Diel activity patterns in nymphs of an Australian mayfly Atalophlebioides sp. (Ephemeroptera : Leptophlebiidae)

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Abstract
Activity of nymphs of Atalophlebioides sp. on the surface of a stone in a laboratory stream was greatest at night, with peak activity occurring at about 2400 h. Most nymphs remained on the lower surface of the stone during the day, but there was gradual movement on to the upper surface at night. Although nymphs showed anocturnal maximum in both drift rates (field observation) and activity. peaks of activity did not correspond with peaks in drift rates. Nymphs at high density (15 per stone) showed a significantly greater level of activity during light hours (1200-1800 h) and the first two hours of darkness compared with nymphs at lower densities (five and two per stone) but there was no significant difference in activity between the two lower densities for the period 1200- 2400 h. Feeding behaviour was observed at all three densities, but activities in the form of rapid darting (duration < 1 s) and a 'swimming' response occurred at the higher density. It is postulated that a relationship exists between the swimming response (as a result of competition for space) and the insect's propensity to drift.

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