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55 DIAGNOSTIC FEATURES AND DESCRIPTION There are two other species of crayfish [the Water Nymph crayfish (Orconectes nais) and the Western Plains crayfish (Orconectes causeyi) that have been confused with the Northern crayfish for decades. Though the Northern crayfish was described in 1870 85 , the Water Nymph crayfish in 1885 59 and the Western Plains crayfish in 1967 123 , it is still not clear as to whether these three are separate species. Hobbs 103 considered the Western Plains crayfish to be a synonym for the Northern crayfish and later stated that "This crayfish [O. causeyi], insofar as I am able to determine, is indistinguishable from O. virilis". 104 Fitzpatrick 66 said that the Northern, Water Nymph and Western Plains crayfishes were "morphologically nearly indistinguishable". In spite of these statements, two old blood serum studies have led to retaining the distinction between the Northern and Water Nymph crayfish. 193, 194 All three of these species have been reported from Nebraska and are present in museum voucher collections. The characters used to separate these species are the physical proportions of the first pleopod and the relative widths of the aureola. I have done an extensive study of these characteristics for several populations in Nebraska. What I found was that there was greater within- population variability in these characteristics than there was between populations. In a nutshell, I concluded that there was no difference between them and have considered the Water Nymph crayfish and the Western Plains crayfish to be synonymous with the Northern crayfish within Nebraska. While the Northern crayfish does have markings, they aren't very distinctive and they tend to disappear as they grow. The coloration of adults is an overall tan-brown- olive with blue-green tinges on the claws. The back can be quite dark which grades into a much lighter belly though the coloration can be variable depending on age and water quality. The juveniles like the one in these photos have the best markings and can be quite light colored. Older individuals can become a very dark brown/olive with age. Colors are their brightest immediately after a molt. Algae