Success! I found a pupa tucked away at the end of a tunnel. Turns out that the miner is a fly, probably Chromatomyia syngenesiae (Agromyzidae) (a.k.a. Phytomyza syngenesiae). This species is widespread and occurs most frequently on sow thistle (Sonchus oleraceus) — exactly where I found it.
Pupa in its mine. Only the spiracles (respiratory structure) project through the cuticle.
Pupa removed from its mine. The spiracles are visible to the lower left.There's a maggot feeding away in its mine at the top right.
At least, it was, until it was evicted. That's the head to the right, in case you couldn't tell.
The mines exposed to reveal a maggot's waste disposal system.It's okay when you're always on the move.
