Datensatz

Zusammenfassung

Field and laboratory studies of Pteronarcys reticulata (BURMEISTER, 1839) and P. sachalina KLAPALEK, 1908 during several years are combined to elucidate development and life history of these two coexisting Far East Russian stoneflies. They were found to be similar in all studied aspects. Adults fly and oviposit mainly in June. Spontaneous egg development occurs at 16°C and over; at 12°C and lower, eggs remain dormant until warming induces development. Speed of development is not distinctly temperature dependent. Ready-to- hatch larvae remain in diapause until warmed again after an exposure to cold, i.e., in the field they apparently hatch the next spring. In the first instar the larva is inactive, its head capsule width (HCW) is 0.4 mm. Qualitative developmental change (gills, pilosity, pronotal comers and abdominal point, antennal and cercal segment numbers, wing pads) is continuous, instars cannot be identified, except the last, mainly by abrupt growth of wing pads. Mode of wing development differs from other Plecoptera studied so far. Larvae in the first two years moult several times per year, larger larvae less; growth is interrupted in winter. In their third summer, larvae enter the penultimate instar and in autumn the last instar which lives for 8 - 9 months, until adult emergence five years after oviposition. Increment of linear measurements at moults varies individually; average increment is 18 % and little dependent on larval size until specimens prepare visibly for metamorphosis, at HCW > 3 nun. During the last two moults, the increment drops to an average 8 %. Specific or sexual differences in growth rate were not detected. The fundamental unsuitability of the standard approach of size class analyses in random population samples using linearly increasing classes is discussed. An alternative geometrically progressing growth model based on first instar HCW and mean increment at moults recognizes 16 hypothetic instars; last instar males (HCW 3.8-4.7 nun) fall into instar 15, females (HCW 4.3-5.6 mm) into instar 16; whether sexes differ by one in actual instar number is uncertain.