258 MEDICAL ENTOMOLOGY 



hatch, but most of the larvae attain maturity and become adults. The 

 practical importance of this is evident, for the wells provide a source 

 of supply from which the temporary collections of water, which must 

 inevitably be formed each year, can be stocked with larvae, and unless 

 they are dealt with the appearance of a large number of mosquitoes 

 at the time of the rains will continue. 



In the Punjab Christophers has found that the main permanent 

 breeding grounds of Anopheles are the large river beds, such as the 

 Jumna and Beas, jheels and large tanks rilled with aquatic vegetation, 

 extensive brick fields and excavated tracts, all of which are common 

 around most large towns and even small villages, and lastly the irriga- 

 tion channels. After the rains temporary breeding grounds are formed 

 by pits and excavations, and shallow accumulations of rain water. 



Christophers found numbers of the larvae of cttlicifacies and fitliginos- 

 ns in large sheets of rain water even when there was no obvious 

 food. As this water begins to dry up the larvae are to be found in 

 the deepest holes, and the next shower of rain enables them to complete 

 their development. These extensive collections of water and the resul- 

 ting deep pools form the most important temporary breeding places for 

 the larvae of Anopheles in the Punjab. 



The seasonal prevalence of mosquitoes depends to a large extent, but 

 not entirely, upon the presence of suitable breeding places. Stegoinyia 



fasciata, for instance, occurs all the vear round in 

 Seasonal Prevalence . , , . , , , ,- 



tropical places with an equable climate ; Cutex fcttigans 



is common throughout the year in Madras, but its numbers are greatly 

 increased in November and December after the rains. A marked 

 seasonal prevalence has been observed among anopheline mosquitoes 

 by workers on malaria in the Punjab. As a general rule all the 

 species are most abundant in the latter half of the year, following on 

 the rains in the early part of July, and diminish as the dry cold weather 

 advances, until there are only a few left in April, when the hot weather 

 commences. The season of the year does not, however, affect the pre- 

 valence of all the species in the same way. A. culicifacies, for instance, 

 disappears entirely during the cold months of the year, and reappears 

 as the hot weather approaches, although the number of breeding places 

 available by that time is necessarily diminished. James found that in 

 Mian Mir it disappeared about the end of November, to return about the 

 middle of March or the beginning of April ; in Ferozpore it is absent 

 from December to May (Adie) ; at Amritsar it appears in February and 

 March, and is to be found, though in gradually diminishing numbers, 



