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 wa sps and cecidomyiid flies, have given rise to numerous obligate inquilines, species which are unable to form galls themselves and survive by inhabitin g galls formed by other species. In contrast, only a single obligate inquil ine, an aphid, is known in the sternorrhynchous Hemiptera, the hemimetabola n lineage in which gall-forming is best developed. We describe the first kn own gall inquiline in psyllids (Sternorrhyncha, Psylloidea), Pachypsylla co habitans Yang & Riemann sp. n. All other members of this genus produce clos ed galls on hackberries, Celtis spp. (Ulmaceae). Newly hatched nymphs of P. cohabitans feed next to nymphs of several species of leaf gall-makers, bec oming incorporated into the gall as the stationary nymphs are gradually env eloped by leaf tissue. In the mature gall, the inquilines occupy separate, lateral cells surrounding a central cell containing a single gall-maker. Pa chypsylla cohabitans is similar in morphology to leaf-gallers, but differs in nymphal and adult colour, allozyme frequency, especially in the malic en zyme, and in adult phenology. Laboratory-reared progeny of side-cell female s, when caged alone, never form galls, while progeny of centre-cell individ uals alone only form galls comprising single individuals. Multiple-cell gal ls 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 individual s. 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-gallin g ancestor. We speculate on selective forces that might favour this evoluti onary transition.