First incidence of inquilinism in gall‐forming psyllids, with a description of the new inquiline species (Insecta, Hemiptera, Psylloidea, Psyllidae, Spondyliaspidinae)

The two largest lineages of holometabolous gall‐forming insects, cynipid wasps and cecidomyiid flies, have given rise to numerous obligate inquilines, species which are unable to form galls themselves and survive by inhabiting galls formed by other species. In contrast, only a single obligate inquiline, an aphid, is known in the sternorrhynchous Hemiptera, the hemimetabolan lineage in which gall‐forming is best developed. We describe the first known gall inquiline in psyllids (Sternorrhyncha, Psylloidea), Pachypsylla cohabitans Yang & Riemann sp. n. All other members of this genus produce closed galls on hackberries, Celtis spp. (Ulmaceae). Newly hatched nymphs of P. cohabitans feed next to nymphs of several species of leaf gall‐makers, becoming incorporated into the gall as the stationary nymphs are gradually enveloped by leaf tissue. In the mature gall, the inquilines occupy separate, lateral cells surrounding a central cell containing a single gall‐maker. Pachypsylla cohabitans is similar in morphology to leaf‐gallers, but differs in nymphal and adult colour, allozyme frequency, especially in the malic enzyme, and in adult phenology. Laboratory‐reared progeny of side‐cell females, when caged alone, never form galls, while progeny of centre‐cell individuals alone only form galls comprising single individuals. Multiple‐cell galls are formed only when adults of side‐cell and centre‐cell individuals are caged together. Experimental removal of centre‐cell nymphs in early stages of gall initiation leads to smaller galls or death of side‐cell individuals. We conclude that the side‐cell individual is an obligate inquiline that is incapable of forming a gall on its own but is derived from a leaf‐galling ancestor. We speculate on selective forces that might favour this evolutionary transition.