The Thermal Resistance of Mayfly Nymphs from Ponds and Streams

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Abstract
1. The thermal resistances of six species of mayfly nymphs, collected from various types of habitat, have been determined. 2. Nymphs from slow or still waters have a greater resistance towards high temperatures than comparable nymphs from swift waters. This is correlated with the more extreme conditions of temperature which are known to exist in slow and still waters. 3. The interspecific differences of thermal resistance cannot be accounted for on the basis either of differences of size or of differences in the efficiency of taking up oxygen dissolved in the water. 4. In one of the stream nymphs (Baetis rhodani) acclimatization to higher temperatures was not possible within 40 hr. 5. The bearing of the results on the ecological distribution of the mayfly nymphs is discussed.