“The compound eye forms beneath the larval eye-spots, and is at first relatively simple and composed of few facets. The number increases by the gradual formation of partial and marginal invaginations, each of which forms a new element. The imaginal antenna grows to a much greater length than that of the larval antenna, and its base is accordingly telescoped into the head, while the shaft becomes irregularly folded.[[115]] Culex, though more modified than Chironomus in many respects, e.g. in the mouth-parts, is relatively primitive with respect to the formation of the imaginal head, and shows a mode of development of the eye and antenna which we may suppose to have characterized a remote and comparatively unspecialized progenitor of Chironomus.”
Formation of the imago in Chironomus.—The development of the head of the imago of Chironomus dorsalis has been discussed by Miall and Hammond. The invaginations which give rise to the head of the fly could not be discovered even in a rudimentary state until after the last larval moult.
“Weismann has given reasons for supposing that invaginated imaginal rudiments could not come into existence before the last larval moult in an insect whose life-history resembles that of Corethra or Chironomus. If the epidermis were invaginated in any stage before the ante-pupal one, the new cuticle, moulded closely upon the epidermis, would become invaginated also, and would appear at the next moult with projecting appendages like those of a pupa or imago. This is actually the way in which the wings are developed in some larval insects with incomplete metamorphosis. In Muscidæ the invaginations for the head of the imago have been traced back to the embryo within the egg,[[116]] but the almost total subsequent separation of the disks from the epidermis renders their development independent of the growth of the larval cuticle and of the moults that probably take place therein.”
The pupal and imaginal cuticles do not follow at all closely the larval skin, but, says Miall, become at particular places folded far into the interior. “The folds which give rise to the head of the fly are two in number and paired. They begin at the larval antenna on each side of the head, and gradually extend further and further backwards. The object of the folds is to provide an extended surface which can be moulded, without pressure from surrounding objects, into the form of the future head. On one part of each fold the facets of the large compound eyes are developed; another part gives rise to the future antenna, a large and elaborate organ, which springs from the bottom of the fold, and whose tip just enters the very short antenna of the larva. The folds for the head ultimately become so large that the larval head cannot contain them, and they extend far into the prothorax. Here a difficulty occurs. If the generating cuticle of the prothorax were also to be folded inwards, the future prothorax would take a corresponding shape. But the prothorax of the fly has a form dictated to it by the limbs which it bears and by the muscles to which it gives attachment. These call for a great reduction in its length, and a peculiar shape, which it is not here necessary to describe. It will be enough to realize that the epidermis of the future prothorax cannot be sacrificed to the folds which are to give rise to the head of the fly. All interference between the two developing structures is obviated by the provision of a transverse fold, which pushes into the prothorax from the neck, and forms a sort of internal pocket. The floor of the pocket forms two longitudinal folds, which prolong the folds originating in the larval head. The roof of the pocket shrinks up and forms the connection between the head and thorax of the fly. Ultimately the head-part is drawn out, leaving the prothoracic structures unaffected.”[[117]]
Fig. 625.—Process of formation of the parts of the head of the fly in the larva of Chironomus (male): A, the new epidermis thrown into complicated folds which have been cut away in places to show the parts within. B, the same parts in horizontal section; lc, larval cuticle; tf, transverse fold; tf′, upper wall of the same; m, cut edge of new epidermis; ant, larval antenna; an, nerve to the same; ant′, antenna of fly; lf, longitudinal fold; o, eye of fly; on, optic nerve; an′, root of antennary nerve; br, brain; œs, œsophagus; b, bulb of antenna of fly; s, s, s′, blood-spaces.—After Miall.
The development of the head of the fly of Chironomus appears, as Miall and Hammond state, to be intermediate between the groups Adiscota and Discota of Weismann; i.e. “between the types in which the parts of the head of the fly are developed in close relation to those of the larva, and the types in which deep invaginations lead apparently to the formation of similar new parts far within the body, the seeming independence of the new parts being intensified by thoroughgoing histolysis,” and they suggest that possibly types may be discovered intermediate between Chironomus and Muscidæ.
Fig. 626.—A, B, C, D, diagrams of transverse sections showing the development of the wings, legs, and the imaginal hypodermis of muscid flies from the imaginal buds of the larva during metamorphosis: lh, chitinous integument of larva from which the underlying hypodermis (lhy) has withdrawn; iid, imaginal buds of wings, iiv, of legs; is, the cords connecting them with the hypodermis; fl, wing-germs; b, leg-germs; ihy, imaginal hypodermis spreading out in D from the imaginal buds. The imaginal rudiments of the hypodermis are indicated by thick, black outlines, the larval hypodermis by two thin, parallel lines.—After Lang.
We are now prepared to consider the extremely complicated changes, in the Muscidæ, leaving out of consideration the origin of the wings from imaginal buds, which has already been discussed on pp. 126–137.