The most conspicuous of the differences between Insects with complete and those with incomplete metamorphosis is, as we have remarked, the existence in the former of a pupa. The pupal state is by no means similar in all the Insects that possess it. The most anomalous conditions in regard to it occur in the Order Neuroptera. In some members of that Order—the Caddis-flies for instance—the pupa is at first quiescent, but becomes active before the last ecdysis; while in another division—the May-flies—the last ecdysis is not preceded by a formed pupa, nor is there even a distinct pupal period, but the penultimate ecdysis is accompanied by a change of form to the winged condition, the final ecdysis being merely a casting of the skin after the winged state has been assumed. In the _Odonata_ or Dragon-flies there is no pupal stage, but the change of form occurring at the last ecdysis is very great. In those Insects where the interval between the last two moults is not accompanied by the creature's passing into a definite, quiescent pupa, the individual is frequently called then a nymph; but the term nymph has merely a distinctive meaning, and is not capable of accurate definition, owing to the variety of different conditions covered by the word. Eaton, in describing this term as it is used for _Ephemeridae_, says, "Nymphs are young which lead an {158}active life, quitting the egg at a tolerably advanced stage of morphological development, and having the mouth-parts formed after the same main type of construction as those of the adult insect."[80] The intervals between the ecdyses are called stadia, the first stadium being the period between hatching and the first ecdysis. Unfortunately no term is in general use to express the form of the Insect at the various stadia; entomologists say, "the form assumed at the first moult," and so on. To avoid this circumlocution it may be well to adopt a term suggested by Fischer,[81] and call the Insect as it appears at hatching the first instar, what it is as it emerges from the first ecdysis the second instar, and so on; in that case the pupa of a Lepidopteron that assumed that condition at the fifth ecdysis would be the sixth instar, and the butterfly itself would be the seventh instar.