Interspecific variation in temperature dependence of egg development of five congeneric stonefly species (Protonemura Kempny, 1898, Nemouridae,Plecoptera)

Embryonal development of the five congeners Protonemura auberti Illies, 1954, P. hrabei Rauser, 1956, P. meyeri (Pictet, 1841), P. nitida (Stephens, 1835), and P. praecox (Morton, 1894) was studied under various laboratory temperatures and different photoperiods.Mean number of eggs in field collected batches was between 470 (P. praecox) and 1211 (P. auberti). Spring species had smaller egg batches than autumn species (Table 1). Mean hatching success in the laboratory was 50–100% at 2–18 °C. In most species hatching success decreased slightly with increasing temperature (Figs. 1a-e). None of the eggs incubated at 24 °C developed. Hatching pattern followed an asymmetric frequency distribution. In general, the hatching periods were the shorter the higher the incubation temperature.Embryonic development of all five species was inversely temperature dependent (Figs. 2a-e), and well described by a power law relationship (Figs. 3a-e). Interspecific differences in incubation periods were notable at nearly all temperatures (Fig. 4). There was a distinct interspecific sequence in length of incubation period (with steps of about 4 days), which was the same as can be seen in the flight periods: The later the species flies the longer the incubation period. Temperature fluctuations and variations in photoperiod had no influence on incubation and hatching periods or hatching success.The thermal demand of the egg stage neither explains the recent geographical distribution of the Protonemura species, nor does it directly correspond to the field temperatures common during their egg development. However, it is optimal in respect to resource partitioning between the five species, with the consequence of temporal displacement of life cycles.Derived from Brittain's (in press) proposal to compare the two constants a and b of the regressions describing the temperature dependence of embryonal development, a new index (Integral Development Time, IDT) indicating the thermal demand was created for easier comparison of numerous species (Table 2). Evaluation of the IDT for various species of Plecoptera (Fig. 5) suggests that species belonging to the family group Systellognatha generally have higher thermal requirements in the egg stage than species belonging to the Euholognatha.