Each mine must at some moment be vacated by its occupant. The opening that is made then often has a very constant shape. In those Agromyzidae species where the mine is left prior to pupation, the larvae make neat semicircular exit slit in the epidermis.
Agromyza alnivora on Alder: exit slit
upper- or lower-surface
Often it is a constant feature of a species whether its exit slits are made in the upper or lower epidermis. In Phytomyza minuscula, a species with an upper-surface mine, the exit slits are invariably in the lower epidermis.
In Stigmella-species the exit slit is more highly arched; here too the the position of the exit slit often just as constant as that of the egg.
Phytomyza minuscula on Lesser Meadow Rue: lower surface exit slits
Most mining larvae spend their entire life within a single mine. There are, however, species that are capable to move to another (part of the) leaf and start a new mine. An odd species goes ‘through the house’, tunnelling through petiole, stem and petiole to a new leave. Most actually leave the protection of the mine, and bore themselves in elsewhere. With a bit of luck it sometimes is possible to see the opening that is made by the larva while reentering.
Many species lack the possibility to reenter a leaf: bracing yourself firmly, gnaw a hole in the epidermis and bore yourself in often goes beyond the capabilities of a larva.
Mompha raschkiella on Fireweed: opening
election of frass
Openings in the mine often have the principal role to enable the ejection of frass.