140 



in 1919. The mosquito attacks during the rainy period in July and on the beauti- 

 ful autumn days in August — September were but slight. 



The regular examination of the above-named 40 ponds gave as main result 

 that there are a few ponds in which only a single species is hatched. This is how- 

 ever an exception; in most of the ponds a greater number of species are hatched. 

 I have made schemes for all forty ponds, but as they resemble each other very 

 much, I will only mention a single one of them as a model. It belongs to the 

 Stenholtsvangs ponds, about two kilom. from Hillerod. This pond, which was under 

 observation for four years 1917—1920, in 1917—1919, in all three years, gave quite 

 the same result. In this pond seven species are hatched; the seven species follow 

 each other with quite the same invariable regularity. In January there live plenty 

 of C. morsitans larva? and a few larva? of O. rusticus; in free water on sunny days, 

 hetween ice and land, diminutive larva? of 0. communis. This fact is unaltered till 

 April when the ponds thaw. Then the ponds teem with larva? of 0. communis, the 

 water is practically a black living mass of larva?; the larva? of Culicella morsitans 

 have retired to the deeper parts of the pond; when dredging in the surface we only 

 get very few larva? of this species. Suddenly in the course of a week the whole 

 bulk of O. comnnm/.s-larva? are altered into pupa?, and in the same weeks the in- 

 sects leave the ponds as imagines. But simultaneously with the metamorphosis into 

 pupa of O. communis, and most probably a little earlier, eggs of 0. cantons have 

 been hatched; already a fortnight later this species, owing to the much higher 

 temperature, leaves the pond as flying insects. A little later the eggs of C. diantwus 

 and Aedes are hatched, and in June these species are imagines. Simultaneously with 

 them C. morsitans leaves the pond and so also 0. rusticus. In June the pond is 

 wholly dry. 



The dry bottom of the pond contains no mosquitoes in any stage in June. In 

 the following three months the case alters again, and all the species find their way 

 to the pond again, laying their eggs on the dry bottom. Side by side, perhaps under 

 the same leaf, the eggs of the different species slumber the long summer sleep; in 

 autumn, when the rain begins, the deepest part of the bottom takes some water, 

 often only a few liters. This is enough to revive the eggs of C. morsitans and a 

 little later 0. rusticus. The larva? of these two species appear, but the moist condi- 

 tions have not the slightest influence upon the eggs of 0. communis, cantans, dian- 

 tceus and Aedes. Before the eggs of these species can be hatched, they must, besides 

 being burned by the sun, also be frozen in the ice, and not till January are any 

 of these eggs hatched. At this time only the larva? of 0. communis appear, the 

 others do not appear before they have passed the whole winter in the egg stage, 

 the larva? not leaving the eggs before April. The two species C. morsitans and 0. 

 rusticus pass the winter as larva? under the ice. 



In 1920 the development of the mosquito life in the pond apparently gave a 

 picture of a somewhat different kind. The facts were the same as regards April; 

 larva? of C. morsitans and 0. rusticus were found on the bottom, and the water 



