7. The process of metamorphosis, at least in the subtropical, temperate, and polar regions, is largely dependent on the change from summer to winter, and, in the tropics, from the rainy to the dry season.
As regards the organization of larval (nepionic) as compared with imaginal forms, the nymphs and larvæ of insects are, with the exception of many Diptera, nearly as perfectly developed as the adult. In this respect the immature insect differs fundamentally from the larvæ of certain worms (for example, the pilidium of Nemerteans) and from the pluteus and brachiolaria stages of echinoderms, which possess only digestive and water-vascular organs.
Insect nymphs and larvæ also differ from the nauplius and zoëa of Crustacea in having at birth all the most important systems of organs (digestive, circulatory, respiratory, nervous, muscular, with sometimes a nearly perfected reproductive system) of the imago, also the same number of cephalic, thoracic, and abdominal segments and appendages. Metamorphism in insects involves (except in the Diptera) rather modifications in the form and functions of organs and appendages already present than the formation of new ones. In larval Crustacea, the thoracic and abdominal appendages do not arise until some time after hatching from the egg.
8. While cases of the suppression or abbreviation of larval characters and direct development are not uncommon in echinoderms and crustaceans, in insects this phenomenon occurs only so far as yet known in the Diptera. In these insects the polypody in the embryo is outgrown, or lost, the embryos and larvæ not having even the temporary rudiments of abdominal appendages. The campodeoid characters also are entirely suppressed, dropped, or lost in the more specialized holometabolous orders, Lepidoptera, Hymenoptera, and Diptera, though retained in the more primitive and generalized Coleoptera. (This proves that the Coleoptera are lower or more primitive and generalized than the other orders mentioned.) This abbreviation or loss of organs is, as Hyatt and Arms claim, due to the prepotency of acquired characters in phylogeny, and are also the result of homochronous heredity.
“The Insecta of the more specialized orders, x.-xvi., afford, next to some parasites, the most notable examples of this mode of evolution. Their larval or nepionic, and pupal or neanic, stages are prolonged at the expense of the ephebic, winged stage, and the reasons for this prolongation are found in the great number of new features introduced into these stages of development in these orders as contrasted with those of the more primitive, and, in large part, more ancient orders, i.-ix. The law of tachygenesis has been at work here, as in the former cases alluded to above, and it is shown in the encroachments of the adaptive characteristics of the caterpillar, grub, and maggot upon the inherited characteristics of the Thysanuran stage, which loses its ancestral characteristics, until in most cases they are either obsolete or recognizable with difficulty.” (Hyatt and Arms, Natural Science, 1896, p. 400.)
9. In the holometabolous insects there is a resting, quiescent stage during the pupal period, when the insect takes no food. In this respect the more specialized insects differ from other metamorphic animals. The larva has an abundant supply of fat lasting through pupal life, while in the quiescent pupa, respiration and circulation is much lessened, the animal being as a rule motionless. This resting stage is also necessary for the histolysis and formation of the adult body from the imaginal buds present in the larva.
10. The hypermetamorphosis of Mantispa, Meloë, Stylops, etc., indicate very plainly that the eruciform type of larva is derived from the campodeoid, since one and the same insect passes through these stages before reaching sexual maturity.
11. As observed by Miall, the larva of insects differs from that of other invertebrate animals in being larger than the adult.
12. The metamorphoses of insects are in some important respects paralleled by those of the Amphibia. The case of pædogenesis of Chironomus affords a parallel with that of the Siredon, or larva of Amblystoma. Also the organs and appendages of the insects, such as caterpillars, are present, just as the skeleton and other organs of the tadpole are the homologues of those of the adult, although these parts undergo a profound modification, and new structures are added. (See the discussion of this point by Miall, and by Hyatt and Arms.)
Theoretical conclusions; Causes of metamorphosis.—It results from a review of the known facts, together with reasonable inductions from such facts, that so far from opposing the theory of descent, the facts of metamorphosis, and particularly of hypermetamorphosis, seem to afford solid foundation for the theory. While natural selection was not the initiative cause, it plays a part as one of several factors; but the fundamental causes are the same as those which have controlled the origin of species and of the larger groups of animals in general. Owing to the struggle for existence, due to overcrowding, the early insects were forced to take to the air, acquiring wings to enable them to avoid the attacks of creeping and running insects. In the end the insects became, owing to this acquisition of wings, and afterwards to the establishment of a complicated metamorphosis, numerically the most successful type of life in existence, the number of species, extinct and living, mounting into the millions.