| Summary: | Twenty-three species of gall midges occur exclusively on hackberries in North America north of Mexico. Twenty-one of them belong to the genus Celticecis and form complex, dehiscent galls on leaves and the current year's twigs. Celticecis species are definitely known only from the typical subgenus of Celtis, distributed through much of the Holarctic Region. Nearctic species do not appear to discriminate among hosts within the subgenus but each has its individual geographic limits. Galls of most species of Celticecis grow quickly in early spring, but some have a delayed, late summer development. When full-grown, larvae form cocoons in the galls that subsequently dehisce at various times during summer and autumn. Larvae pupate early the following spring and adults appear shortly after. Some larvae remain in the galls through the entire next year to pupate only in the following spring. Larvae of the second and third (ultimate) instars present some of the best characters for determining species and their affinities. Seven of the ten previously described species of North American Celticecis are valid: Celticecis capsularis (Patton) (Celticecis painteri (Felt), new synonym), Celticecis celtiphyllia (Felt), Celticecis oviformis (Patton), Celticecis pubescens (Patton) (Celticecis texana (Felt), new synonym), Celticecis semenrumicis (Patton), Celticecis spiniformis (Patton) (Celticecis unguicula (Beutenmüller), new synonym), and Celticecis wellsi (Wells). Fourteen new species of Celticecis are named and described: Celticecis aciculata Gagné, Celticecis acuminate Gagné, Celticecis conica Gagné, Celticecis connata Gagné, Celticecis cornuata Gagné, Celticecis cupiformis Gagné, Celticecis expulsa Gagné, Celticecis globosa Gagné, Celticecis ovata Gagné, Celticecis pilosa Gagné, Celticecis pyriformis Gagné, Celticecis ramicola Gagné, Celticecis subulata Gagné, and Celticecis supina Gagné. Celticecis is to date known also from the Japanese Celticecis japonica and an undescribed species from Turkey. Peracecis Gagné new genus, is described for a new species, P. fugitiva Gagné, responsible for an integral leaf gall found in southern U.S. Parallelodiplosis acernea (Felt), a widespread inquiline in cecidomyiid and psyllid galls of hackberry, is redescribed and compared to its congeners. Keys to galls and larvae (second and third instars) of Nearctic gall midges on hackberry are presented. Other cecidomyiid galls previously reported on Celtis outside North America are noted.
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