Photoperiodic control of larval development in Leucorrhinia dubia (Vander Linden): a comparison between populations from northern and southern Sweden (Anisoptera: Libellulidae)

The larval photoperiodic responses at 15° and 20° C, and, to a limited extent, the larval development in the field, were examined in a population of L. dubia from N of the Arctic Circle. The results were compared with similar data from a S Swedish population. In the latter, the life cycle duration was 3 years; in the northern population it was probably longer. The basic effects of photoperiod on development were of the same type in both populations. Larvae in at least the 4 instars preceding the final one showed a simple photoperiodic reaction of the long-day type, and a hibernation diapause was induced by short days. A long-day diapause starting in the early part of the final instar, and a short-day diapause chiefly effective at a later interecdysis stage in the same instar, ensure that emergence is restricted to the spring, and that most larvae spend the last winter before emergence in a suitably advanced stage within the final instar. The remaining final instar development is stimulated by the long days during the spring. Larvae overwintering in pre-final instars normally enter diapause in the final instar. The differences between the populations are primarily related to the photoperiodic regime and the length of the summer in the respective areas. Compared to the southern population, the northern population displays a 4-6 hours longer critical daylength, a generally weaker diapause in the final instar, and somewhat different responses in the penultimate instar. Northern larvae often showed responses typical for short days at LD 21.5:2.5 and (in particular in the penultimate instar, and at 15° C) at continuous light. The induction of hibernation diapause in the penultimate instar is thus likely to occur early in the northern population, thereby restricting the entry into the final instar to the early part of the season. Thereafter, the weaker final instar diapause of the northern larvae permits them to spend their last winter in a more uniform and advanced state than the larvae in the southern population despite the shorter summers. This produces an early, brief emergence period believed to be essential for L. dubia in N Sweden. In the southern population a later entry into the final instar is produced by the different response pattern of the penultimate instar: long days produce a slight prolongation of development, and the short-day (hibernation) diapause is less readily induced than in the preceding instars. When a potentially dangerous ’last minute" entry into the final instar occurs in the autumn, it leads to overwintering in an early interecdysis stage and a certain delay in emergence, but it pays off by a shorter development than with an early induction of penultimate instar diapause.