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-15- Aquatic Insects A total of 66 known general of aquatic insects were collected from the river and tributary streams. Identification of the forms collected was made by Guy Musser, G. F. Edmunds and other members of the faculty, Department of Biology, University of Utah. Generally, there was a similarity in the insect fauns, of most of the tributary streams. The differences in species composition iri collections from the various tributaries appears to be due largely to sampling procedure. Most of the streams sampled were very productive in both numbers and kinds of insects (Table 5), although flash floods doubtlessly reduce their numbers in certain areas. A reduction in the number of aquatic insects occurred during the fall, which appeared to be due to the emergence of many of the forms observed during the summer. The abundant insect fauna makes up a large proportion of the diet of the dace and bluegill which occur in the tributary streams. Although mayfly nymphs were abundant in the streams and ponds, no stone flies (Plecoptera) were collected, a few cast skins being the only evidence of-this group. The paucity of these forms in the area may be due to the fact that most of the tributaries are relatively small slow streams with high summer water temperatures. G.F. Edmunds collected a few stonefly nymphs at Aztec Greek in April 194g• It is quite possible that these insects are con- fined to some of the larger and swifter tributaries. In contrast to the las^ge numbers of aquatic insects present in the trib- utaries, the Colorado and San Juan Rivers support sparse, though fairly di- verse, populations of aquatic insects. Most of the aquatic insects collected from the rivers were found attached to rocks, especially in the riffle areas, or in small bays. Very few insects were found among the shifting sands of the river bottom. This dearth of aquatic insects is doubtlessly one of the factors responsible for the scarcity of fish, other than the herbivorous flannelmouth sucker and omniverous channel catfish, in these rivers.