A five year study of the larval life history of Coenagrion hastulatum (Charpentier) and C. armatum (Charpentier) in northern Sweden (Zygoptera: Coenagrionidae)

The life cycles of the 2 spp. were studied in a productive pond during 5 yr. In both spp. a 2-yr life cycle was most common. Emergence of both spp. was fairly synchronous and peaked mainly in early June, often starting in late May. C. armatum seemed to be slightly earlier than C. hastulatum. Eggs probably hatched in July-Aug. Most larvae first overwintered in rather early instars, and a second and last time in the final instar. However, a small but variable part of the populations (best documented for C. hastulatum) showed a 1-yr life-cycle with the winter spent in large pre-final instars. This unexpectedly rapid development is probably explained by favourable local conditions. In both spp. a low recruitment of larvae in 1987 is explained by severe weather conditions in that year. It is discussed how detailed information about development and cohort splitting can emerge from limited material of this type.