iS93. COLOUR CHANGES IN INSECTS. 291 



leaves and with certain green pigments is thus explicable; the light 

 reflected from the leaves possesses yellow rays, and, consequently, 

 produces light effects on the insects, while the green pigments absorb 

 these rays. So also the red paper backgrounds absorb the rays and 

 produce dark effects, but red glass or gelatine transmits the rays and 

 causes green or light colour in the insects. Petersen (6) seems to have 

 independently arrived at similar results, and to have given the same 

 explanation of them. 



The only imagine made the subject of experiment by Poulton 

 was Gnophos obscurana. This moth is mentioned in " The Colours of 

 Animals " as being commonly light on chalk and dark on peat. A 

 number of these insects were reared from the egg, some amid dark, 

 others amid light surroundings, but in no stage was the colour of the 

 insect affected by its environment, and the moths all turned out a 

 light-grey hue. Poulton is led, therefore, to suggest that the pre- 

 valence of varieties in nature appropriate to the soil where they occur 

 is the result of natural selection. 



The cause of the darkening of moths in various localities has 

 been for several years past frequently discussed, and numerous 

 theories have been propounded. Lepidopterists have noticed that 

 British moths are, as a rule, darker than specimens from the Conti- 

 nent, and that, within the British isles, western, northern, and 

 mountain varieties are darker than insects from the southern and 

 eastern lowlands, the melanic tendency being most strongly marked 

 on the west coast of Ireland, and in the northern and western islands 

 and highlands of Scotland. Yorkshire naturalists have noticed also 

 a special tendency to melanism near the large manufacturing towns 

 of the north, and the dark varieties are said to be increasing, while 

 the lighter types are dying out. It has been suggested that the 

 darkening is, in this case, due to the soot which the unhappy larva? 

 are compelled to eat with their food ! 



Natural selection, moisture, cold, and the absence of sunlight, 

 liave been put forward as serious suggestions for the cause of 

 melanism. The districts where the phenomenon is most marked are 

 the wettest in the country, and it is evident that constant moisture 

 tends to render dark most objects on which insects rest, and so to 

 favour dark varieties. Natural selection would therefore tend to the 

 preservation of melanic moths in moist districts, but the general 

 impression is that something in moist districts tends also to the pro- 

 duction of such varieties. Tutt (7) and many other naturalists, 

 consider moisture of itself to be a cause of melanism. INIerrifield 

 believes low temperature to be effective in the same direction, and 

 has, during the last few years, conducted numerous experiments on 

 the effect of temperature in producing dark colouration (8, 9) analo- 

 gous to the well-known experiments of Weismann. By cooling the 

 pupae of Selenia illnstniria (summer brood) and Ennomos aidumnaria, he 

 has obtained striking results, the moths being rendered decidedly 



U2 



