Dec., 1892. 
DOMICILES. 
271 
would come. This rudder stem is the exit for the moth ; 
but the moth has not, as in most) cocoons, to gnaw its way 
jihrougli; the sides of the boat—so to speak—are brought 
close together, but they are not spun together or in any way 
joined ; the moth, when fully formed, has only to push from 
within and the boat’s stern bulges with the pressure and lets 
her through. Why then, you may ask, can not an enemy 
or a thief get in where the tenant gets out ? That is the 
very point of the ingenuity; as long as the chrysalis is inside 
the pressure on the elastic sides of the cocoon acts as a spring, 
and keeps the edges together ; it is a genuine valve, giving easy 
passage out, but absolutely barred to the foe who would 
attempt to force an entrance from without. It is quite easy 
to see the mechanism with an empty cocoon ; and I have 
brought two empty ones for the purpose. All you have to 
do is to put a slight pressure with a finger on the top, and 
the rudder is at once bent from a single line into the two 
sides of a circular opening. 
But there is another point of interest in these cocoons. 
You will observe in my three specimens that the two which 
are spun on leaves are dark or darkish brown ; while the one 
which is constructed on the fragment of a chip box is quite 
white. In fact, in each case the colour of the surroundings is 
almost exactly imitated by the colour of the cocoon. You will 
also notice that the same is the case with two other of the sets 
of cocoons exhibited in this case, namely the Hop-dog and the 
Small Eggar. Now the question arises at once, is it conceiv¬ 
able that the caterpillar has the power to change the colour 
of the silk it spins according to the tint of its surroundings ? 
It seems to be proved by recent researches, in which 
Mr. Poulton has taken a distinguished part, that the pigment 
on caterpillars can certainly be determined—not by natural 
selection only, though, of course, this acts as usual—but by 
some sensitiveness of the organs in the life-time of the 
individual larva, so as to resemble the surroundings ; but the 
case is different with the silk that forms the cocoon, and would 
require, of course, to be separately proved. The question is 
at this very moment being hotly debated. Take the case of 
the Small Eggar: you will see that the cocoon which I 
show which was spun in a chip box is white ; whereas the 
usual colour is dark brown. Other experiments were made 
by Mr. Poulton and others ; and when the larva was put into 
a box with white surroundings he always seemed to spin a 
white or whitish cocoon. Tnis looked conclusive ; but the 
next experiment showed the need of care in drawing con¬ 
clusions. Last week, Mr. Bateson showed before the Entomb- 
