INSECT METAMORPHOSIS
must remain at the stage it had reached when the cuticula hardened. Only by 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 requires 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 of a large group of insects have acquired an extra stage in their life cycle, namely, a final reconstructive stage beginning some time before the last molt of the young and completed with a final added molt which liberates the fully formed adult. The insect in this stage is called a pupa. The entire pupal stage is divided by 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 transformation from the young into the adult is direct or is completed in an intervening pupal stage. Insects of the first class are said to have incomplete metamorphosis; those of the second class, complete metamorphosis. The expressions 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 life cycle is called a larva, and the young of an insect that does not have a pupal stage is termed a nymph, according to the modern custom of American entomologists. But the term "larva" was formerly applied to the immature stage of all insects, a usage which should have
[ 245 ]
