Aquarius najas (DE GEER, 1773) has a univoltine life cycle above the latitude of 53º47’ N, (BRINKHURST 1966) but may be bivoltine in warmer climates. On the Grabia River (51º34’ N; 19°15’ E) in central Poland its life cycle is probable partially bivoltine. The peak abundance of overwintering adults occurs in late April. The first early summer generation occurs from May till November. Second generation nymphs appear in August and September and overlap with later instars of the first generation. Only the first three instars of the second generation are present, and it is unlikely that adults of the second generation were produced. Because only adults are present in early spring, this indicates that nymphs of the second generation do not overwinter.
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