Many organisms have complex life cycles with distinct life stages that experience different environmental conditions. How does the complexity of life cycles affect the ecological and evolutionary responses of organisms to climate change? We address this question by exploring several recent case studies and synthetic analyses of insects. First, different life stages may inhabit different microhabitats, and may differ in their thermal sensitivities and other traits that are important for responses to climate. For example, the life stages of Manduca experience different patterns of thermal and hydric variability, and differ in tolerance to high temperatures. Second, life stages may differ in their mechanisms for adaptation to local climatic conditions. For example, in Colias, larvae in different geographic populations and species adapt to local climate via differences in optimal and maximal temperatures for feeding and growth, whereas adults adapt via differences in melanin of the wings and in other morphological traits. Third, we extend a recent analysis of the temperature-dependence of insect population growth to demonstrate how changes in temperature can differently impact juvenile survival and adult reproduction. In both temperate and tropical regions, high rates of adult reproduction in a given environment may not be realized if occasional, high temperatures prevent survival to maturity. This suggests that considering the differing responses of multiple life stages is essential to understand the ecological and evolutionary consequences of climate change.

Most terrestrial ectotherms experience diurnal and seasonal variation in temperature. Because thermal performance curves are non-linear, mean performance can differ in fluctuating and constant thermal environments. However, time-dependent effects -effects of the order and duration of exposure to temperature -can also influence mean performance. We quantified the non-linear and time-dependent effects of diurnally fluctuating temperatures for larval growth rates in the tobacco hornworm, Manduca sexta L., with four main results. First, the shape of the thermal performance curve for growth rate depended on the duration of exposure: for example, optimal temperature and thermal breadth were greater for growth rates measured over short (24 h during the last instar) compared with long (the entire period of larval growth) time periods. Second, larvae reared in diurnally fluctuating temperatures had significantly higher optimal temperatures and maximal growth rates than larvae reared in constant temperatures. Third, for larvae maintained at three mean temperatures (20, 25 and 30°C) and three diurnal temperature ranges (±0, ±5 and ±10°C), diurnal fluctuations had opposite effects on mean growth rates at low versus high mean temperature. Fourth, both short-and long-term thermal performance curves yielded poor predictions of the non-linear effects of fluctuating temperature on mean growth rates (compared with our experimental results) at higher mean temperatures. Our results suggest caution in using constant temperature studies to model the consequences of variable thermal environments.

Summary1. Rapid evolution of physiological traits in response to novel thermal environments has rarely been demonstrated in natural populations. 2. We studied the temperature dependence [thermal performance curves (TPCs)] of larval feeding rate for two populations each of Colias eurytheme and Colias eriphyle in North America that occur over a range of elevations and climates. Using historical data for two of the populations, we assessed changes over time in both air temperatures and TPCs for larval feeding. 3. Populations at lower elevations with longer growing seasons had broader TPCs for larval feeding. In contrast, higher elevation populations with shorter growing seasons had higher optimal and maximal temperatures for feeding. 4. Overall mean air temperatures during the growing season showed little change at the two sites, but the frequency of high air temperatures (>28°C) has increased markedly at both sites over the past 40 years. This climatic shift was associated with increased rates of larval feeding at higher temperatures (>28°C) in both populations. 5. These results suggest that recent climate warming has led to physiological shifts in the TPCs for larval feeding in this system, indicating that thermal adaptation can occur rapidly in response to changing thermal conditions.

Summary1. Adult size and development time are the outcomes of growth and differentiation throughout the life of an individual organism -its developmental trajectory. Correlations among ages and sizes across different life stages may influence both direct and indirect components of selection on body size and age during development. 2. We used two field studies in experimental gardens in fall and summer to evaluate phenotypic selection on size and age across larval, pupal and adult stages in the tobacco hornworm, Manduca sexta, and how this may vary across seasons. 3. Rapid larval development was positively associated with survival to pupation and adulthood, in part because it allowed escape from larval parasitoids. Larval mortality owing to parasitoids was greater in the fall than in the summer. Egg production was positively correlated with adult mass, but not with development time. 4. Principal component analyses of size and age throughout development showed that adult size and development time were not negatively correlated, contrary to life-history expectations. As a result, selection favouring larger adult size (via female reproduction) and selection favouring rapid larval development (via juvenile survival) do not act in opposition in this system. 5. We discuss the physiological mechanisms that may underlie the independence of adult size and early larval development for holometabolous insects, and the implications for selection on body size and developmental trajectories.