Ephemeroptera, popularly known as mayflies, are the most primitive and ancient of the extant insect groups. Mayflies undergo hemimetabolous metamorphosis, having a unique maturation stage between the larva and adult, the subimago. Prosopistomatidae is a distinct, enigmatic family of mayflies presently confined to the Palaearctic, Afrotropical, Oriental, and Australasian realms and represented by a single genus viz. Prosopistoma Latreille, 1833. Neoephemeridae is a small group of pannote mayflies presently confined to Holarctic and Oriental regions. Heptageniidae is a family of mayflies with around 509 described species and distributed mainly in the Holarctic, Oriental, and Afrotropical regions. The homogenous family Baetidae commonly known as minnow mayflies encompasses around 100 genera and 900 species constituting one-quarter of the global Ephemeroptera diversity with a cosmopolitan distribution except for Antarctica and New Zealand. Mayflies are occupying freshwater and brackish water habitats across the world, with the exception of Antarctica.