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 re- 

 quires only a short time for the expansion ot 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 trans- 

 formation 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 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 

 life cycle is called a larva, and the young of an insect 

 that does not have a pupal stage is termed a nymph, ac- 

 cording to the modern custom of American entomologists. 

 But the term "larva" was formerly applied to the im- 

 mature stage of all' insects, a usage which should have 



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