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ponds in the first days of April, which look as if they were newly-hatched, are 

 really about three months old. In animals of such a small and delicate structure, 

 the need for atmospherical air is by no means as high as for larger ones which, 

 for a long time of their life, are covered with a much thicker cuticula; the respira- 

 tory conditions for cutaneous respiration in these ponds are also much better at 

 low temperatures than at higher. In early spring, immediately after the melting 

 of the ice, all our small ponds in our woods, the bottom of which is covered with 

 decaying leaves, teem with mosquito larvae; at that period in most of the explored 

 localities these almost all belong to one single species, the most common of all our 

 mosquitoes 0. communis. In the three years 1917 1919 the ponds thawed at diffe- 

 rent times, in 1917 from 5/iv | o 25/ IV , in 1918 already in the last days of March, 

 and in 1919 Viv. Nevertheless C. communis was hatched as imago almost at the 

 same time all three years, in 1917 about 6/y, in 1918 about 15/v, and in 1919 

 about 10/v. In 1920 the temporary ponds were open already by Wn and then con- 

 tained huge masses of halfgrown larvae of 0. communis. In a mild period in the 

 first half of April they pupated; then a very cold period came, tp. of air hardly 

 ever more than five to seven dg., and the development was great retarded. The 

 ponds were often visited; a layer of living pupae covered the surface but ima- 

 gines did not appear before lo/v. I got the impression that an enormous amount 

 of pupae were destroyed by larvae of Dytiscs, by Phryganids (Phryganea minor) by 

 Hydrometridce and Notonecta. 



At the beginning of the period we often find newly-hatched and halfgrown or 

 even fullgrown larvae among each other; side by side we further find ponds in one 

 of w r hich all larvae are fullgrown, whereas in the other many larvae are almost of 

 the same size; the pupation takes place almost on the same day, or at any rate 

 in the same week. Further it is very interesting to see, that the hatching of ima- 

 gines in the different ponds, but in the same latitude, almost everywhere in the last 

 part of the period takes place simultaneously; in North Seeland hundreds of forest 

 ponds have sent out their O. nemorosus material almost in the same week, and in 

 the years 1917 1920 this always lay between 6/ v and 15/v. 



The different development at the beginning of the period is intelligible, because 

 heavy showers suddenly raise the watermark line in the ponds, carrying a new 

 border of eggs outwards, giving them conditions for development. But also the 

 peculiar fact that, in spite of the different time for hatching from eggs, all larvae 

 are ready to pupate simultaneously, is intelligible. For it must be remembered that 

 these latest hatched larvae grow up under much higher temperatures and much 

 better conditions for rapid nutriment than those hatched in winter in ice and snow. 

 From my observations in my numerous cultures in my laboratory I can show that 

 I am always able to control the rapidity with which I wish the development of 

 the larvae to take place. Kept at temperatures between and 5 degrees, the develop- 

 ment lasts about four months; kept at 15 20 degrees it is finished in at fortnight; 

 at low temperatures the water in my aquaria is very limpid and undoubtedly poor 



