In some cases (many species of Ascaris) the metamorphosis is reduced to a simple process of growth.
The freeliving differ from the majority of the parasitic forms in undergoing no metamorphosis; they also possess certain structural peculiarities which led Bastian (Trans.
The differences in appearance between the caterpillar and the butterfly, striking as they are to the eye, do not sufficiently represent the phenomena of metamorphosis to the intelligence.
This is a most remarkable case, but unfortunately very little information exists as to the details of metamorphosis in this group.
Miller discovered that they undergo a metamorphosis, and that the minute worm-like lamperns previously known under the name of Ammocoetes, and abundant in the sand and mud of many streams, were nothing but the undeveloped young of the river-lampreys and small lamperns.
A common result of metamorphosis is that the larva and imago differ markedly in their habitat and mode of feeding.
Meanwhile the English naturalist, John Ray, was studying the classification of animals; he published, in 1705, his Methodus insectorum, in which the nature of the metamorphosis received due weight.
Moreover, in many insects with imperfect metamorphosis the change from larva or (as the later stage of the larva is called in these cases) nymph to imago is about as great as the corresponding change in the Holometabola, as the student will recognize if he recalls the histories of Ephemeridae, Odonata and male Coccidae.
The post-embryonic growth of Hexapods with or without metamorphosis is accompanied in most cases by the acquisition of wings.
It appears.probable that this is only a further simplification of the more complicated metamorphosis described above.
As regards growth after hatching, all beetles undergo a "complete" metamorphosis, the wing-rudiments developing beneath the cuticle throughout the larval stages, and a resting pupal stage intervening between the last larval instal1 and the imago.
Metamorphosis is, from this point of view, the sum of the changes that take place under the cuticle of an insect between the ecdyses, which changes only become externally displayed when the cuticle is cast off.
(I) the larger part of the hypodermis that exists in the maggot or caterpillar and is disf e b solved at the metamorphosis; (2) parts that remain comparatively quiescent previously, and that grow and develop when the other parts degenerate.
Insect metamorphosis may be briefly described as phenomena of development characterized by abrupt changes of appearance and of structure, occurring during the period subsequent to embryonic development and antecedent to the reproductive state.
In the former position the suckers are developed and growth proceeds for 8 to Io weeks until the metamorphosis of its host.
Others again consider that the whole cycle is a metamorphosis which, beginning in the Heterocotylea as a direct development, has become complicated in the Holostomidae by a larval history, and finally in the Malacocotylea has acquired additional complexity by the intercalation of two larval forms, and is thus spread over several generations.
The formative force in this process of evolution (or " metamorphosis ") is conceived as an intellectual principle (idee generatrice).
Metamorphosis.It has already been pointed out that each kind of member of the body may present a variety of forms. For example, a stem may be a tree-trunk, or a twining stem, or a tendril, or a thorn, or a creeping rhizome, or a tuber; a leaf may be a green foliage-leaf, or a scale protecting a bud, or a tendril, or a pitcher, or a floral leaf, either sepal, petal, stamen or carpel (sporophyll); a root may be a fibrous root, or a swollen tap-root like that of the beet or the turnip. All these various forms are organs discharging some special function, and are examples of what Wolff called modification, and Goethe metamorphosis.
The word metamorphosis cannot, in fact, be used any longer in its original sense, for the change which it implied does not normally occur in ontogeny, and in phylogeny the idea is more accurately expressed by the term differentiation.
Metamorphosis in Diptera is complete; the larvae are utterly different from the perfect insects in appearance, and, although varying greatly in outward form, are usually footless grubs; those of the Muscidae are generally known as maggots.
The process of destruction of the larval tissues was first studied in the forms where metamorphosis is greatest and most abrupt, viz.
Amongst insects with imperfect metamorphosis the nearest approximations to the true pupa of the Holometabola are to be found in the subimago a From Chittenden, Bull.
Brauer in his arrangement of these orders laid special stress on the nature of the metamorphosis, and was the first to draw attention to the number of Malpighian tubes as of importance in classification.
It is now, in fact, generally admitted that metamorphosis has been acquired comparatively recently, and Scudder in his review of the earliest fossil insects states that " their metamorphoses were simple and incomplete, the young leaving the egg with the form of the parent, but without wings, the assumption of which required no quiescent stage before maturity."
During this period he published his poetical satire called Metamorphosis (1726), his Epistolae ad virum perillustrem (1727), his Description of Denmark and Norway (1729), History of Denmark, Universal Church History, Biographies of Famous Men, Moral Reflections, Description of Bergen (1737), A History of the Jews, and other learned and laborious compilations.
Both agree in having nothing that can be termed a metamorphosis; they are active from the time of their exit from the egg to their death, gradually increasing in size, and undergoing several moults or changes of skin.
His cult survived the metamorphosis of the ancient Vedic nature-worship into modern Hinduism, and there still are in India fire-priests (agnihotri) whose duty is to superintend his worship. The sacred fire-drill for procuring the temple-fire by friction - symbolic of Agni's daily miraculous birth - is still used.
He has observed that in young specimens of Siren lacertina (the larva is still unknown) the gills are rudimentary and functionless, and that it is only in large adult specimens that they are fully developed in structure and function; he therefore concludes that the sirens are the descendants of a terrestrial type of batrachians, which passed through a metamorphosis like the other members of their class, but that more recently they have adopted a permanently aquatic life, and have resumed their branchiae by reversion.
The eruciform larva of the Orthorrhapha leads on to the headless vermiform maggot of the Cyclorrhapha, and in the latter sub-order we find metamorphosis carried to its extreme point, the muscid flies being the most highly specialized of all the Hexapoda as regards structure, while their maggots are the most degraded of all insect larvae.
The Siphonaptera appear by the form of the larva and the nature of the metamorphosis to be akin to the Orthorrhapha - in which division they have indeed been included by many students.
The other groups of the old Linnean order (such as lacewing-flies and caddis-flies)--which are hatched as larvae markedly unlike the parent, develop wing-rudiments hidden under the larval cuticle, and only show the wings externally in a resting pupal stage, passing thus through a " complete " metamorphosis and falling into the sub-class Endopterygotawere retained in the order Neuroptera, which thus became much restricted in its extent.