The transformations in the case of the blue-bottle are typical of the house-fly and others of closely related families and genera which are many-brooded within the year; these creatures develop very rapidly immediately after emerging from the egg. Some other kinds of dipterid maggots, which are single-brooded, pass a very prolonged and obscure early period of skin-shedding and non-feeding, a preparatory sort of baby-hood metamorphosis; then at last they begin to feed voraciously and to follow the general habits of other maggots. Some maggots curiously refuse to feed except in company; probably some are unable to feed on dung except where other species are providing the necessary dissolving juice.
When the common maggots or gentles have ceased feeding, they burrow into the ground or crawl away, often to a considerable distance, apparently seeking a secluded, a more wholesomely clean, and a dryer spot. During this migrating time, they are palatable food for many birds, which would not eat them in their former food-loaded or unscoured state. Indeed, it is doubtful whether either a vulture or a raven could eat a fly-blown carcase without danger of myiasic punishment. The skin of the larva whilst growing is transparent, but, when about to pupate, it thickens and becomes an opaque creamy white.
The most marvellous part of the metamorphosis of the blue-bottle is concealed, when the gentle becomes the pupa; according to Réaumur the embryonic fly develops most curiously inside the puparium by a procedure not exactly like the change from the caterpillar to the chrysalid in the case of the butterfly. After a pause of a day or two, the front segments of the fully fed maggot contract, so that the body assumes a barrel-like shape; the skin then hardens, and turning a reddish brown it becomes a much contracted shell or case called the puparium. However, the long slender maggot has done something more than merely shrink and shape itself conformably to the case; it has withdrawn its embryonic head, so small as to be hardly distinguishable microscopically, together with its embryonic legs, wings and thorax into its embryonic abdomen! As the development proceeds, and the embryonic members of the future perfect insect acquire their destined shape, the immensely increased head and the thorax with its appendage members slowly emerge, and the partly inverted integument of the abdomen rolls back, disclosing the shape of a fly not before recognisable.
Other naturalists would have it believed that the true account of the transformation is as follows,—when the maggot has shrunk and freed its body inside its skin which forms the case or puparium, all its pre-existing internal organs become absolutely dissolved; then out of the fluid mass a new growth ensues, constituting the pupa with its recognised shape. This account is the one represented in most modern entomological books, and is based partly upon B. T. Lowne's monographic work on the blow-fly.
The comparative embryologist of our day is inclined to be a hyper-theorist, and so it seems that some have not remained content with either of the above accounts; to them, apparently, the production of the large and complex head of the imago out of a single small anterior segment of the maggot requires a more recondite explanation, and must be brought into harmony with analogous facts. To this end some degree of linked support is found by the investigations of microscopic anatomy, and it has been conjectured that not one or two head segments, but five are lying blended and embryonically hidden in the larvæ, all ready to bud forth. However, for fear of wearying too much with the theories of advanced erudite scientists, the following jeu d'esprit is presented, instead of a more elaborate and sober attempt, to lure the unscientific lay reader to an extreme hypothetical conception of the "essential unity" underlying the apparent diversities of Nature within that vast domain of the Kingdom of Fauna, which is obviously outside the later creation of a vertebrate Animalia.
Fig. 5. Illustrating the debatable continuity of a
12-segmented structure throughout the metamorphosis.
The futurist's dogmatic CREDO of creative progress, "For him who would meritoriously pass his histological examinations, and qualify as a Professor and Doctor of Science, above all it is necessary that he should acknowledge the unicellularity of the primæval OVUM (or egg), whence proceeds the seventeen-segmented boneless ANNELID (or worm), out of which there develops the quadrangular articulated crustacean INSTAR (or shell-encased aurelian), which metamorphosises into the winged IMAGO (the angelic? or diabolic? fly); in the contemplation of this knowledge alone is there supreme Darwinian Modernismal salvation and felicity." Amen.
In view of the prosaic illustration of transmutation, figure 5 above, the futurist disciple will have to accept the seventeenness of segmentation by something like faith without sight.
The quadrangularity of the crustacean stage is based upon the idea that the wings bud out from the two upper corners, whilst the legs develop from the lower corners of the transmuting instar.