134 YORKSHIRE NATURALISTS' UNION. 



It would have been interesting to compare the actual 

 amount of uninterrupted sunlight in the districts to which I 

 have referred, with that of others less affected by mist or smoke ; 

 but I regret to find that there are no sufficient records available 

 to enable me to lay this comparison before you. If, as there is 

 reason to suspect, the incidence of direct sunshine has become 

 sensibly diminished in certain districts, we may perhaps be 

 justified in predicting that yet more darkened or suffused 

 varieties of our common insects may be found within those 

 districts in years to come. 



Since butterflies are in the habit of resting with their wings 

 erect, whereas the wings of moths are placed in a different 

 position ; we might expect to find the change, if any, more 

 noticeable on the under surface of the former, rendering them, 

 (as is the case especially in the frequently hybernating genus 

 Vanessa, and in the greenish Arctic forms of Colias), well fitted 

 to absorb heat in repose ; and on the upper surface of the latter, 

 for the same purpose as has already been observed. 



Summing up the conclusions to which I have endeavoured, 

 however imperfectly, to guide your attention, it may be granted 

 that a tendency to variation in the direction of melanism is 

 noticeable not only in Lepidoptera of high latitudes, but also 

 in others exposed to approximately the same climatic con- 

 ditions. 



It is known that black, green, and red are the most rapid 

 heat absorbers in ordinary substances, and it is proved that in 

 the case of black this applies also to insects. 



The advantage which insects would derive from an increased 

 power of rapidly acquiring warmth in cold or sunless situations 

 can scarcely be disputed. 



The chemical importance of diffused sunlight, probably 

 through the action of the ultra-violet or invisible rays, which 

 show a preference for blacky must not be overlooked. 



Trans. Y.N. U., 1883 (pub. 1SS5). Series D 



