Malpighian tubules of the nymph of Baetis rhodani (Ephemeroptera, Baetidae)

The ultrastructure of the Malpighian tubules of the final instar nymph of t he mayfly Baetis rhodani is described. Numerous Malpighian tubules enter th e alimentary canal at the transition from midgut to hindgut. Each of them i s made up of a cylindrical distal portion and a proximal narrow duct emptyi ng into the gut. At the junction between the two regions, the cylindrical p ortion enlarges and curves, to continue into the thin duct. The wall of the cylindrical portion consists of primary cells characterized by typical fea tures of the secretory/absorbing epithelia. These cells show a large polyte ne nucleus, an apical microvillated border, a high number of mitochondria a nd basal infoldings. The narrow duct is formed of large cells joined by hig hly convoluted junctions, and their cytoplasm is Riled with electron-transl ucent vesicles. The apical border of the cells of narrow duct lacks microvi lli but bears several finger-like extensions. Each narrow duct empties into a canal interposed between groups of gut epithelial cells, which constitut e the transition area between midgut and hindgut. The Malpighian tubules of B. rhodani represent an ancestral condition and their structural arrangeme nt provides important information for tracing mayfly evolutionary trends.