Multiple Developmental Pathways of Agnetina capitata (Plecoptera:Perlidae) in a Temperate Forest Stream

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Abstract
We determined the phenology and life-cycle of a population of the perlid stonefly, Agnetina capitata (Pictet), in Cascadilla Creek, Tompkins County, New York, including the pattern of larval growth throughout the year, the period of adult emergence, and the development of eggs under laboratory conditions. Eggs did not develop at 5°C and hatching was negligible at 10°C, suggesting that the lower threshold for embryonic development in A. capitata is above those temperatures. Median egg development time significantly decreased with increasing temperature from 15 to 20-25°C. Independent of temperature, however, there was considerable variation in the development time within egg clutches. Thus, the delay in hatching of A. capitata eggs is complex, showing characteristics of both quiescence and diapause-mediated dormancy. Changes in larval size were not observed during the winter, suggesting that stream temperatures are too low for growth and development. Last instars first appeared in May, and pharate adults were collected in emergence traps throughout June and July. The distribution of emergence was characterized by two peaks in both males and females. Slight protandry was observed in both emergence peaks, and a decrease in size throughout the emergence season occurred for both sexes. Growth patterns of A. capitata larvae were consistent with a complex life cycle that is completed in at least two years and characterized by multiple developmental pathways (cohort splitting). Delayed hatching within egg-clusters, an extended emergence period for adults, and an oviposition period spread out over the life span of the female lead to an extended larval recruitment period and large variation in larval size in the population. Those larvae that hatch early, soon after the beginning of oviposition during the summer, seem to complete the life cycle in two years; but they emerge later in the season than those that hatched from eggs in the fall or early spring, and emerge after three years from oviposition.

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