Thermal adaptation in insects : effects of inbreeding environment and selection Insekten Künstliche Auslese Umweltfaktor Photoperiode Genetische Korrelationen evolutionäres Potential The ability to withstand thermal stress is considered to be of crucial importance for individual fitness and species survival. Thus organisms need to employ effective mechanisms to ensure survival under stressful thermal conditions. Responses to environmental challenges may occur quickly through phenotypic plasticity or through genetic adaptation needing longer periods of time. Beside thermal stress other environmental factors might have a similar impact on temperature stress resistance. We first investigated phenotypic adjustment in temperature stress resistance following different environmental manipulations in the butterfly Bicyclus anynana. We found that temperature induced plasticity quickly and strongly modulated temperature stress resistance and that such responses are readily reversible. Short time hardening responses revealed more complex patterns with e.g. cold stress resistance being highest at intermediate hardening temperatures. However we found resistance traits also to be affected by food availability age and light cycle. We further investigated whether temperature stress resistance is affected by photoperiod in the fly Protophormia terraenovae. Indeed variation in temperature stress resistance can be triggered by photoperiod (and temperature) with shorter day lengths inducing more cold and longer day lengths more heat tolerant phenotypes. We suggest that short term photoperiod mediated changes in insect thermal tolerance represent a mechanism ...