A positive correlation between photoperiod and development rate in summer species of Odonata could help to make emergence date appropriate to latitude: a testable hypothesis*
In the western Nearctic and the Palaearctic Regions several species of Odonata occur, without evident gaps in distribution, from latitude 50° N northwards to the Arctic Circle (66°30'N) and beyond. The decline in incident solar radiation along this latitude gradient does not appear to be reflected, as might be expected, in progressively later emergence, despite the progress of metamorphosis being dependent on ambient temperature. On the contrary, reports indicate that, in some species, northernmost populations may emerge at least as early as, and sometimes even earlier than, more southerly populations, suggesting that some mechanism exists that enables larval developmental rate to compensate for latitude. Reported responses by late-stadium larvae to photoperiod, placed in the context of seasonal changes of photoperiod at different latitudes, make it plausible to postulate the existence of a single, fixed response to photoperiod that would continuously adjust developmental rate to latitude, at least between 50° and 70° N. In Odonata such a response, to be effective, would be confined to species possessing a Type-2 or Type-3 life cycle, in which more than one stadium precedes metamorphosis in spring or early summer. The hypothesis proposed here does not invoke genetic heterogeneity of response in populations at different latitudes, such as has been detected in certain other insects. The response predicted by the hypothesis may complement, rather than substitute for, other mechanisms of seasonal regulation. Steps are described by which the hypothesis could be tested in Odonata.*Text based on an invited, oral contribution to the Workshop "North American Dragonflies", included in the Joint Meeting of the Entomological Society of Canada and the Entomological Society of Manitoba, Winnipeg, Manitoba, 9 October 2002
