Wing Pad and Tergite Growth of Mayfly Nymphs in Winter

Abstract

Growth rates of nymphal wing pads and tergites of Epeorus (Ephemeroptera) during the winter from a West Virginia stream were established. The average specific growth rate of the wing pad was 0.193 mm/week/mm and the average growth rate of the tergite was 0.017 mm/week. The pre-emergent tergite growth is slower in con- trast to the rapid increases of wing pad size. The effect of temperature on the rate of growth appeared to be minimal, as the growth rates were achieved during periods of lowest water temperature. Most stream bottom fauna studies have been conducted under warm temperature conditions. Little information concerning the winter ecology of stream benthos is available. Some abundant species appear as early instars in the autumn and undergo a spring emergence. A study of the characteristics of physical growth during the period of low temperatures is important for an understanding of the life cycles of aquatic insects. Brown (1953) found that freezing tempera- tures seemed to have little effect on certain aquatic organisms pro- viding the water immediately surrounding them is not frozen. There is a lack of agreement as to the effect of winter conditions on the growth of the Ephemeroptera. Macan (1957) was unable to de- termine if the growth rate was affected by temperature, but recog- nized some nymphal growth during the winter. Illies (1952) found that Rhithrogena sp. did not grow during the winter. The objective of this investigation was a winter study of physical growth in Epeorus (Ephemeroptera). This genus constituted 30 per cent of the total bottom fauna sampled.