Mystacides azureus is the spitting image of Mystacides niger above. Yet, you can tell them apart quite easily. Even in sunshine Mystacides niger is black all over, while Mystacides azureus has a blueish sheen. And in the right light the eyes of Mystacides azureus are brown. Those of Mystacides niger are always just plain black. While Mystacides niger prefers standing water, Mystacides azureus prefers running water. In behaviour though the two species do not differ much. Both appear from May onwards (probably in two breeds) and the males of both species like to swarm in the evening. Both are quite small, reaching a length of no more than 10 mm.