71 



soft hairs, an inconspicuous row of thorns along the margin. Palpe well developed 

 with four apical digits. Mentum triangular with median tooth and about 14 small 

 teeth on each side. 



Colour commonly greyish or brown, the head almost black. 



Biology. Even in January, when the lakes are covered with thick ice, and 

 the landscape white with the snow, we are, on bright frosty days, ahle to feel the 

 heating power of the sun. If then on such a day, we examine the south exposed 

 borders of small ponds, lying in the beech-forest but in bright sunshine we shall 

 see that there is often a little stripe of water between the borders of the pond and 

 the ice edge; if the frost is severe, the stripe will disappear a few hours after 

 sunset, but will he formed again the next day if the weather is fine. By means of 

 a thermometer we are further ahle to ascertain the peculiar fact that the tempera- 

 ture in this stripe of water, only few inches from the ice edge, at the brightest 

 hour of the day, may rise to 7 — 10° C. ; sometimes I have even found 17° C. In 

 this stripe of water we find very many hibernating organs of the freshwater fauna : 

 Statoblasts, resting eggs, turions of water plants (Stratiotes, Myriophyllum etc.); 

 besides we also find a great part of the freshwater fauna which has hibernated 

 under the ice and now, in the water stripe, get the first feeling of approaching 

 spring. (Larv;e of Odonata, of Ephemerida? etc.; many small Mollusca, Crustacea etc.). 

 The enjoyment is but short; at four or five o'clock the temperature at the south 

 exposed borders of the ponds falls again, often below zero; but the above-named 

 temp, has been enough to hatch many of the hihernating organs, swept in autumn 

 by the wind on to the borders of the ponds, where they have been frozen in the 

 ice in November — December. This is the case with many Cladocera, and I suppose 

 also with the eggs of Phyllopoda. Simultaneously with them some insect eggs, which 

 have hibernated with them are hatched; among these eggs we shall in this connec- 

 tion especially pay attention to the eggs of 0. communis. 



It is a rather common fact, at all events for my district of exploration, to 

 find in January the borders of the partly ice-covered ponds teeming with newly- 

 hatched Ci'/e.v-larva?; all the larva 3 still carry their egg tooth; they are hatched on a 

 sunny day from twelwe to three o'clock, but run the risk of not meeting this 

 temperature again for months. In the winter of 1918 — 1919 the ponds thawed in 

 the first days of January and were open till -2/i; the larvae were observed in 

 these three weeks. In this period the air-temperature never rose above 2° C; we 

 had only very few sunny days, and the hours in which the temperature in 

 ponds was above one or two degrees, were certainly very few. By - 2 i the ponds 

 were frozen again, and did not thaw before the first days of April. Examining the 

 same ponds again, we still find the borders of the ponds teeming with very small 

 newly-hatched mosquito larva?. All are almost of the same size, a few have passed 

 the first ecdysis. From material taken into my laboratory on 13/i and wintered here 

 at low temperatures, near zero, we learn that the larvae do not grow at these 

 temperatures. I therefore suppose that a great part of these' larvae found in the 



