We document for the first time the larval food plants of Anoncia mosa Hodges (Cosmopterigidae) mining the leaves of Collinsia heterophylla and C. tinctoria (Lamiales: Plantaginaceae) in Madera County, California. We describe and illustrate the mine, larval feeding habits, last instar larva and pupa of Anoncia mosa, then compare the immature stages to other Cosmopterigidae to help identify this species and call attention to some overlooked morphological characters. These include the larval P seta, larval cuticular texture and the genital opening of the pupa in a pit. We partially describe the previously unknown pupa of Euclemensia. We also use characters of the immature stages to briefly discuss the systematic placement of three taxa. The family status of the Chrysopeleiidae and the inclusion of Triclonella in the Cosmopterigidae are supported by the pupa but not the larva. Conversely, the systematic placement of Metharmostis in Cosmopterigidae is better supported by the larva than the pupa. Our study is an example of the need to save larval and pupal exuviae for morphological descriptions.