|Looks as though he got a chill when he was but a mere youngster!|
The Small Tortoiseshell appears to be particularly sensitive to temperature shock, so that extreme shocks of either heat or cold during the last 24 hours of the larval stage and the first 48 hours of the pupal stage, can disrupt the natural process of metamorphosis and inhibit the normal processes in which organic chemicals create the colouration of the wing scales.
It is difficult to ascertain how frequently any of these aberrations occur in the wild, however it is a rare event that exposes the newly formed pupa or transitional larva to the necessary conditions for metamorphosis to be disrupted in this way, and this is supported by the paucity of historical sightings – especially of the extreme aberrations in the wild. This particular butterfly is known as a “partial aberration” – the really extreme aberration can be an almost entirely melanistic black.
It has been speculated that severe late frosts could possibly cause instances of these aberrant forms, as well as a larva/pupa being exposed to particularly strong sunlight after having the normally sheltered pupation site disturbed in some manner.
So thanks Dan – fascinating stuff which I thought I would share!