The relationship between the palaearctic and nearctic faunas 



275 



c d 



(Photo P. Ardo.) 



FIG. 44. Bembidion grapei Gyll., a Carabid beetle showing polymorphism of the hind- 

 wings. Types a-b are counted as macropterous, c-d as brachypterous. — Ice- 

 landic specimens. 



springs, whatever its effect otherwise, will never be able to withstand an approach- 

 ing land-ice. But thermal activity, in Iceland often giving the springs a temperature 

 above boiling-point, may considerably ameliorate the loco- and microclimatical 

 conditions of a possible ice-free refuge. 



This opinion is confirmed by the occurrence of a Carabid beetle, Bembidion 

 grapei Gyll., in Iceland. It shows dimorphism or, rather, polymorphism with 

 respect to the hind-wings: in some specimens these are full and no doubt function- 

 ing (2 specimens were found in wind-drift material in Finland; Palmen, 1944, p. 

 37), in others they are more or less reduced and cannot be used (fig. 44). Breeding 

 experiments with another Carabid beetle, Pterostichus anthr acinus 111. (Lindroth, 

 1946) have shown that the phenomenon has a heriditary base, "short wing" 

 being a dominant. The full-winged individuals are therefore homozygotes and 

 owing to their superior powers of dispersal usually form a genetically pure zone 

 at the periphery of the species' area. At any rate, predominance of short-winged 

 specimens is characteristic of old populations. 



Possibly more than one gene mutation is involved in the case of Bembidion 

 grapei (fig. 44), since different stages of wing-reduction occur, but the macro- 

 pterous individuals also in this species are homozygotes, as is quite evident from the 

 distribution in Fennoscandia (Lindroth, 1949, p. 401-405). The brachypterous 

 form is predominant along the Norwegian coast, within the area of ice-free refuges 



