7893. 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 obscuvavria. ‘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 outa 
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 te 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 larve 
are compelled to eat with their food ! 
Natural selection, moisture, cold, and the absence of sunlight, 
have 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. Merrifield 
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 
pupee of Selenia illustravia (summer brood) and Ennomos autumnaria, he 
has obtained striking results, the moths being rendered decidedly 
U2 
