MELAXIftM IN YORKSniRE LEPinorTERA. 023 



Enough has probably been said as to the indisputable fact of uielanisni, 

 and it is time to get on to our discussion as to the ' Why ' and the Cause 

 or Causes of the phenomenon. As we all know, the generally accepted 

 theory as to the ' Why ' is that it is a means of protection. But is it 

 really a protection ? And from what 1 We have been told over and over 

 again that birds are the natural enemies of our moths, and that the pale 

 forms, being so much more conspicuous on our darkened tree trunks, the 

 birds pick them off, and the darker specimens, being less conspicuous, are 

 more likely to escape detection, and so escape in proportionately greater 

 numbers and are left to pei'petuate still darker forms. But do birds feed 

 to any extent on moths 1 My own experience certainly does not warrant 

 any such conclusion. Many lepidopterists here will bear me out when I 

 say that if, in an early morning's walk in the woods, one espies any moth, 

 however pale and conspicuous, on a tree trunk, and does not happen to 

 have a box in his pocket, it may safely be left there until towards 

 evening, when one has time to fetch it. In ninety-nine cases out of the 

 Imndred the moth will be there just as when left in the morning, notwith- 

 standing that the wood may be alive with insectivorous birds. Birds, as 

 we all know, feed largely, many species almost exclusively, on caterpillars, 

 and I have always maintained that the chief use of the Lepidoptera in 

 the economy of Nature is to provide food for birds as larvae ; but if the 

 birds fed on the moths themselves, would it not be a case of ' killing the 

 goose that lays the golden eggs ' ? You may go on to one of our heaths on 

 a fine late afternoon or early evening, and see the place absolutely alive 

 with Micro-lepidoptera on the wing, and at the same time the swallows 

 feeding on the Diptera high up ; but they are not taking the moths, 

 which are flying in myriads a few inches above the heather. The only 

 bird I remember which seems to live upon moths is the goatsucker, and 

 he undoubtedly does get rid of an enormous quantity. But the goat- 

 sucker only feeds at night, and always catches his meal on the wing, so 

 the colour assimilation to tree trunks cannot apply to it. The same 

 remark applies to bats, which also account for the slaughter of myriads of 

 moths ; but it is only at night, and we can scarcely conceive that a dark 

 moth would have any better chance of escape than a pale one from the 

 eyes of either a goatsucker or a bat. Dragon-flies certainly take moths 

 in the daytime, but they hawk for them and take Ihem on the wing, 

 never, T should say, from a tree trunk. Besides, we have comparatively 

 few dragon-flies in Yorkshire, and certainly far less in the melanic area 

 than in any other part of the county. The big green grasshopper {Locusta 

 viridissima) is a deadly enemy to moths, but it does not occur in York- 

 shire at all. On the Deal sandhills, and on the South Devon coast, 1 

 have often seen them perched on the tops of sugared posts, waiting for 

 the moths to come up, and have seen one seize and devour immediately a 

 large Xylophasia polyodon. Only last August, on a marshy part of the 

 Deal sandhills, I saw these grasshoppers in profusion perched after dark 

 on the tops of the thistles, deliberately waiting to pounce on the moths 

 aa they visited, as they do in large numbers, the thistle flowers. There 

 was no melanism in the moths there, but if there had been it would not 

 have protected them one atom. Hence I cannot see my way to assent to 

 the theory that the primary reason of melanism is for protection against 

 such enemies. 



Melanism is with us, strongly, and is still increasing ; but what do wc 

 know as to its cause ? 



Y2 



