Euclidium syrian mustard
Dichotomous table for leafminers
1a larva a, seemingly headless, maggot: Scaptomyza flava
1b larva with well recognisable chitinised head => 2
1c galls, etc => Tables for all parasites per species
2a larva without feet: Ceutorhynchus minutus
2b larva with thoracic feet: Phyllotreta nemorum