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• May samples from upper Dry Creek contained the largest number of taxa and also the <br />greatest abundance of benthic animals as shown on Tables 11-30 and 11-31, These were <br />dominated by the sp heariid clams Pi si dium sp. and Spha erium sp. Other abundant benthic <br />animals were baetid mayflies. Midge larvae of the family Chironomidae and black fly <br />larvae of the family Simvlidae ranked third and fourth in abundance, respectively. <br />As mayflies of the family Baetidae, midge larvae, and black fly larvae dominated in the <br />majority of locations sampled, a brief description of their life histories and <br />environmental requirements is presented below. <br />Because of the altitude, flow, and temperature regimes of local streams, mayflies <br />inhabiting these waters probably exhibit yearly cycles. Considerable numbers of mature <br />specimens were collected from most sites in May and from lower Sage Creek (Site 5-3) in <br />November, indicating that peaks of emergence, mating, and egg laying probably occur <br />during late spring and early summer. According to Berner (1959), however, it is possible <br />for baetid mayflies to have two generations per season in the northern portions of their <br />range and two or more generations per season in the southern portions of their range. <br />• Baetid mayflies are considered by Needham et al. (1935) to be rheophilic and free ranging <br />species, living in shallow running water under rocks and debris or on vegetation along the <br />banks of streams. Feeding habits of these nymphs appear to be entirely herbivorous with <br />periphyton being the principal food source (Berner, 1959). Baetid mayflies are known to <br />exist under wide ranges of natural chemical concentrations in their environment, but <br />appear to need dissolved oxygen levels greater than 4 mg/1 and a pH range of 5.6 to 8.5 <br />(Roback, 1974), <br />Midge larvae of the Chironomidae were abundant to common at all stations sampled. These <br />benthic animals are quite ubiquitous, being found in almost every type of aquatic <br />environment. It is necessary to identify members of this family to the species level in <br />order to determine particular environmental requirements and pollution tolerances. <br />During stress or in polluted environments, tolerant species may replace sensitive species <br />of the same genera. Species of chironomids may produce several generations per year, <br />Feeding habits range from omnivorous to herbivorous, with some species being predaceous. <br />Larvae of the family Simulidae have more stringent environmental requirements than do <br />• chironomid larvae. The simul id larvae require fast flowing, well-oxygenated water with <br />stable, stoney substrates. Feeding is accomplished by large mouthbrushes or fans which <br />27 <br />