The life cycle and larval photoperiodic responses of Coenagrion hastulatum (Charpentier) in two climatically different areas (Zygoptera: Coenagrionidae)

The life-cycle of C. hastulatum was studied in southern (58° 42'N) and northern (67° 5074) Sweden. In the southern area, the life cycle duration was 1-2 yr. The univoltine and semivoltine larvae separated as cohorts already before the first winter. During the second summer, the semivoltine cohort continued to develop slowly, while larvae that had overwintered in the last 3 instars, i.e. the univoltine cohort and older semivoltine larvae, rapidly proceeded to emergence, which mainly took place in June. Egg development was direct. In the northern area, the life-cycle duration was 3-4 yr, and almost all larvae overwintered in the final instar before emergence. A very low frequency of penultimate instars in the overwintering population seemed to be generated during the late part of the third summer by a cohort splitting suggestive of the one in the southern area. Experiments performed at different photoperiods partly explained the life-cycle pattern. Larvae overwintering below a certain critical size, associated with the frequency minimum produced by the cohort splitting, showed a slow development at subsequent long-day photoperiods, in particular in the penultimate instar. Larvae overwintering above the critical size rapidly proceeded to emergence under the same conditions. A 19,3 hr daylength produced a long-day response in the southern population only. The shorter days of September (southern population) produced a burst of moulting activity before a hibernation diapause, present in at least the 3 last instars, was induced. The winter critical size was concluded not to be a result of the previous cohort splitting, which was assumed to be under separate environmental control. The early cohort separation may serve to reduce intraspecific competition and to add further refinement to seasonal regulation.