 

 from the humanistic standpoint which can be said about mosquitoes. Of course 

 there is no doubt as to the benefits derived from the larvae of Psorophora, Megon 

 rliinus, Lutzia and the like, since they destroy the larvae of other biting mosqui- 

 toes, including those forms which carry disease. As to the others, their effect 

 upon the water on the whole must be purifying, and there is an idea in parts of 

 Texas and others of the southern United States that where mosquito larvae occur 

 in the rain-water tanks and wells — the tanks in many places furnishing the 

 drinking water supply — this water is purer and safer than water which does not 

 contain these larvae. We have had drinking water served at table in small hotels 

 in the South, which contained Culex or Aedes larvae, and, on requesting a glass 

 of water without larvae in it, have been assured by the waiters that this was an 

 evidence of the purity of the water. This possibly restricted popular idea may 

 have a sound basis, since certainly mosquito larvae eat many organisms and 

 among these must often be such as are injurious to the human species. It must 

 be remembered, however, that with bacilli this function may not in the least 

 reduce their vitality and they may pass through the alimentary canal of the 

 mosquito larva unharmed. 



Recently Boyce and Lewis have attempted to show that bacteria thrive better 

 in the presence of mosquito larvae, and this may be true under some circum- 

 stances. They point out that there is some opposition to the destruction of the 

 larvae of the yellow-fever mosquito (Aedes calopus) in receptacles with drinking 

 water on account of their supposed purifying function. To test the truth of this 

 belief they made a series of experiments. These consisted in placing larvae of 

 Culex and of Culiseta annulatus " in a flask of non-sterilized drinking-water, 

 and comparing from day to day the number of bacteria present in the water with 

 the number present in a control flask to which no larvae had been added. The 

 number of bacteria was estimated by plating 1 cc. of the water in gelatine and 

 incubating at 21° C. for 72 hours." In all but one of the four experiments the 

 bacteria showed greater increase in the flasks with mosquito larvae. This in- 

 crease might easily have been due to the introduction of further bacteria together 

 with the mosquito larvae, although it finds its readiest explanation in the organic 

 substances discharged by the larvae. The experiments are inconclusive as they 

 were carried out under artificial conditions and as, furthermore, it is impossible 

 to avoid all sources of error. The fact that the larvae of different species dilfer 

 in feeding habits also must not be lost sight of as it will affect the bacteria. 



There is great diversity in the behavior of mosquito larvae. Some live almost 

 wholly at the surface of the water, others at the bottom, while many rise and 



