It would be meaningless to assert that the two stages above mentioned
were _completely_ independent of one another. It is obvious that the
amount of organic and living matter contained in the caterpillar
determines the size of the butterfly, and that the quantity of organic
matter in the egg must determine the size of the emergent larva.
The assertion in the above heading refers only to the structure;
but even for this it cannot be taken as signifying an absolute, but
only a relative independence, which, however, certainly obtains in
a very high degree. Although it is conceivable that every change of
structure in the imago may entail a correlative change of structure
in the larva, no such cases have as yet been proved; on the contrary,
all facts indicate an almost complete independence of the two stages.
It is quite different with cases of _indirect_ dependence, such, for
example, as are brought about by ‘nurse-breeding.’ This phenomenon is
almost completely absent in Lepidoptera, but is found in Diptera, and
especially in Hymenoptera in every degree. The larvæ of ichneumons
which live in other insects, require (not always, but in most
instances) that the female imago should possess a sharp ovipositor,
so that in this case also the structure and mode of life of the larva
influences the perfect insect. This does not depend, however, on
inherent laws of growth (correlation), but on the action of external
influences, to which the organism endeavours to adapt itself by natural
selection.