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exit slit

Many species pupate outside their mine. Before vacating the mine they use their mandibles to cut a slit in the epidermis; this slit mostly has a very fixed, more or less semicircular shape. Often it is species-specific whether the slit is made in the upper or lower epidermis.

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Phytomyza minuscula, vacated mine: illumination from behind clearly shows the exit slit.