Insect transformation / by George H. Carpenter.

  • Carpenter, George H. (George Herbert), 1865-1939.
Date:
1921
Catalogue details

Licence: In copyright

Credit: Insect transformation / by George H. Carpenter. Source: Wellcome Collection.

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    broken down by histolysis, and new regions arise from groups of imaginal cells. The degree of replacement necessary increases with the increase of divergence between imago and larva. In wasps and bees the cellular walls of the main air- trunks pass over from the grub to the adult with considerable increase in capacity brought about by the multiplication of cells, and even in the transformation of the maggot into the blue-bottle the great bladder-like tracheal enlargements known as air-sacs have a similar relation to the larval condition. The excessively fine tubular terminations of the respiratory system, which are known as tracheoles and have no spiral thickening to their chitinous wall, the product of intracellular activity of the living tracheal epithelial layer, become greatly multiplied in preparation for the increased oxidation necessary during the aerial life of the adult, for it is through the exquisitely delicate walls of these tracheoles that the gaseous exchanges between the air and the tissues are carried on. From the system of air-tubes with their cuticular lining we pass naturally to consider a few special features in the origin of the outer cuticular structures of the adult in relation to the groups of living ceils through whose activity they are formed. It has already been explained how in metamorphic insects the cuticle of the pupa is formed beneath the larval cuticle, and later the cuticle of the imago beneath the pupal. As to the main regions of the body, it is found that in cases where these regions are typically represented in the larva, the cuticle of each is formed ready for pupation, from discs derived from the skin of the corresponding part of the larva. These discs may however grow so as to lie beneath other regions of the larval cuticle. For example, in the grub of the midge Chironomus^ the brain is situated in the first thoracic segment, and the imaginal discs of the head, including those of the eyes and feelers, grow backwards towards the brain. In the maggots of blue-bottles2 and their allies, the mouth, opening at the narrow front extremity of the body, leads into a spacious chitin-hned cavity usually styled the pharynx (Fig. 86 A h), but probably to be regarded as the inpushed larval head. ^ L. C. Miall and A, R. Hammond : “ The Structure and Life-history of the Harlequin Fly (Chironomus) ”. Oxford, 1900. 2 J, Van Rees : “ Beitrhge zur Kenntniss der inneren Metamorphose der Musca VOmitoria ”. Zool. Jahrb. {Anat.),lll. 1888.