I didn't think they would make it through pupation, considering I had so rudely uprooted their homes, but the plastic bag is full of dried up Sonchus and active little flies. (And they really are little — less than 2mm body length.) I worked through the Lucid key and confirmed the original identification. More or less. Unfortunately, the two introduced species of Chromatomyia are indistinguishable from one another on external features, so I'll have to be satisfied with an either/or ID.
Lest you think that putting a name to these minute beasts was a matter of great ability I should let you know that it was done using two attributes — the pupation site (within the mine) and colour of the third antennal segment (black). Given the host plant, identification was pretty straightforward. Entomological skillz: I do not haz dem.
Okay. That's one fly down and only another gazillion to go. I was better off with the bivalves.
(That's not an ex-fly, by the way. I stuck it in the fridge to slow it down, but that didn't have an effect, so I put it in the freezer. That worked. For a few minutes. Then the fly was moving around as if nothing had happened.)