THE VARIATION OF THE IMAGINES OF THE LEPIDOPTERA. 61 



insects are, in any of their stages, precisely similar. Variation is 

 general throughout every stage of an insect's existence, i.e., in the egg, 

 larval, pupal and imaginal stages. 



Every living animal seems to exist for two distinct purposes to 

 eat and to be eaten. Nature provides everything with a means of 

 offence or defence, or both. Among insects, weapons of offence are rare, 

 and, generally speaking, their safety lies rather in their defensive 

 characters. These are usually of the most inactive kind, and consist 

 essentially of various disguises, by means of which, when in repose, 

 they bear a strong resemblance to the various objects on which they 

 rest the bringing into harmony, as it were, the colours of insects 

 with their environment, so that they may agree in tint with the object 

 on which they rest, or that they may bear a close resemblance in hue 

 and shape to some object common upon their resting-place. This 

 bringing into harmony presupposes the possibility of a change in the 

 colours of insects, in order that they may respond to the varying con- 

 ditions under which they may be placed, and in which they have to 

 live. This further presupposes a plastic condition of the colours them- 

 selves, otherwise they would not be able to respond to differences of 

 environment. These differences are so many and so varied, that we 

 find variation in the colours of insects occurring under a multitude of 

 different conditions, and to be presented in a variety of ways. In 

 these notes we shall confine ourselves to the brief consideration of 

 a few of the principal phases of variation exhibited by the imagines of 

 certain Lepidoptera. 



The colours of the wings of butterflies and moths are due largely 

 to the scales found on the wing membrane, and, in a less degree, to 

 the colours of the wing membrane itself. The scales themselves are 

 hollow chitinous cells, united by a ball and socket joint to the mem- 

 brane of the wing. They are epithelial expansions, which, having 

 attained the size and shape peculiar to the species, become hardened 

 externally by a chitinous deposit. In the process of their develop- 

 ment, they go through a regular series of changes. They are at first 

 transparent, then they become whitish, then a secretion from the pupal 

 haemolymph, called " pigment factor," enters the scale, and it becomes 

 yellow ; lastly the pigment-factor is elaborated, and the scales assume 

 the coloration that they will have in the wing of the perfect insect. 

 These changes, of course, all take place in the pupa, before the imago 

 emerges, and no development takes places afterwards ; any change that 

 then occurs being due to exposure, the influence of light, etc. There 

 can be no active response, whatever, in the perfect lepidopterous insect, 

 to any change of environment, i.e., no change can occur in its colora- 

 tion once the insect has emerged from the pupal state. 



Ordinary white light can be decomposed. Popularly, we say, it 

 can be broken up into a number of differently coloured lights red, 

 orange, yellow, green, blue, indigo and violet, and we call these the 

 colours of the solar spectrum. These colours, in fact, represent the 

 effect produced on the optic nerve by the variable rate of vibration of 

 the constituent waves, of which white light is really composed. If a 

 substance has the power of absorbing some of the light waves, from 

 the white light which ordinarily falls upon it, and of reflecting others, 

 only the reflected portion can possibly affect the optic nerve. If the 

 red rays only be reflected, then the colour of the substance appears to 



