The genus Argia appears to have had a tropical American origin, but the range of some species now extends to Canada. This colonization of latitudes with very different seasonal regimes from those of the ancestral environments has been made possible, at least in A. vivida, by the selection of relatively warm habitats for oviposition, and by a larval diapause which restricts the cold-intolerant adult and egg stages to a favorable time of year. The ability of larvae to adapt their metabolism individually to different temperatures, and the habit of sun-basking in adults, are also components of a strategy which has allowed northward colonization. But it is the widespread occurrence of diapause in the Odonata which is seen as the important contributant to their success in colonizing higher latitudes, a phenomenon which contrasts with the other principal groups of aquatic insects (Ephemeroptera, Plecoptera, Trichoptera) in which diapause has only occasionally been demonstrated. The Plecoptera and Trichoptera, at least, appear to have generally taken an alternative route, namely the development of cold tolerance in all stages.