READING IN CLASS


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THE NATURE 0f METAMORPHOSIS

There are two ways of regarding metamorphosis.

(I) It may be supposed that the insect is undergoing a progressive development towards the adult forms that it is subject during its larval stages to some inhibition which prevents the completion of this development until it is fully grown; that the restraint is then removed and differentiation la completed.

(II) Alternatively, it may be supposed that metamorphosis involves a switch into a line of development which la qualitatively different from that of the larva. In other words, that a new system of genes, previously latent in the chromosomes, la brought into action at a given point in development and it la -this system that is responsible for the control of the adult form. According to .this conception, the origin of metamorphosis has consisted In the independent evolution of two genetic systems, one controlling larval form and the other controlling adult form. The successive stages of the insect are thus comparable with the different forms in a polymorphic species, or with the different forms of the parts of the body, which are likewise believed to result from the activation of different sets of genes.

Hormonal differences certainly exist in the blood at metamorphosis, and these are responsible for the morphological changes. If young larvae of Rhodnius are decapitated and transfused with the blood of 5th-stage larvae, which are In process of moulting to become adult, they undergo a precocious metamorphosis. Even let-stage larvae recently emerged from the egg, if treated in this way, will develop the cuticle and pigmentation rudimentory wings, abdominal structure and genitalia characteristic of the adult. Pieces of the integument from larvae of Galleria (Lep.) implanted into pupating larvae of the same species, or another species such as Achroea, moult to form pupal cuticle synchronously with their new host; further development of these implants to form imaginal cuticle takes place when the host pupa completes its metamorphosis. By implanation of isolated fragments in the mature larva, the integument of newly hatched Galleria larvae can be caused to become pupal and then imaginal.

The insect seems to be capable of developing its imaginal characters, that is, of undergoing metamorphosis at any stage – provided that the appropriate hormones are circulating in the blood.