The development stages of a species may have an identical lower development threshold (LDT) and proportionally different durations. This phenomenon called "rate isomorphy" (RI) has been demonstrated for a number of insect species. In contrast, the growing day degrees accumulated over the period of larval development (sum of effective temperatures SET) should be plastic and vary with environment conditions. The prediction from RI is that, with changing conditions, the uniform LDT should be accompanied by differences in development time which remain proportional at different temperatures. This was tested by investigating the effect of diet on thermal requirements for development of larvae of the polyphagous species Autographa gamma (L.) (Lepidoptera: Noctuidae). The larvae were kept at 15.0, 20.3 and 26.7°C and fed on leaves of 13dicotyledoneous herb and tree species. The proportion of total development time spent on a particular diet was plotted against temperature. The existence of RI was inferred from a zero change in development time proportion with changing temperature. This rigorous test supported RI for 3 of 9 diets where development was completed in all temperatures. The LDT observed on 11 diets where the larvae completed development in at least 2 temperatures varied between 9.3 and 11.0°C while SET varied between 167 and 353 day degrees (dd). Assuming RI, LDT and SET for those 9 diets were recalculated. The recalculated LDT was 10.0°C and SET varied between 177-257 dd. The SET increased with decreasing water content and decreasing nitrogen content of food. Worsening food quality decreased food consumption, metabolic and food conversion efficiency, and the relative growth rate of the larvae. Increasing metabolic costs of development were thus positively correlated with SET. The standardized rate of growth (mg.dd-1) was typical for particular diets. Pupal mass decreased with increasing temperature and, within each temperature, with development length.
Development rates of the eggs of 9 species, larvae of 6 species and pupae of 6 species of the genus Amara (Coleoptera: Carabidae) were recorded at five constant temperatures between 17 and 28°C, and thermal constants for each development stage calculated. The lower development threshold varied between 9.2-13.5°C for different stages and species. Rate isomorphy, which implies the existence of a common temperature threshold for all development stages, was demonstrated in 5 species. The sum of effective temperatures differed between stages. On average the egg stage took 18%, the first larval instar 13%, second instar 13%, third instar 35% and pupa 21% of the total development time. A poor diet increased the SET of the larvae. The results are compared with published data on Carabidae.