Vv COLOURS OF ANIMALS 347 
Many other cases are known among insects in which the 
same species acquires a different tint according to its sur- 
roundings; this being particularly marked in some South 
African locusts, which correspond with the colour of the soil 
wherever they are found. There are also many caterpillars 
which feed on two or more plants, and which vary in colour 
accordingly. A number of such changes are quoted by Mr. 
R. Meldola, in a paper on “ Variable Protective Colouring in 
Insects” (Proceedings of the Zoological Society of London, 1873, 
p. 153); and in some cases it has been shown that green 
chlorophyll remains unchanged in the tissues of leaf-eating 
insects, and being discernible through the transparent 
integument, produces the same colour as that of the food 
lant. 
: In all these insects, as well as in the great majority of 
cases in which a change of colour occurs in other animals, the 
action is quite involuntary; but among some few of the 
higher animals the colour of the integument can be modified 
at the will of the individual, or at all events by a reflex 
action dependent on sensation. The most remarkable case of 
this kind occurs with the chameleon, which has the power of 
changing its colour from dull white to a variety of tints. 
This singular power has been traced to two layers of movable 
pigment-cells deeply seated in the skin, but capable of being 
brought near to the surface. The pigment-layers are bluish 
and yellowish, and by their contraction or concentration these 
can be forced upwards either together or separately. When 
the animal is passive the colour is dirty white, which changes 
to various tints of bluish, green, yellow, or brown, as more or 
less of either pigment is forced up and rendered visible. 
The animal is excessively sluggish and defenceless, and its 
power of changing its colour so as to harmonise with sur- 
rounding objects is essential to its safety. Here too, as with 
the pupa of Papilio Nireus, colours such as scarlet or blue, 
which do not occur in the natural environment of the animal, 
cannot be produced. Somewhat similar changes of colour 
occur in some prawns and flat-fish, according to the colour 
of the bottom on which they rest. This is very striking 
in the chameleon shrimp (Mysis chameleon), which is 
gray when on sand, but brown or green when among sea- 
