The relationship between the palaearctic and nearctic faunas 



291 



FIG. 51. Libellula qiiadrimaculata L., a generally 

 boreal, circumpolar Dragonfly with excellent means 

 of dispersal by active flight combined with wind- 

 drift. It is a well-known migratory insect. 



(According to Bartenef, 1935, Valle, 1952, Needham 

 & Westfall, 1955.) 



'-V, 



(From Svensk Insektfauna.) 



(c) The sea and the sounds between the continents of the North are regularly 

 closed by drifting ice, at least in the first half of the year, joining the Siberian coast 

 with Novaja Zemlja and via Spitzbergen with Greenland and, usually, northern 

 and eastern Iceland. On the American side, continuous drifting ice every spring 

 goes right down to Newfoundland and the Bering Strait is regularly closed by ice. 



The ice of the Polar basin, stable or drifting, is often traversed in winter by 



