Nearly all current hypotheses on the phylogeny of pterygote insects are unsatisfactory because they do not take into account the constraints resulting from the palaeozoic environments where the first steps of this evolutionary history were accomplished. Present knowledge about freshwater and terrestrial environments in the Middle and Upper Palaezoic suggests that the evolutionary history of insects is quite like that of vertebrates. Therefore, we agree with Handlirsch in maintaining that the pterygote insects came direct from aquatic ancestors; the transition from a truly aquatic to a truly terrestrial life being represented by amphibiotic habitants, as in mayflies, or dragonflies. Our hypothesis is supported by many arguments, mostly concerning (a) strategies for protecting eggs and juveniles from desiccation, (b) structural and functional traits of the sense organs, (c) mechanisms of sperm transfer, and (d) mechanisms for breathing.