METAMORPHOSIS
must remain at the stage it had reached when the cuticula hardened. Only bi a subsequent separation of this cuticula, allowing another period of growth in the cells of the body wall, can the form and the external organs of the adult be perfected. With another molt, therefore, the fully formed insect is at last set free, and it now re- quires only a short time for the expansion of the legs and wings to their normal size and shape and for the hardening of the final cuticular layer which will preserve the contours of the adult. It thus comes about that the members qf a large group of insects haie acquired an extra stage in their lire cycle, namely, a final reconstructive stage beginning some time be_[ore the last molt qf the young and completed v«ith a final added molt a'hich liberates tt?e fully .formed adult. The insect in this stage is called a pupa. The entire pupal stage is divided bi the last molting of the young into a propupal period, still occupying the loosened cuticula of the insect in its last adolescent stage, and a true pupal period, which is that between the shedding of this last skin of the young and the final molt which discloses the matured insect. All insects that undergo a metamorphosis may be divided, therefore, into two classes according as the trans- formation from the young into the adult is direct or is COlnpleted in an intervening pupal stage, lnsects of the first class are said to have ineomplete metamorphosis; those of the second class, eomplete metamorphosis. The ex- pressions are convenient, but misleading if taken literally, for, as we shall see, there are many degrees of "complete" metamorphosis. The young of any insect that has a pupal stage in its lire cycle is called a larva, and the young of an insect that does not haie a pupal stage is termed a n.vmph, ac- cording to the modern custom of American entomologists. But the terre "larva" was formerly applied to the im- mature stage of all insects, a usage which should haie
l ?45 1
INSECTS
