(5) Through flying insects which are aquatics even when fully developed and 

 which come winging over from neighboring waters ( Notcnectae. water bugs), 



A repopulation after drainage is rapidly achieved and in many ways. Aa a matter of 

 fact, aquatics are plentiful again, shortly after replanting. 



It is for this reasoi that, personally, I do not believe that a two years' period of 

 operation, as proposed by Nordquist, will materially increase the fertility in German 

 ponds. The advantage, therefore obtained of greater preservation of aquatic life during 

 the winter months has its disadvantages, to wit: It interferes with proper stock regul- 

 ation and also will result in incon^ilete mud deterioration. 



Also in^jortant for mass productivity of aquatics, aside from their mode of intrusion 

 after the yearly drainage, is their rapid propagation, i.e., the rate of their generation* 

 per annum. Wundsch (1919) made the following classifications from this vlewpolnti 



(1) Forms of polyannual generations (large Ephemerae and Trlchopterae. some 

 species of dragcKi flies — Aeschnae — and a few large water beetles. 



(2) Forms of regular, yearly reproduction (small Ephemerae and Trlchopterae. 

 most of the dragon flies, larger Chironomidae. gnats, genuine water 

 butterflies, all Notcnectae and mollusks ) . 



(3) Forms of many yearly swarm periods (every month or every other month) like 

 small Chironomidae. genuine flies, mosquitoes, Amphipodae , 



(A) Forms of numerous yearly swam, periods (shore and bottom dwelling 

 Cladocerae). 



As regards Chironomus plumosis . the recent investigaticsis by Potonle. concerning their 

 yearly cycle — later contested — seem to attest one yearly swarm period, i.e one generation 

 per annum. This period occurs especially during the end of the summer but stretches— with 

 less intensity — all through the summer months, thus assuring a relatively regular coloni- 

 zation. 



The generative periodicity of insect larvae is also dependent upon food and weather 

 conditions. The better these are, the more swarm periods. 



The possibility of many generations per annum exist in the case of Cladocera where 

 parthogenesis during the sunmer months occurs. Their winter eggs fonn often only in the 

 fall or — ^in the case of unfavorable conditions of existence — issue from fertilized eggs. 



The discussed generative modus explains in part the following indlvidiial oaxliiae, 

 as formulated by TCundsdi. 



S 



Species of a spring mnYjimim (water fleas) 



Species of an early summer mnYimiim (larvae) of Chironomdae and of most 



Dipterae, mollusks 

 (3) Species of a surmer maximum (larvae of Ephemerae and Trlchopterae ). 

 (i) Species of late summer maximum (larvae of dragon flies). 



We have a characteristic example of the varying rate of productivity of fish food 

 during the summer months in the larvae of Chironomidae. the principal natural "provender". 



For a basis I refer to an investigation by Contag (1931), conducted for me at the 

 hatcheries at Eberswalde. Its results are almost completely Identical with iAvestigationa 

 made by Ifundsch (1919) in Sachsenhausen, who went about them by means of "stake scrapers", 



Contag's investigations were carried out with "bottom scrapers", which is the most 

 modem implements for quantitative examinations. 



36 



