50 



BR. J. ENT. NAT. HIST., 7: 1994 



Table 3. Numbers of two forms of /. aversata taken in paired m.v. light traps at Juniper Bottom, 

 Box Hill, Surrey, 22.vi-6.vii.1990. Traps of a pair were sited 20 m apart, one inside and one 

 outside mature yew woodland. (From Jones et al., 1993.) 



Unbanded Banded Total 



Inside yew 51 27 78 



Outside yew 23 3 26 



The banded form is the nominate form. The unbanded form is f. remutata. 

 Test of unbanded vs banded, inside vs outside yew woodland: x, =5.05, P<0.05. 



higher than in the chalk grassland, the converse being the case for both the typical 

 and half-melanic classes, but the differences were very slight. 



Discussion 



Jones et al. (1993) suggested two possible causes for the presence of higher 

 frequencies of melanic and dark banded forms under dark canopies than in more 

 open habitats. Either, as a result of selective predation by birds in the past, 

 morph-specific habitat preferences have evolved so that their degree of crypsis and 

 thus protection from visually hunting predators is maximized. Alternatively, in the 

 absence of such preferences, continual bird predation, by eliminating light forms 

 in dark habitats, and dark forms in open habitats, may maintain the frequency 

 differences year on year. Several authors make the assertion that the latter is an unlikely 

 explanation because the level of differential bird predation would have to be very 

 high to maintain abrupt frequency differences over such short distances (Majerus, 

 1989; Jones et al., 1993; Aldridge et al., 1993). However, Jones (1993) has shown 

 that sharp clines in morph frequencies may be maintained by relatively weak 

 differential bird predation, as long as dispersal distances are not high. 



Aldridge et al. (1993) put forward a third hypothesis, namely that differential 

 habitat selection has evolved for reasons not associated with bird predation and the 

 degree of crypsis. They note, as a possibility, that the night temperature would 

 generally be higher under closed woodland canopies than in the open. As the degree 

 of irradiation from a surface is at least in part dependent on its colour, a dark surface 

 radiating heat faster than a pale one, they suggest that melanics may be at a 

 disadvantage if they fly in cooler more open situations. However, in this context it 

 must also be pertinent to ask why some species appear not to show morph-related 

 habitat selection. 



While the data available are not sufficient to allow more than speculation on 

 the three alternative explanations of the data, consideration of the origin and 

 genetic control of melanism in the species which do, and those which do not show 

 abrupt frequency differences, may be informative. In all the species in which 

 sharp morph frequency differences over short distances have been recorded previously, 

 the melanism is thought to be controlled by one or two genetic loci, and to be of 

 relatively ancient origin (Jones etal., 1993; Aldridge et al., 1993). This is significant 

 because Howlett (1989) has demonstrated that morph-specific behavioural differences 

 will take substantial amounts of time to evolve. The likelihood of morph-specific 

 habitat preferences evolving will thus increase with time, and be more frequent in 

 species in which melanism is of relatively ancient origin. 



In E. bistortata, Kettlewell (1973) cites melanism as being '? industrial', from 

 a number of sites in southern England and Wales. The inheritance of melanism 



