260 MEDICAL ENTOMOLOGY 



Similarly many species ' aestivate ' during the hot and dry months in 

 the tropics, when, either from lack of water, or because the high tem- 

 perature and the dry atmosphere are in themselves unfavourable, the 

 eggs cannot be matured and laid. The adults which survive in this 

 manner are almost, if not quite, all females. 



In other cases, and particularly in the tropics, adverse conditions 

 are met by a retardation of the larval stage. The cold weather does 

 not kill off all the larvae, but it inhibits their growth and develop- 

 ment, so that this stage may last several months instead of the normal 

 period of a week or ten days. When the weather becomes warmer the 

 larvae again become active and complete their development ; the imagines 

 so produced give rise to successive broods with increased rapidity as the 

 weather becomes warmer. It is in this way that cut id fades is carried 

 over the cold months of the year in the Punjab. 



The eggs themselves, in some species at least, are able to resist 

 unfavourable conditions, such as dryness, for a considerable time ; the 

 case of Stegomyia fasclata has already been quoted. Cazeneuve records 

 some observations which indicate that they can also resist the action of a 

 very low temperature. He succeeded in hatching out eight Anopheles 

 larvae from a block of ice taken from a marsh in North China, where the 

 temperature frequently falls as low as -32 C. 



The question as to the manner in which mosquitoes spread from their 

 breeding grounds towards the places where they feed is an important 



one. As a general rule they do not travel far, as they 



Method of Distnbu- J . J 



tjon are -not structurally adapted for long nights or tor 



travelling in rough weather. At times, however, they 

 are found at a considerable distance from the nearest breeding places, 

 having been either carried there by the wind, or having performed a 

 series of short flights on successive days. One of the writers had 

 experience of the first method of spread in the Aden Hinterland, where, 

 on account of the proximity of infected Arab children and the great 

 prevalence of Anopheles, the British and Indian troops were suffering 

 severely from malaria. The camp was moved to another place about a 

 mile from the only breeding ground, the Wady Tiban, but unfortunately 

 it had to be placed on its leeward side. In spite of the distance numerous 

 Anopheles could be found in the tents daily, and the conditions were 

 almost as bad as before, although every precaution was taken to prevent 

 larvae being brought into the camp in the drinking water. In this case 

 the mosquitoes were blown across the open country. 



James records having found gravid females of fuliginosus in Mian Mir 



