PDF - Wallace Online

v COLOURS OF ANIMALS 847Many other cases are known among insects in which thesame species acquires a different tint according to its surroundings;this being particularly marked in some SouthAfrican locusts, which correspond with the colour of the soilwherever they are found. There are also many caterpillarswhich feed on two or more plants, and which vary in colouraccordingly. A number of such changes are quoted by Mr.E. Meldola, in a paper on " Variable Protective Colouring inInsects" (Proceedings of the Zoological Society of London, 1873,p. 153); and in some cases it has been shown that greenchlorophyll remains unchanged in the tissues of leaf -eatinginsects, and being discernible through the transparentintegument, produces the same colour as that of the foodplant.In all these insects, as well as in the great majority ofcases in which a change of colour occurs in other animals, theaction is quite involuntary; but among some few of thehigher animals the colour of the integument can be modifiedat the will of the individual, or at all events by a reflexaction dependent on sensation. The most remarkable case ofthis kind occurs with the chameleon, which has the power ofchanging its colour from dull white to a variety of tints.This singular power has been traced to two layers of movablepigment-cells deeply seated in the skin, but capable of beingbrought near to the surface. The pigment-layers are bluishand yellowish, and by their contraction or concentration thesecan be forced upwards either together or separately. Whenthe animal is passive the colour is dirty white, which changesto various tints of bluish, green, yellow, or brown, as more orless of either pigment is forced up and rendered visible.The animal is excessively sluggish and defenceless, and itspower of changing its colour so as to harmonise with surroundingobjects is essential to its safety. Here too, as withthe 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 colouroccur in some prawns and flat-fish, according to the colourof the bottom on which they rest. This is very strikingin the chameleon shrimp (Mysis chamseleon), which isgray when on sand, but brown or green when among sea-