Adam Sedgwick

“ It must not be supposed that in all insects in which the sexual animal has a different habitat from the young form, there is a metamorphosis of the kind just described. In the may-flies and dragon-flies, in which the larva is aquatic, the change is prepared for some time before the actual metamorphosis, the organs which are necessary for the aerial existence being gradually acquired during larval life. In such cases, the metamorphosis belongs to our first type and consists of the act by which the organs previously and gradually acquired suddenly become functional. ”

“ The larva itself does not form any of these organs and carry them about during its active life, though it does possess the very minute centres of growth known as the imaginal disks which burst into activity after the larval life is over. ”

“ In the Vertebrata a metamorphosis occurs in the lamprey and the Amphibia. The metamorphosis of the lamprey is peculiar. It lives for three or four years as a sexless larva, known as the ammocoete. It then quite rapidly (in three or four days) undergoes a series of changes and becomes converted into the adult. The metamorphosis affects the alimentary canal, the eyes, the respiratory apparatus and other organs, and especially the reproductive organs, which become mature. The adult lives for a few months only, spawning soon after the metamorphosis. ”

“ The adult body is, in the main, fashioned out of the larval body, and it takes over most of the organs of the latter; but as a rule the adult mouth, oesophagus and anus are new formations, and the central nervous system of the larva when present shares the fate of the larval locomotory apparatus. In Asteroids and Crinoids the metamorphosis is accompanied by fixation to foreign objects, the fixation being effected as in Cirripedes by the preoral lobe. ”

“ During its life it undergoes much growth, but no important change in structure. When it leaves the egg it is adapted to live and feed on a particular species of plant, on or near which the eggs are deposited by the parent butterfly. It has powerful biting jaws by which it procures its vegetable food. The adult, on the other hand, is a winged creature which also lives on plants but in quite a different way to the larva (fig. 2, A) . ”

“ They persist practically unchanged through larval life and become active as centres of growth in the pupa. The pupal stage in such a metamorphosis may be compared to a second embryonic stage in which the organs of the adult assume their final shape. In this kind of metamorphosis the larval organs are entirely got rid of in the pupal stage, during which the insect is as a rule incapable of locomotion and takes no food ”

“ This passes into a later protozoaea stage (C) in which the rudiments of the compound eyes and of the abdominal segments are visible beneath the cuticle and in which certain functional changes (jointing, &c.) have appeared in the limbs. ”