Fig. 623.—Late larva of Simulium, showing the rudiments of the pupal structures within the larval skin: l1, l2, l3, fore, middle, and hind legs of the fly; ra, respiratory appendages of pupa; w, wing of fly; h, halter of fly.—After Miall.
Fig. 624.—Imaginal buds in larva of Corethra (diagrammatic cross-section of thorax): invaginations (fe and be) of the larval hypodermis (lhy) in whose bases the rudiments of wings (fa) and legs (ba) arise; lh, chitinous integument of the larva.—After Lang.
These imaginal buds may be regarded as evaginations of the outer surface of the body. The only difference is that the buds of the appendages as a whole seem sunken below the level of the surface of the body, being situated at the bottom of an evagination, as in the buds of the head and trunk in the Pilidium larva of nemertean worms, and in the rudiments of the lower surface of the body of Echinus present in the pluteus larva.
The lumen of the invagination in which the appendages of Corethra (and other Holometabola) are situated is called by Van Rees the peripodal cavity, and the external sheath bordering it, which is naturally continuous with the hypodermis of the body, the peripodal membrane (Fig. 636, p).
We must adopt the view that the rudiments of the appendages (imaginal buds) are from the first divided into ectodermal and mesodermal portions, which are derived from the corresponding germ-layers of the larva. The ectoderm of the rudiments of the appendages is continuous with the peripodal membrane, and through it with the hypodermis. Weismann was inclined to derive the organs (tracheæ, muscles, etc.) developing within the germs of the appendages from a hypertrophy of the neurilemma of a nerve passing down from within into the imaginal bud, and held that nerves and tracheal branches soon after passed into the inner surface of the imaginal bud. (Korschelt and Heider.)
When the imaginal buds of the appendages enlarge, then the peripodal membranes become correspondingly distended, and the limbs within assume a more or less crumpled position, and in Corethra are spirally twisted, while the rudiments of the wings are folded. The completion of the rudimentary limbs is accomplished simply by their passing out of the invagination in which they originated. The limbs thus gradually become free, the peripodal membrane is seen to reach the level of the rest of the hypodermis and become a part of it, and the base of the extremity is no longer situated in a cavity.
The internal organs of Corethra undergo but to a slight degree the destruction (histolysis) which is so thoroughgoing in the Muscidæ. Kowalevsky states that in the mid-intestine of Corethra a histolysis of the larval and reconstruction of the imaginal epithelium goes on in the same way as has been described in Musca. Most of the larval organs pass without histolytic changes directly over into those of the pupal and imaginal stages; the muscles in general are also unchanged, but those of the appendages and wings are made over anew. The last arise, according to Weismann, in the last larval stage from strings of cells which are already present in the embryo.
When we consider how insignificant the internal transformations are during the metamorphosis of the Tipulidæ, of which Corethra serves as an example, we can scarcely doubt that we here have before us conditions which illustrate the passage between an incomplete and a complete metamorphosis. Thus, among other things, should be mentioned the short duration of the pupa stage and the activity of the pupa, as also the early appearance of the germs of the compound eyes, a character which Corethra has in common with the Hemimetabola. (Korschelt and Heider.)
Formation of the imago in Culex.—In respect to the formation of the imaginal head, Culex is still more primitive than Corethra. Miall and Hammond find from Hurst’s partly unpublished descriptions and preparations that there are no deep invaginations for the compound eyes or antennæ of the imago.