Mayflies are insects assigned to the Order Ephemeroptera. They are related to dragonflies and damselflies. Their development takes place in freshwater and typically takes a year. During that time, they are known as "nymphs". The adults are short-lived, from a few hours to a few days depending on the species. About 2,500 species are known worldwide. Depicted here is a female subimago of the March Brown, Rhithrogena germanica, which is a faunistic rarity and on the red list of endangered species.
The plant shown is a horsetail, Equisetum arvense.