The Orthopteroid insects (sens. lat.) include all such forms as have mouth-parts of the mandibulate type and undergo a gradual or “incomplete” metamorphosis. As in other orders of insects, the extent to which the immature stages (“nymphs”) diverge from the adult in form and structure is more or less proportional to the differences in environment and habits. Accordingly they may be divided into two groups, (1) those which are terrestrial throughout life, and (2) those in which the early stages are aquatic.