none no species The generation-packing hypothesis on the origin of monoecious life cycles in Pemphigus predicts that 1st-instar aphids deposited by alate emigrants of heteroecious Pemphigus species have the potential to grow in the poplar gall. To see whether this prediction is true of P. matsumurai , its galls parasitized by an aphidiid species were examined in early August. Emigrants were forced to larviposit in these unopened galls, and one 2nd-instar aphid of the 3rd generation (a root generation) was found in one of them. Reproduction; Reproductive productivity; Fecundity; Life cycle and development; Evolution; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Host parasite relationships; Land and freshwater zones; Palaearctic region Aphidiidae (Braconidae); Hemipteran hosts; Pemphigus matsumurai; Host life cycle & development; Japan; Hokkaido; Hemipteran host life cycle & development Pemphigus (Aphidoidea); Evolution; Monoecious life cycle, implications from heteroecious species development Pemphigus matsumurai (Aphidoidea); Number of generations; Life cycle, viability in primary plant host; Development; Heteroecious taxon development in primary plant host; Hymenopteran parasites; Aphidiidae; Host life cycle & development; Plant hosts; Populus (poplar) & herbaceous dicotyledons; Heteroecious taxon, viability in primary host; Implications for origin of monoecious life cycle; Reaction of host to parasite; Populus (plant) gall, parasite development; Effect of parasitism by Aphidiidae (Hymenoptera); Japan; Hokkaido; Development in primary plant host, effects of parasitism, implications