Fig. 85.—Butterfly (Pieris): A, the newly hatched young, or larva magnified; B, pupa (natural size) just antecedent to last ecdysis; C, perfect Insect.
Every Insect after leaving the egg undergoes during the process of growth castings of the skin, each of which is called a moult or ecdysis. Taking for our present purpose five as the number of ecdyses undergone by both the locust and butterfly, we may express the differences in the successions of change we portray in Figs. 84 and 85 by saying that previous to the first ecdysis the two Insects are moderately dissimilar, that the locust undergoes a moderate change before reaching the fifth ecdysis, and undergoes another moderate change at this moult, thus reaching its perfect condition by a slight, rather gradual series of alterations of form. On the other hand, the butterfly undergoes but little modification, remaining much in the condition shown by A, Fig. 85, till the fourth, or penultimate, ecdysis, but then suffers a complete change of form and condition, which apparently is only inferior to another astonishing change that takes place at the fifth or final moult. The chief, though by no means the only, difference between the two series consists in the fact that the butterfly has interposed between the penultimate and the final ecdyses a completely quiescent helpless condition, in which it is deprived of external organs of sense, locomotion, and nutrition; while in the locust there is no loss of these organs, and such quiescent period as exists is confined to a short period just at the fifth ecdysis. The changes exhibited by the butterfly are called "complete metamorphosis," while this phenomenon in the locust is said to be "incomplete." The Insect with complete metamorphosis is in its early stage called a larva, and in the quiescent state a pupa. The adult state in both butterfly and locust is known as imago or perfect Insect.
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 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.
Various terms are used to express the differences that exist in the metamorphoses of Insects, and as these terms refer chiefly to the changes in the outer form, we will here mention them. As already stated, the locust is, in our own language, said to have an incomplete metamorphosis, the butterfly a complete one. The term Holometabola has been proposed for Insects with complete metamorphosis, while the appellations Ametabola, Hemimetabola, Heterometabola, and Paurometabola have been invented for the various forms of incomplete, or rather less complex, metamorphosis. Some writers use the term Ametabola for Insects that are supposed to exhibit no change of external form after quitting the egg, the contrasted series of all other Insects being then called Metabola. Westwood and others use the word Homomorpha for Insects in which the condition on hatching more or less resembles that attained at the close of the development, and Heteromorpha for those in which the form on emergence from the egg differs much from what it ultimately becomes.
Hypermetamorphosis.
There are certain unusual changes to which the term hypermetamorphosis has been applied; these we can here only briefly allude to.
Insects that have complete metamorphoses, and are not supplied with food by their parents or guardians, are provided during their larval life with special modifications of extremely various kinds to fit them for the period of life during which they are obtaining food and growing. Thus caterpillars possess numerous adaptations to fit them for the period during which they live on leaves, while maggots have modifications enabling them to live amongst decomposing flesh. Some larvae are greatly modified in this adaptive way, and when the adaptations change greatly during the life of the larva, hypermetamorphosis is said to exist. As an instance we may mention some beetle larvae that are born with legs by whose aid they can cling to a bee, and so get carried to its nest, where they will in future live on the stores of food the bee provides for its own young. In order that they may be accommodated to their totally different second circumstances, they change their first form, losing their legs, and becoming almost bladder-like creatures, fitted for floating on the honey without being injured by it. Such an occurrence has been described by Fabre[[82]] in the case of Sitaris humeralis, and his figures have been reproduced in Sir John Lubbock's book on the metamorphoses of Insects,[[83]] as well as in other works, yet they are of so much interest that we give them again, especially as the subject is still only in its infancy; we at present see no sufficient reason for the later of these larval states. Little is, we believe, known as to the internal anatomy of the various instars in these curious cases.
