The numbers of nymphal instars of several Australian Plecoptera

Abstract

Abstract The instar number of ten Australian stonefly (Plecoptera) species representing three families (Gripopterygidae, Eustheniidae, Austroperlidae) was found to range from 10–14. The conformance with Dyar's (1890) rule of the individual species was also determined and is discussed in terms of their life histories. Neboissoperla alpina has 11 instars and does not conform with Dyar's rule. Riekoperla rugosa, Acruroperla atra and Austropentura victoria have 10–11, 12 and 14 instars respectively, and conform well with Dyar's rule. Dinotoperla arenaria, Trinotoperla irrorata, Leptoperla kimminsi, Illiesoperla australis, Eunotoperla kershawi and Eusthenia venosa show uniform growth through the early instars, but do not conform with Dyar's rule in the later instars. D. arenaria has 11 instars, T. irrorata and L. kimminsi 12 and I. australis and E. kershawi 14. Instar number is different for male and female nymphs in E. venosa; the males have 13 and the females, 14. Instar number also varies with environmental temperature; the Aberfeldy River population of R. rugosa has 10 instars, while the population from a cooler habitat, Cement Creek, has 11. The application of the simple size frequency histogram and Janetschek (1967) methods in determining instar number in the Plecoptera are discussed. Interpretation of the simple histogram was found to be easier and as accurate as the Janetschek method. The application of Dyar's rule was useful in determining true instar peaks for both methods and in indicating the number and position of missing instar peaks for Stenoperla australis. S. australis appeared to have 12–14 instars.