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Neaspilota aenigma Freidberg and Mathis is a multivoltine, oligophagous fruit fly (Diptera: Tephritidae) developing in the flower heads of Erigeron foliosus Nuttall and E. divergens Torrey and Gray in the subtribe Asterinae of the tribe Astereae in southern California. This tephritid also has been reported in the southwestern United States bordering Mexico from five other genera and plant species belonging to the subtribe Solidagininae of the Astereae. The egg, first-, second-, and third-instar larvae, and puparium are described and figured. The dorsal sensory organ is well defined in all three instars. The caudal segment of the first and second instars is ringed by four stelex sensilla, but by six stelex sensilla in the third instar. In turn, each stelex sensillum in the first instar is ringed by several hemispherical, minute acanthae and one upright, apically rounded acanthus. The mouth hooks of the first and second instars are bidentate, but in the third instar are tridentate. The integumental petal is fused with the sternal sense organ in the first instar, but these structures are separate in the second instar. The ventrally-toothed oral ridges number seven or eight in the third instar, which compares to six oral ridges in three other congeners examined to date. All instars feed mainly on the ovules and soft achenes, but towards the end of the third stadium, the larva tunnels deeply into the receptacle, sometimes continuing through it into the pedicel. Pupariation occurs inside the mature flower heads, but no protective cell is formed, as with congeners that overwinter as a prepuparium. Instead, F-1 adults emerge from flower heads of desert shrubs in late spring (May) and early summer (June), mate, and complete a summer F-2 generation on late-flowering E. divergens growing at higher elevations along with E. foliosus (July-August) and a fall F-3 generation in different species of late-flowering desert shrubs (September-October). Some of these F-2 and probably all F-3 adults overwinter, and those that survive the winter aggregate the next spring (April-May) on preblossom host plants to mate and subsequently oviposit. Pteromalus sp. (Hymenoptera: Pteromalidae) and an unidentified species of Braconidae (Hymenoptera) are the principal parasitoids of N. aenigma in its Erigeron hosts.