Anax junius (Odonata: Aeshnidae) is a widespread North American dragonfly with a terrestrial adult stage and an aquatic larval stage. Over large spatial scales, the species exhibits two distinct but sympatric emergence phenologies. One larval cohort (traditionally referred to as migrants) develops rapidly from egg to adult within two to four months, emerging in late summer. Another larval cohort (nonmigrants) develops in eight to twelve months, emerging in late spring. Adults from each cohort are believed to have different dispersal phenotypes, but the relationship between these groups has long been unclear. Molecular measures show no difference between migrants and nonmigrants, suggesting phenotypic plasticity determines developmental trajectory. Such a plasticity must be triggered by a reliable cue capable of functioning over 30 degrees of latitude. Siblings were placed in an environmental chamber, split into photoperiod treatment groups (increasing, decreasing, and constant), and raised from egg in single cups. Within two months, significant growth rate differences had emerged, with increasing photoperiod siblings growing more quickly than constant and decreasing treatments. These results suggest that dispersal phenotypes and developmental phenologies observed over large spatial scales may result from ecological rather than locally adaptive processes, though inflexible cues such as photoperiod may also prove to be traps in the face of such threats as anthropogenic climate change.