had the largest number of species. Generally caddisfly distribution is 

 similar to that of the Plecoptera with a steady downstream decline in species. 

 The genera Hydropsyche (figures 24 and 25) and Cheumatopsyche are abundant 

 throughout the river, but dominate in the lower 10 stations. 



Other Orders 



The distribution of the remaining aquatic orders is given in figure 26. 

 The order Diptera is widely distributed throughout the river, with the 

 family Chironomidae being the most abundant and diverse. Protanyderus 

 margarita, a Diptera species previously unreported from Montana, was captured 

 at several stations. Representatives of the remaining orders illustrated 

 no distributional trends and, with the exception of the Oligochaeta, were 

 never abundant. 



TONGUE RIVER 



The distribution of macroinvertebrates found in the Tongue River, shown 

 in table 3, is complex and not easily explained. The fauna is similar to 

 the Yellowstone fauna in many ways, but there are several differences. The 

 stonefly Aovoneuvia abnovmis, the elmid beetle Stenelmis sp. and the mussel 

 Lampsilis sp. are abundant in the Tongue but rare in the Yellowstone. 

 Odonates are more abundant and diverse in the Tongue River. 



INSECT EMERGENCE 



MAYFLIES 



Emergence times were determined for only 13 species of mayflies (figure 27), 

 generally the species common in the lower reaches of the Yellowstone River. 

 Most mayfly adults emerge at dawn or dusk and live from a few hours to a 

 few days. Emergence of mayfly adults in the lower river is concentrated in 

 the June-September period. Adult Ephovon album emerged so late in the summer 

 that many adults, influenced by cold morning temperatures, were observed 

 fluttering on the beaches, unable to fly. 



One of the largest mayfly emergences observed occurred in late August 

 1974 at Huntley (station 11), where adult Traverella albertana (figure 28) were 

 emerging. The adults were so thick on the water surface (probably hundreds of 

 thousands of insects were involved) that carp were surface feeding on them. 

 It was a wet day, and the adults hovered over the wet highway from Huntley to 

 Miles City. The conspicuous emergences of Tricorythodes minutus (figure 29) 

 and Ephoron album also involved large numbers of individuals. 



STONEFLIES 



The emergence of adult stoneflies, occurring from March to August (figure 

 30), covers a longer time span than does that of mayflies. Three species, 



42 



