178 GNATS OR MOSQUITOES — CHAPTER VIII 



It is needless to remark that besides the above almost 

 every variety of soil is to be met with in so large a country 

 as the Indian peninsula, but the above are the only two 

 cases with which I have any personal familiarity which are 

 sufficiently extensive and peculiar to require any special 

 mention. In all probability what has been remarked with 

 regard to the Gangetic alluvium will apply also to other 

 tropical and subtropical alluvia, such as those of the Nile 

 and Mississippi, but I have not been able to obtain any 

 special information on such points, except as regards 

 India. 



Infiuence of Vegetation. — As the three essentials to the 

 well-being of all species of Mosquitoes, vegetable food, 

 shelter from the sun during the heat of the day, and the 

 presence of puddles wherein to rear their young, are all 

 either dependent upon or greatly favoured by the presence 

 of vegetation, it is obvious that its amount and character 

 is a factor that must always be considered in estimating the 

 potentialities of malaria in any given place. 



Open grassy plains have long been known to be 

 unfavourable to malaria. In such situations gnats can find 

 no sufficient shade, and such puddles as form are hidden 

 from them by the closely crowded stems. I am inclined to 

 believe that the hiding of the surface of pools by close 

 vegetation tends to prevent Mosquitoes from using them as 

 breeding places, and that this fact might often be made use 

 of as a cheap and effective means of sanitation. 



There can be no doubt e.g., that the presence of certain 

 water plants in some way prevents the appearance of 

 Mosquito larvae in the water covered by them, and the 

 only reason I can suggest for this is that the plants act in 

 this way because they hide the water from the female gnat 

 searching for a suitable place in which to deposit her eggs. 

 At any rate it is difficult to otherwise account for the 

 curious fact that in the Benares public gardens, where' there 

 are some scores of the small irrigation tanks described 

 below, Culex and A^iopheles larv£B, alone or in company, 

 were present in every tank save those that were covered 

 with a peculiar floating water plant, looking much like a 



