1. Metamorphosis of the external form of the larva commences with a voiding of the mid-gut contents; thus arises the prepupa in which the external features of the imago are more nearly revealed. In the prepupa the rostrum, legs, and wings grow out. In the pupa the whole body, and particularly the appendages, shrink and differentiate into the elegant form of the imago.
2. The larval epidermis (hypodermis) is not divisible into dormant imaginal and functional larval cells. The ‘imaginal disks’ of the appendages are functional epidermis in the larva and are distinguishable only by their stronger basiphil staining especially in the late larva; as, indeed, are other parts of the epidermis (e.g. rostrum ‘Anlage’) where rapid growth is to occur. The epidermal cells divide in the growing larva. During metamorphosis they extrude chromatic material and some cytoplasm, and undergo renewed cell-division. Cell degeneration is rare. Epidermal gland-cells disintegrate and are partly phagocytosed.
3. The mid-gut disintegrates and is regenerated from the larval fore-gut, i.e. from ectoderm(?); the mid-gut ‘replacing cells’ survive at the tips of the mid-gut caeca. The cells of foreand hind-guts behave like those of the epidermis. Mycetocytes wander from the mycetoma into the anterior mid-gut caeca.
4. Salivary glands disintegrate, are partly phagocytosed, and regenerate from cells at the openings of the larval ducts.
5. The Malpighian tubes disintegrate and regenerate from local mid-gut cells; there is no phagocytosis.
6. The heart and alary muscles pass into the imago. The nephrocytes survive from the early larva and without further division into the imago; there is occasional chromatic globule extrusion early in the metamorphosis. Leucocytes multiply in the blood; they seem to arise largely from masses of ‘pericardial tissue’ in the dorsal sinus.
7. The fat-cells divide in the growing larva, and accumulate food reserves, which are partly depleted during metamorphosis. Destruction of fat-cells does not occur. In the pupa the clumps of fat-body break into individual cells. The fat-body is phagocytic during metamorphosis. In the imago the fat-body retrogresses.
8. The oenocytes grow in size in the larva; there is no multiplication. In the pupa they undergo histolysis, partly by phagocytosis ; some survive for a time into the imago. The imaginal oenocytes arise apparently from the epidermis.
9. The tracheal system undergoes extensive elaboration during metamorphosis, particularly to meet the needs of the enlarged thorax and of the appendages. The cellular changes in the larva and during metamorphosis are similar to those of the epidermis. The chitinous intima, even of minute tracheoles, is withdrawn, at the last larval moult, through the stigmata.
10. The larval musculature (somatic and intestinal) degenerates and is reformed from myoblasts that have proliferated in the larval period. The myoblasts may be (a) parts of the larval muscle-fibre syncytium, as in all the muscles that regenerate in connexion with pre-existing larval muscles, and (b) scattered cells, in cases where they regenerate independently of larval muscles (leg muscles). Tendons are epidermal invaginations. The highly specialized condition of the imaginal musculature is mainly the outcome of changes in the region of insertion and origin of the muscles, owing to the altered form of the imago. The histogenesis of the wing-muscles is quite different from that of all other muscles.
11. The thirteen ganglia of the larval nerve-cord enlarge and become concentrated into five ganglia, not extending beyond the thorax. In the brain there occurs an elaboration of all its parts: the optic ganglia are new structures. Dormant neuroblasts occur in the larval cord in addition to nerve-cells. The former (and latter?) exhibit chromatic globule extrusion and proliferate during metamorphosis. There is no histolysis of larval nerve-tissue; specialization of the nervous system seems to be mainly on its sensory side.
12. The larval ocelli survive as pigment spots on the brain of the imago. Compound eyes develop from larval epidermis; they are of the exocone type.
13. Growth of the gonads proceeds throughout the larval stage, but is greatly accelerated in the prepupa and early pupa. The copulatory organs are developed as invaginations probably entirely from the ninth segment.
14. The hypothesis is offered that the abrupt changes that occur in the larval tissues at metamorphosis are the outcome of the hypertrophy of their cells.
Molecular detection of Chlamydia psittaci and Chlamydia felis in human keratoconjunctivitis cases