64 THE entomologist's eecord. 



The variation of Polia chi, Linn. 



By J. W. H. HARRISON, B.Sc, F.E.S. 



I do not wish, in these few notes, to tabulate the many forms 

 of Polia chi, but I desire to point out what are, in my opinion, the 

 causes of the variation. I have worked at this species for about five 

 years, and during the present season (1905), I have captured and classified 

 considerably over a thousand specimens. Most of these, after inspection, 

 have been liberated. When I first began to take the species, I thought, 

 as many do, that the latitude and the mean temperature to which the 

 insect was exposed in all its stages, had to do with the change in 

 colour, but I have since seen the error in that. I have seen specimens 

 from Nenthead, Cumberland, which were the whitest I have ever had. 

 Yet that place is so bleak and cold, that even the common sparrow 

 fails to maintain itself. This seems to indicate that low temperature 

 is not the determining factor. It is generally offered as an explanation 

 of the evolution of the aberration olivacea, that it is intended to further 

 protect the species, by causing it to assimilate with the lichens on 

 stone walls. I have several objections to that statement. We generally 

 admit that the coloration of Bryophila perla, which undoubtedly 

 mimics the lichens on walls, is a fairly effective protection. Yet, when 

 I first saw Bryophila perla at rest several years ago, and even after I 

 took it, I was under the impression that it was a dwarf form of P. chi, 

 for, since B. perla does not occur here, I had only experience with set 

 specimens before. Is then the coloration of P. chi, which, if not 

 exactly that of B. perla, yet bears a very close general resemblance to 

 it, less a protection than that of B. perla I One would conclude not. 

 Now let us consider a few localities in which I have taken the insect, 

 or have obtained it. Four at once separate themselves from the others, 

 as being the districts for lichen-clad walls of my acquaintance. These 

 are Menethorpe, Yorkshire; Alston, Cumberland; Upper Weardale, 

 Durham; and Brampton, Cumberland. In all four the type form of 

 P. chi alone occurs. Take now other districts, in which I have (or a 

 Birtley friend has) taken the insect, ciz., Birtley, Dunston, Ouston and 

 Killingworth. All these produce the aberration in the ratio of four to 

 one of the type. In one locality, adjoming Birtley, the proportions are 

 reversed, and as one gets southward, some five miles, the variety does 

 not occur ; I shall consider this case later. In the above four districts 

 the walls are absolutely free from lichen. Hence, if the lichen theory 

 of the colours and markings of P. chi ab. olivacea be true, we have the 

 anomalous position of the protected variety failing to occur in districts 

 where its protection would (presumably) be of most use and vice versa. 

 Lastly, I think, from my own notes, that P. chi rarely ascends a wall 

 to dry its wings after emerging, but is found in such places on the 

 second day after its emergence. It is very rare, indeed, to find perfect 

 specimens on walls ; they are generally more or less rubbed. Let me 

 now put forward my theory of the colours of P. ab. olivacea. Consider 

 the localities in which the species is found, not only those localities I 

 have named, but also those recorded in books on the British lepi- 

 doptera. We see that P. ab. olivacea occurs in thickly populated 

 manufacturing districts, whereas the type occurs in places which are in 

 their natural wooded condition. In the former, owing to the absence 

 of trees, the larvae are driven more to a diet of low plants, such as 



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