RES Handbook Vol 1 Part 9. Ephemeroptera

NORMALLY two pairs of wings; anterior pair much larger than the posterior which in some genera are much reduced in size or even entirely absent. Venation is of considerable importance in classification. The areas between the veins take the name of the main vein preceding them. Small additional veins between the main veins are termed intercalary veins. Three pairs of legs, the anterior pair in the male often very long. The abdomen terminates in two or three long, many-jointed setae, the median one in certain genera being atrophied, and the male carries a pair of jointed claspers or forceps arising from a plate on the ninth sternite. Mouth-parts completely atrophied. Compound eyes of male larger than of the female, sometimes divided, and the upper part elevated turret-like (turbinate eye).