Mayflies are sound indicators for good water quality as they are very vulnerable to pollution. Frequent car drivers in the Netherlands are very familiar with them due to the ‘summer snow-swarms’ the insects may cause. As adults, they leave the mysterious ‘aquatic society’ almost simultaneously,1 after having spent their entire lifetime below the watermark, hidden for most human beings who might not be aware of their presence. Hibernation is a natural aspect of their life cycle (Gysels 1991).