223 



strata of the water, the temperature becomes largely independent of 

 the depth. Thus larvae of A. maculipennis were found in places where 

 the water was 7 feet deep but completely invaded by Potamogeton 

 pectinatus. In the canals the deposition of eggs and the appearance 

 of the larvae usually occur at a later date than in the marshes ; the 

 water is colder and suitable vegetation is lacking at the end of winter 

 and early in spring. In the hot months the contrary obtains ; the 

 marshes diminish in importance and the canals become serious foci. 

 They produce Anophelines up to the autumn, and the last winged 

 specimens issue from them in November. Spirogyra and some other 

 plants appear in some canals and form in July and August a covering 

 that prevents access to the surface. It is for this reason that Lemna 

 has been recommended against Anophelines, but too much must not 

 be expected of this plant. It cannot thrive if oiling is carried out, 

 and thus nothing impedes the development of the larvae until the 

 plant completely covers the whole of the water ; indeed as long as 

 the covering is scanty it favours the larvae. 



Experiments with solutions of common salt showed that a degree of 

 salinity greater than 9 per mille is not suited to the larvae of A. maculi- 

 fennis or A. hifurcatus, though the pupae are able to stand stronger 

 concentrations, such as 24*8 per mille. In the laboratory tests there 

 was nothing to show that the adults could detect the salt dissolved 

 in the water and regulate oviposition as a result. From the practical 

 point of view it was worth ascertaining thf time required l)y a con- 

 centrated solution to kill the larvae. It was found that a strength 

 of 22 per mille caused death in 10 hours, and a strength of 18 per 

 mille caused death in 24 hours. Unfortunately, such high strengths 

 are not always feasible, and an initial high strength is gradually diluted 

 by fresh water filtering in. In a further experiment it was found 

 that such larvae as had not been killed by immersion in a salt solution 

 survived after transfer to fresh water. 



The adults of A. ynacnlipennis begin to hibernate in mid-October. 

 Reproduction usually ceases at this time, and food taken on several 

 consecutive occasions may be used for forming adipose tissue instead 

 of for developing the eggs. 



From February to April the hibernated adults of A. macuUpennis 

 oviposit and die. Early in April the new adults appear. There is 

 therefore no interruption in oviposition, and thus adult specimens- 

 of A. macttlipeymis occur throughout the year, which is not the case 

 with A. bifurcat'Hs. Whereas in winter there is no relation between 

 feeding and fgg-development, in summer the relation is very close, 

 and the eggs are matured and laid after one meal. Anophelines do 

 not bite again until they have digested the blood previously sucked. 

 Generally speaking a given mosquito is unable to infect more than 

 one person in the same night, its capacity for another feed being 

 determined by the time required for digestion in the different months. 

 Under experimental conditions Anophelines may feed while their 

 eggs are in various stages of development. Under natural conditions 

 in summer, A. macnlipemiis, having matured its eggs by one feed, 

 does not feed again until the eggs have been deposited. It appears 

 certain that both the winter and summer achdts are able to oviposit 

 several times. In natural surroundings the adults emerge both by 

 day and night. 



