30 



found in enormous masses. Then the larvae have only one thing to do: to dig 

 their holes near the borders of the pond and pupate. 



It is further of interest to see how the larvae of a pond which belong to the same 

 species almost all follow each other with regard to the rate of development. In 

 periods of cold rainy weather the larvae can be forced towards their last ecdysis 

 and then stop in their further development, awaiting the days when the tempera- 

 ture is higher, when it is bright sunshine and calm weather. When such days 

 come, suddenly the whole bulk of larvae are transformed into pupae in the course 



Textfig. 1. My mosquito cultures. 



Textfig. 2. My mosquito cages. 



of only a very few hours. When at nine o'clock in the morning I have passed 

 one of my experimental ponds, I have often found several larvae; when at twelve 

 o'clock I passed the pond again, the majority of the larvae had been transformed 

 into pupae. More than once when I have taken material from such a pond, in which 

 there still remained plenty of unaltered larvae, it proved that these larvae belonged 

 to another species whose temperature of transformation lay at a higher point than 

 that of those first transformed. This has more especially been the case with the 

 larvae of the two species 0. communis and 0. cantans, the latter species being always 

 about one or two weeks later than the former. 



Now when we remember that the whole period of life which the mosquitoes pass 

 in the pupa stage is normally restricted to a few days, and in the tropics often to 



