We have seen that the embryo rapidly passes through extraordinary changes of form, and now, after hatching, especially in the insects with a complete metamorphosis, the animal continues to undergo striking changes in form, in adaptation to different modes of life.
The life of a winged insect, such as a butterfly, fly, or bee, may be divided into four stages: the embryo, or egg state, the larva, pupa, and imago,—the term metamorphosis being applied to the changes after birth, or post-embryonic stages of life. The transformations of the more specialized orders of insects involve wonderful changes of form, which are only paralleled in other types of animals by the metamorphoses of the echinoderms, of certain worms, and of the Crustacea, as well as by those of the frog. An insect, such as a butterfly or bee, during its post-embryonic life lives, so to speak, three different lives, having distinct bodily structures and existing under quite dissimilar surroundings and habits; so that a caterpillar is practically a different animal from the pupa, and the latter from the imago, with different organs, the appendages and other structures being so modified as to be, so far as regards their functions, radically different. These changes of functions or of habits have also been plainly enough the exciting cause of the divergency in structure of what fundamentally is one and the same organ, the change having been brought about by adaptation of the same organs to quite different uses.
The changes are not only observable in the body and its appendages, but also in the internal organs, and consequently are both structural and physiological. The term larva, as applied to the first stage of animals, is a very variable and indefinite one, that of insects in general being a much more highly organized animal than the larva of a worm, starfish, or crustacean.
a. The nymph as distinguished from the larval stage
As there is no marked difference between the different stages of the young in the insects with an incomplete metamorphosis (Heterometabola), the chief difference being the possession of the rudiments of wings and the absence of a resting stage, the terms larva and pupa are in reality scarcely applicable to them, and we much prefer the term nymph, first proposed by Lamarck for the active “pupa” of Orthoptera, Hemiptera, the Odonata and Ephemeridæ, and adopted in part by many. Indeed, in the more generalized and older orders, the larval and pupal stages are not differentiated, though the term larval, in its general sense, will probably always be used; just as we speak of the larval stages of worms, echinoderms, or Crustacea.
Eaton in his elaborate work on the Ephemeridæ employs the term nymph to designate all the aquatic or early stages in the development of the young after hatching, and he urges that the old-fashioned usage of larva and pupa seem scarcely worth retention. “Nymphs are young which live 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.” The word nymph is used in the same sense by McLachlan, and by Cabot. Calvert also applies the term nymph “to the stage of odonate existence between the egg and the transformation into the imago.” On the other hand, Brauer applies the term nymph to the pupa of holometabolous insects. For larval Hyatt proposes the term nepionic.
b. Stages or stadia of metamorphosis
The intervals or periods between the moults or ecdyses of caterpillars and other eruciform larvæ are called stages or stadia; thus, as most caterpillars moult four times, we have five stages or stadia, or stage (stadium) I to V. As observed by Sharp, there is, unfortunately, no term 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.” Hence he adopts a term suggested by Fischer,[[86]] and calls the insect as it appears after leaving the egg the first instar, and what it is after the first moult the second instar, and so on; hence the pupa, or chrysalis, which assumed that condition after moulting five times would be the sixth instar, and the butterfly itself would be the seventh instar.
c. Ametabolous and metabolous stages
In the Synaptera development is direct, the young differing neither in form, structure, or habits from the adult. Hence they are said to be ametabolous. Since there is an absence of even a tendency to a partial metamorphosis, it is evident that the insects have not inherited a tendency to undergo a transformation, but that it is an adaptation induced in the hexapod type after the first winged insects appeared, and which became more marked in the more specialized insects and at a period comparatively late in geological history, i.e. perhaps at or soon after the beginning of the Carboniferous period.[[87]]