Phyllonorycter pastorella op Salix babylonica (Amsterdam)
Generally, Phyllonorycter-mines can only be identified reliably when the mine contains pupa, or has a old pupal skin attached to it. In most (all?) species the pupa, just before emergence, works itself half out of the mine through an opening that has made in de epidermis. After emergence the empty skin remains (fig.). When this gently is pulled out, taking care not to damage the cremaster, it is even more practical than a fresh pupa:after gently warming it in a few drops of water it is delicately transparent.
Within the mine the pupa is envelod in a silken cocoon. This can be so flimsy as to be nearly invisible, but may a dense and tough as parchment. Depending on the species, the cocoon is fastened to the upper side, lower side or both sides of the mine.
The frass may be neatly collected in one lump in a corner of the mine, held together with some silk; but it may also be arranged in a horse-shoe fashion around the cocoon, of it may for the largest past be incrustrated in the wall of the cocoon itself.
to complicate matters, in most Phyllonocters have two generations (at least in temperate regions), and most hibernate as a pupa in the cocoon. The general aspect of the cocoons of the two generations may differ considerably.