The history of the early descriptions of Prosopistoma is reviewed, and the steps taken to trace the original material described as Prosopistoma variegatum Latreille, 1833 are outlined. Eaton (1884) designated P. variegatum as the ‘‘type’’ of the genus Prosopistoma. Because only the larva was described by Latreille (1833) and this material is no longer available, a larva of this species was selected as the neotype. Scanning electron micrographs of some of the key larval characters provide additional detail for morphological characters not previously described in the Prosopistomatidae. The associated adult male and subimaginal male and female collected from the same sites as verified larvae of P. variegatum are described for the first time.

The extant global Ephemeroptera fauna is represented by over 3,000 described species in 42 families and more than 400 genera. The highest generic diversity occurs in the Neotropics, with a correspondingly high species diversity, while the Palaearctic has the lowest generic diversity, but a high species diversity. Such distribution patterns may relate to how long evolutionary processes have been carrying on in isolation in a bioregion. Over an extended period, there may be extinction of species, but evolution of more genera. Dramatic extinction events such as the K-T mass extinction have affected current mayfly diversity and distribution. Climatic history plays an important role in the rate of speciation in an area, with regions which have been climatically stable over long periods having fewer species per genus, when compared to regions subjected to climatic stresses, such as glaciation. A total of 13 families are endemic to specific bioregions, with eight among them being monospecific. Most of these have restricted distributions which may be the result of them being the relict of a previously more diverse, but presently almost completely extinct family, or may be the consequence of vicariance events, resulting from evolution due to long-term isolation.

Mayfl y larvae are usually characterised by the presence of paired abdominal gills on the fi rst seven abdominal segments. Numerous taxa assigned to different families however possess additional membranous cuticular outgrowths on different body parts that are generally referred to as accessory gills. These accessory gills can be located on maxillae, labium, thoracic sterna or coxa. The present study compares the different structures of those accessory gills that occur in some taxa of Siphlonuroidea and Eusetisura. The homology of these outgrowths is discussed, and their possible phylogenetic relevance is evaluated.

A heptageniid mayfly species Paegniodes dao sp. nov. (Ephemeroptera: Heptageniidae) is described from Vietnam. The larva is distinguished by round anterolateral margins of pronotum, triangular-shaped labrum, rudimentary lamella and well developed fibrillae of gills 1, and light brown body that lacks distinct markings. Diagnosis, line drawings, material, and habitat data are provided.