2 93 



THE GEOGRAPHICAL DISTRIBUTION OF THE 

 MOTHS OF THE SUBFAMILY BISTONINAE. 



J. W. HESLOP HARRISON. D.Sc. 



XL— THE GENUS BISTON (LEACH). 



Biston strataria (Hufn.). Distribution : — Central Europe, 

 The British Isles, Southern Scandinavia, Spain and Portugal, 

 Morocco, Algiers, North and Central Italy, Dalmatia, South 

 Russia and Asia Minor. 



Biston comitata (Warren). Eastern Siberia, China and 

 Japan. 



Biston robustum (Butler). Japan. 



Biston regalis (Moore). North India, Himalayas. 



The name Biston has, unfortunately, always been mis- 

 applied, Lycia hirtaria uniformly being quoted as Biston 

 hirtaria — a very natural mistake — for Leach included Lycia 

 hirtaria and Biston strataria, as well as Amphidasys betularia, 

 within his genus. Very early in the ' thirties,' however, both 

 Stephens and Westwood restricted the name to strataria which, 

 in consequence, becomes the type and carries with it the ad- 

 ditional species cited above ; and, in truth, comprising these 

 forms the genus is a very natural one. 



Just as when one throws a stone into a pool, of the ripples 

 formed, the furthest away from the centre of disturbance or 

 dispersal is the oldest, so B. strataria, located thousands of 

 miles away from what we shall demonstrate to be the metropolis 

 of the genus, is the most primitive species ; this fact its com- 

 paratively slight development of sexual dimorphism confirms. 



Indeed, so little has the physiological affinity between 

 our familiar B. strataria and the genus Amphidasys as ex- 

 emplified by A. betularia been diminished, that hybrids between 

 them have been successfully reared. Nor is the morphological 

 difference between them very striking, the only feature of any 

 importance being the absence in Biston of the posterior middle 

 tibial spurs. Now, as there exists no similar pair of species 

 within the groups, we must assume that Biston, in a form not 

 differing widely from strataria, arose from some exponent of 

 the Amphidasys of the A. betidaria-cognataria type by a muta- 

 tion resulting in the loss of the second pair of spurs. 



At first sight, we might venture upon the conjecture that 

 this may have occurred at any station in the European habitats 

 common to the two insects, but to any adoption of this view 

 many serious objections may be advanced. Amongst them 

 there is none greater than the fact that, of the two significant 

 betularia forms, B. strataria appears to be nearer the Palas- 

 typical insect, i.e., A. cognaiaria, and this, as we have seen 

 previously, is absent from Europe and, furthermore, has its 



1917 Sept. 1. 



