Figure 21. Mature nymph of the mayfly (Heptagenia elegantula) , 



Stoneflies 



The longitudinal distribution of the stoneflies (Plecoptera ; in figure 22 

 differs considerably from that of the Ephemeroptera (figure 19). Thirty-seven 

 species were identified in the study area. Data available for this order are 

 probably the most accurate because of the work of Stadnyk (1971) and Gaufin 

 et al . (1972). Only one species was collected at every station. Most of 

 the fauna are probably adapted to the conditions found in the upper river. 

 Twelve species drop out in the transition zone, and five could be classified 

 as prairie stream forms. Aaroneuria abnormis probably washed out of the 

 Tongue River, where it is abundant, and was collected only at station 15. The 

 number of Plecoptera species decreases steadily downstream (figure 20). 

 Generally the nonprairie stoneflies appear to have habitat requirements 

 similar to those of the salmonid fishes. 



Caddisfl ies 



Caddisfly (Trichoptera ; distribution in the Yellowstone River is presented 

 in figure 23. The present list contains 36 species; more will probably be 

 collected if additional studies are performed. Distributional patterns are 

 less distinct than with the Ephemeroptera and Plecoptera. In most cases 

 caddisfly larvae cannot be identified to species; adult males are necessary. 

 The present distribution data are incomplete because all stations were not 

 sampled with equal frequency. For example, station 9, sampled more intensively, 



39 



