In a pamphlet entitled “Burrowing Mayflies of Our Larger Lakes and Streams,” published in 1920 in the Bulletin United States Bureau of Fisheries, Vol. XXXVI, pp. 269-292, Professor Needham, discussing the species of the genus Hexagenia, makes the statement (op. cit. p. 279) that he is “unable to recognize more than two good and distinct species in the Eastern United States—a lowland species from lakes and rivers, Hexagenia bilineata Say. and an upland bog-stream species, H. recurvata Morg.” He amplifies this statement in previous paragraphs (p. 278) by applying the name bilineata Say. “to all the variants of the species that occupies the beds of our larger lakes and streams. The color differences appear to be only differences of degree. Even the differences of male genitalia—usually our ultimate criteria of species—are intergradient.”