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THE INFLUENCE OF DROUGHT UPON MOSQUITO LIFE IN SURREY. 



By Malcolm E. MacGrrgor, 



Wellcome Bureau of Scientific Research 

 {Wellcome Field Laboratory, Wisley, Surrey). 



The unusual and prolonged drought that we have experienced this year has had 

 a marked effect upon the mosquitos. So much so, that while last year I found no 

 difhculty in obtaining any of the eleven species which so far have been found to exist 

 in this locality, yet for many weeks now it has been impossible to obtain any 

 but a few Anopheles maculipennis (adults). Anopheles bifurcatus (larvae), Culicella 

 morsitans (larvae, pupae, and adults), and Culex pipiens (larvae, pupae and adults). 



As I have been anxious to get specimens of the other species, which were common 

 around here last year, in order to continue certain observations, I have made prolonged 

 searches for them in the usual breeding-places, only to find that with most of the water 

 collections dried up none of the species required was obtainable. 



Anopheles bifurcatus. 



Normal conditions. — As larvae and pupae ; plentiful in slowly moving streams 

 throughout the year. As adults ; fairly numerous in pigsties from end of March 

 to June, but unobtainable later. 



Conditions during drought.— \s larvae and pupae ; unobtainable, except in an 

 abnormal breeding-place (well). As adults ; unobtainable since late in June. 



Last year I found that large numbers of Anopheles bifurcatus larvae were to be 

 found in a certain stream at Ockham throughout the summer and winter of 1920 and 

 until the streams dried up in June of this year. The only permanent water in the 

 vicinity was a small lake in which no Anopheline larvae were discovered last year. 



I thought it probable that the A nopheles bifurcatus females after the stream had 

 dried up would be forced to oviposit on the water of this lake, but repeated search has 

 failed to discover larvae in the lake even under the present conditions of water 

 shortage. 



Soon after, having made an extensive search for Anopheline larvae along the banks 

 on all sides of the lake, I came across a disused well in an open field near-by. Its 

 top was covered by a loosely fitting lid, and the water in the well was about five feet 

 deep, standing at about ten feet below the well-mouth. When examined, the wafer 

 was found to be crowded with the larvae of Culicella morsitans, associated with which 

 were numerous larvae of Anopheles bifurcatus in all stages of growth. Such an 

 unusual situation for Anopheles bifurcatus larvae, moreover in water that was turbid 

 and foul, is surely an outcome of the scarcity of water and the loss of normal breeding- 

 places. It is nevertheless difficult to account for the females' choice of this situation 

 instead of the water of the lake, around whose weed-protected bays the eggs might 

 be laid in security against attack by fish. Under normal conditions. Anopheles 

 bifurcatus chooses water that is clear and cool on which to lay her eggs. 



The well, however, is the only situation where Anopheles bifurcatus larvae have 

 been found, and the proportion of adults of this species which can be emerging at 

 present must be infinitesimal compared with the normal. 



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