Cases in which the variability depends entirely upon the pupa, while the larva and imago are extremely constant, are of great rarity. Vanessa Io is a case in point, the pupa being light or dark brown, or bright golden green, whilst in the two other stages scarcely any light shades of colour or variations in the very complicated marking are to be met with.
The facts thus justify the above view that the individual stages of development change independently—that a change occurring in one stage is without influence on the preceding and succeeding stages. Were this not the case no one stage could possibly become variable without all the other stages becoming so. Did there exist a correlation between larvæ, pupæ, and imagines of such a nature that every change in the larva entailed a corresponding change in the imago, as soon as a large number of larval characters became fluctuating (i.e. as soon as this stage became variable), a large number of imaginal characters would necessarily also become fluctuating (i.e. this stage would also become correspondingly variable).
There is one other interpretation which might perhaps be attempted from the point of view of the old doctrine of species. It might be said that it is a special property of certain larval or imaginal markings to be variable whilst others are constant, and since the larval and imaginal markings of a species are generally quite distinct, it may easily happen that a butterfly possessing markings having the property of constancy may belong to a caterpillar having variable markings.
There is a soul of truth underlying this objection, since it is true that the various forms of markings which occur in Lepidoptera apparently reach different degrees of constancy. If we speak of the constancy or variability of a species, a different meaning is attached to these expressions according as we are dealing e.g. with a species of Sphinx or a species of Arctia. That which in the latter would be estimated as a high degree of constancy, in the former would be taken as a considerable amount of variability. It is of interest, in connection with the question as to the causes of constancy, to note that the power of any form of marking to attain to a high degree of constancy is by no means inversely proportional to the complication of the marking, as would have been expected à priori.
Thus, the species of Sphinx and of allied genera possess on their fore-wings, which are mostly coloured with a mixture of dull grey, white and black, an exceedingly complicated arrangement of lines which, in constant species, show a high degree of uniformity: on the other hand, the checquered fore-wings of our Arctiidæ, which are far more coarsely marked, always show, even in the most constant species, well-marked individual differences. The different types of marking must therefore be measured by different standards.
But in granting this, we decidedly refute the statement that constancy and variability are inherent properties of certain forms of marking.
This reasoning is based on the simple fact that a given type of marking comprises both species of great constancy and of (relatively) great variability.
Thus, the fore-wings of Sphinx Ligustri and S. Convolvuli are extremely constant, whilst the very similarly marked Anceryx (Hyloicus) Pinastri is exceedingly variable. Similarly Deilephila Euphorbiæ is known by its great variability of colouring and marking, whilst D. Galii, which resembles this species so closely as to be sometimes confounded with it, possesses a high degree of constancy, and further, the Corsican and Sardinian D. Dahlii is very variable. Among the family Arctiidæ, Callimorpha Hera and the Alpine Arctia Flavia are cases of constancy, whilst A. Caja, which is so similar to the last species, is so generally variable that two perfectly identical specimens can scarcely be found together.
The same can be shown to hold good for the markings of caterpillars. Thus, the larva of D. Dahlii shows very considerable variability, whilst that of D. Galii is very constant in marking (disregarding the ground-colour). So also the larva of Vanessa Urticæ is very variable and that of V. Antiopa very constant, &c.