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in organic matter; at higher temperatures the sides of the aquaria are covered with 

 algae, and the water contains numerous Infusoria. 



On the other hand we often find ponds where life conditions for the mos- 

 quito-larva? are so difficult that they highly influence the development. 



On an excursion in Gripskov 15/iv 1919 I found many ponds with almost 

 fullgrown 0. communis larvae; a single pond lying in thick forest, and never getting 

 a single sunbeam, was still ice-covered; in the free stripe of water near the border 

 were many newly hatched 0. communis larvae, hatched almost a fortnight later 



Textflg. 8. Pond in Store Dyrehave. O. communis, rusticus, cantons. 



than in all ponds in the neighbourhood. The cold influenced the size of the ima- 

 gines, this being only half of the normal. 



Of two of my Mochlonyx-ponds one lies free on the top of a hill, without 

 forest and fully exposed to the sun; the other lies in a deep hole, overshadowed 

 by willows and hardly ever sun-lit; the first is very shallow, has never more than 

 about one or two decim. water and dries up in May, the other is about one m. 

 deep, and dries up three or four weeks later; on sunny days the tp. in the former 

 is about 10 degrees above that of the latter. In 1919 the imagines were hatched 

 already on 6/y in the first pond, and the whole vegetation was covered with numer- 

 ous 0. communis; a few days later, the pond w r as almost quite dry. In the second 

 pond the pupae did not arrive before 26/y then the pond was covered with a thick 

 layer of millions of pupae, and not before 29/y were the pupae hatched. 



D. K. D. Vidensk. Selsk. Skr., naturvidensk. og mathem. Afd. 8. Rrekke, VII, 1. 



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