Climate features that influence life cycles, notably severity, seasonality, unpredictability and variability, are summarized for different polar zones. The zones differ widely in these factors and how they are combined. For example, seasonality is markedly reduced by oceanic influences in the Subantarctic. Information about the life cycles of Arctic and Antarctic arthropods is reviewed to assess the relative contributions of flexibility and programming to life cycles in polar regions. A wide range of life cycles occurs in polar arthropods and, when whole life cycles are considered, fixed or programmed elements are well represented, in contrast to some recent opinions that emphasize the prevalence of flexible or opportunistic responses. Programmed responses ale especially common for controlling the appearance of stages that are sensitive to adverse conditions, such as the reproductive adult. The relative contribution of flexibility and programming to different life cycles is correlated with taxonomic affinity (which establishes the general lifecycle framework for a species), and with climatic zone, the habitats of immature and adult stages, and food., Hugh V. Danks, and Lit
First stadium juveniles of P. angustus were reared under controlled seasonal conditions to maturity, reproduction and death. Individuals born in any one breeding season either had a 1-year or a 2-year life cycle (cohort-splitting). The life cycle was annual for individuals born in the first part of the breeding season (May-August), but became biennial for those born later (August-October). Two phenomena were involved: (1) Only individuals reaching the penultimate stadium (stadium VII) before a critical period at the end of spring could become adult in the breeding season following that of their birth. After this time, stadium VII individuals entered into aestivation and only became adult in the second autumn after their birth. (2) Females becoming adult in autumn entered reproductive dormancy and only laid eggs in the following spring. Overall, individuals born at the start of the breeding season easily reached stadium VII before the critical period and were able to breed at I year, whereas individuals born at the end of the breeding season reached stadium VII after the critical period, then had two consecutive periods of dormancy and only bred at 2 years age. Individuals from the same nest born in the middle of the breeding season (August) could have either annual or biennial life cycles, depending on whether they reached stadium VII before or during aestivation. The environmental factors capable of triggering aestivation in subadults and reproductive dormancy in autumn-maturing females are discussed., Jean-Francois David, Marie-Louise Celerier, Jean-Jacques Geoffroy, and Lit
There is much current discussion about the factors that control the distribution and abundance of animal species, particularly at the edges of their range. The significance of temperature for survival and development is compared in two closely related psyllid species (Craspedolepta nebulosa and C. subpunctata) living on the same host plant (Chamerion angustifolium) (Onagraceae) but displaying different distributions along latitudinal and altitudinal gradients. The following measurements were made at critical periods during the life cycle (a) winter supercooling points (SCPs), (b) tolerance of short (1 min) and long term (1-25) days exposure to sub-zero temperatures above the SCP, (c) tolerance of short term exposure to high spring/summer temperatures and (d) comparative field development rates among species and sites during the early critical part of the growing season. Successful completion of the life cycle is related to heat availability during the growing season. This appears to limit the distribution of the Craspedolepta species, rather than their survival response to thermal extremes. No significant differences were found between the two species in the supercooling point or in their long and short term survival responses at low or high temperatures., Jeremy M. Bird, Ian D. Hodkinson, and Lit