Eastern dobsonfly (Corydalus cornutus) pupae after first metamorphosis. A final molting will take place within a few days, when the adult dobsonflies will emerge after discarding their pupal skins. This transformation occurs within a "pupa cell", which is usually under a log or stone. The pupa cell is excavated by the larva (hellgrammite) after it crawls out of a stream or river. This is a spring occurrence and the timing is based on exterior climate conditions such as temperature and moisture. Ludlow, Massachusetts. Wild.