Mature larvae of Gonioctena leprieuri (Pic) and Gonioctena aegrota (Fabricius) are described and illustrated for the first time, based on specimens collected on Genista spp. (Fabaceae) in Spain. A key to known larvae of subgenus Spartoxena Motschulsky is provided. Diagnostic characters for the identification of species within this subgenus are shape of labrum, arrangement of microtrichia of epipharynx, shape of tarsal claws, and numbers of tubercles on abdominal segments. Notes on distributions and host plants are provided.