Substrate selection as a factor inHexageniadistribution

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Abstract
Our observations demonstrate that in laboratory aquaria Hexagenia bilineata (Say) nymphs will leave clay, gravel, and sandy clay and migrate to adhesive mud or fine sandy mud with no response to coarse sandy mud indicated. The nymphs were able to burrow into all substrates offered, but the time required for penetration varied significantly. Burrowing times ranged from 30 s in adhesive mud to 168 s in clay. Ease of penetration (as measured by burrowing time) correlated well with observed substrate preferences. None of the physical or chemical parameters of the substrates which we measured correlated significantly with either selection or burrowing time. Ease of penetration seemed to be dependent upon a combination of factors including particle size and compactness and cohesiveness of the substrates. The characteristics of the two selected substrates include high moisture content (40% for each), low bulk density (0.91 and 0.94 g/ cm3), high organic content (7.1 and 7.9%), and predominantly silt composition (29% clay, 60% silt, and 11 % sand for adhesive mud and 26% clay, 51 % silt, and 23% sand for fine sandy mud). Our data suggest that substrate selection may play an important role in observed distribution patterns.