The relationship between the palaearctic and nearctic faunas 243 



ance with the proportions displayed by the faunas of countries bordering the North 

 Atlantic;! 



Macrolepido- Butterflies Noctuid Geometrid 



ptera, total (Diurna) moths moths 



Newfoundland (Krogerus, 1954) 280 38 = 14% 126 = 45% 89=32% 

 Greenland (Henriksen, 1939) 29 4=14% 17 = 59% 6 = 21% 

 Iceland^ (Lindroth, 1931) 28 — 14 = 50% 12 = 43% 

 Norwayi (Haanshus, 1933) 720 96=13% 257 = 36% 237 = 33% 

 British Islesi (Kloet & Hincks, 1945) 850 66= 8% 336 = 40% 285 = 34% 

 Amphiatlantic and "Westarctic" spe- 

 cies 16 1= 6% 13 = 81% 2=13% 



Evidently, Noctuid moths are strongly over-represented among Amphiatlantic 

 (and geographically related) Lepidoptera. The explanation is no doubt that the 

 members of this group are superior to other Lepidoptera in the art of flying, v^^ith 

 the exception of the Hawk-moths (fam. Sphingidae) which, however, are no resi- 

 dents of arctic and subarctic regions. The Anartas numbering no less than 6 

 of the 13 species, are extremely rapid flyers, on the wing both night and day, 

 though, it must be admitted, their perseverance of flight has not been investigated. 

 Also, among the Butterflies, the genus Colias contains some of the most capable 

 flyers; especially nastes and hecla, almost confined to the arctic region, are able 

 to withstand even rather strong winds.— No records were available to me of the 

 flight powers of the two Geometers concerned. 



Crymodes exulis (fig. 38) is the member reaching farthest east of a series of 

 three Noctuids of American origin which via Greenland have proceeded at least 

 as far as Iceland on their air-borne advance (p. 254). 



An instance of at least partly active dispersal across the North Atlantic is formed 

 by the famous Monarch Butterfly {Danaus plexippus), a North American species 

 regularly appearing in Britain in spite of the fact that its food plant (Asclepias) 

 is not indigenous there {vide Ford, 1945, p. 157-159). 



2. Animals especially suited for passive aerial transport. This applies to all of 

 the Araneae listed above, 21 in number, which, at least as young, disperse them- 

 selves by "ballooning". Even if it is admitted that our present knowledge of the 

 distribution of spiders is far from complete, implying that certain of the listed 

 species actually belong to a more or less circumpolar type (but also that others, not 



^ Some of the figures are not quite up to date but this hardly affects the records of per- 

 centage. 



2 I have later received a revised list of Icelandic Lepidoptera by Mr. Niels L. Wolff, 

 Hellerup (Denmark), which, however, does not essentially alter the percental figures given 

 above. 



