Temperature and photoperiod are among the major abiotic factors that control all the aspects of insect life histories, especially in temperate climates. Oftentimes, an insect is sensitive not to each of these two factors separately but rather to their joint action on the organism. For example, photoperiod can modify thermal reaction norms for development and body size, i.e., it does not simply change the average values of these traits but affects the very pattern of their temperature-dependence. Identification of such interactive effects requires a dedicated experimental design in which insects are being reared under various combinations of temperature and day length. However, in some insect species coming from seasonal climates, active development and growth seem to be insensitive to photoperiod over the whole nonstressful temperature range. Here, we analyze and summarize the results of our laboratory experiments with four phytophagous chrysomeloid beetles: Chrysolina hudsonica from Canadian forest-tundra, Zeugophora subspinosa from urban greenbelts of St. Petersburg, Chrysomela populi from Bryansk forests of southwestern Russia, and Cassida vittata from Moroccan sugar beet plantations. These beetles differ in their seasonal occurrence, host plant preferences, and thermal phenotypes, and experience natural variation in daylength throughout their developmental period in the field. Nevertheless, under experimental conditions, their survivorship, developmental rate and body mass at maturity turn out to be insensitive or extremely weakly sensitive to photoperiod. Such counterexamples provide a key to better understanding how insect life histories evolve in seasonal climates. We conclude by discussing the possible causes of the discovered photoperiodic insensitivity and some nontrivial implications of our findings for future research.