In their response to environmental stimuli, terrestrial isopods show various trends that are correlated with their ecology and physiology. With the transition from sea to the littoral zone ( Ligia ), orientation to light changes from positive to negative. Yet, since these isopods are positively hygrokinetic even at very high humidities, their hygroreaction is of greater significance than their photoreaction. In isopods from mesic habitats ( Oniscus, Porcellio, Armadillidium ) photoreaction becomes of less significance than in littoral species, until in some species it reverts to positive ( Armadillidium ). Sometimes the positive photoreaction occurs only at high temperatures ( Porcellio ), a pattern of behavior correlated with thermoregulation by evaporative cooling. Similarly, in mesic species the response to humidity becomes less significant than in littoral species, resuming importance mainly when the isopods become dehydrated ( Armadillidium ). Finally, in isopods from xeric habitats in semi-arid and desert regions ( Armadillo, Venezillo ), photoreaction is strongly negative except in Hemilepistus at lower temperatures. All of these isopods are positively hygrokinetic only at low humidities and are strongly negatively thermoactive, indicating a drop in activity at high temperatures. The photonegative response in these isopods is stronger than the hygroreaction.