“The formation of the head-vesicle proceeds in a way similar to that in Musca. The ventral disk fuses early at its lateral edges with the dorsal pair. The communications between both ventral and dorsal disks and the pharynx rapidly widen (in the old larva they have already become very large), and soon the disks and pharynx form together a single vesicle, which is the head-vesicle.” The imaginal buds of the abdomen Pratt finds to be exactly as in the corresponding ones of Musca.
In the embryo of Melophagus the cephalic and thoracic imaginal buds first appear as local thickenings, followed by the invagination of the ectoderm; the cephalic buds first appear very early in the ontogeny of the insect (Fig. 636, C), just as the germs of the digestive canal, nervous system, and tracheæ are appearing. The single median thickening (v) is destined to form the ventral cephalic bud, while the pair of thickenings behind (d) become the dorsal buds, those homologous with the cephalic buds of Musca.
Fig. 636.—Imaginal buds in Musca,—A, in Corethra,—B, in Melophagus,—C, in embryo of Melophagus: dorsal view of head; b, bud; p, peripodal membrane; c, cord; hy, hypodermis; cl, cuticula; st, stomodæum; v, ventral cephalic bud, behind are the two dorsal cephalic buds (d).—After Pratt.
The thoracic buds, which arise as hypodermic thickenings, do not appear until late in embryonic life, until the time of the involution of the head.
Pratt did not observe in the embryo the buds of the internal organs and of the abdominal hypodermis, and thinks it probable that they appear first in the larva.
c. General summary
We have seen that in Coleoptera, Lepidoptera, Diptera, and Hymenoptera, and with little doubt in all the holometabolous insects, the parts of the imago originate in single formative cellular masses (imaginal buds) already present in the larva, and often even in the later embryonic stages. There are such imaginal buds for each part of the body,—for the appendages of the head, for the legs and wings, for the ovipositor, and probably for the cercopods, for the hypodermis of the abdomen, and for the different sections of the digestive canal. We have seen, as Korschelt and Heider state, that the formation of the mesodermal organs of the imago (muscles, connective tissue, fat-body) begins in the mesodermal part of the imaginal buds, whose first origin is still obscure. Simultaneously with the formation of the imaginal organs, there goes on under the influence of the leucocytes the destruction of the larval organs. Both processes (destruction and regeneration) therefore go on hand in hand, so that the continuity of the organs in question in most cases remains perfect, inasmuch as the complete destruction only ensues after the formation of the final organs. The only exceptions are most of the muscles of the larva, which are destroyed at a very early period.
Moreover, it is evident that the sharp division into larval, pupal, and imaginal stages only applies to the external surface of the body, since they follow one another after successive moults. The processes of the internal development, on the other hand, form an entirely continuous series of transformations between which is no sharp line of demarcation. Yet as a whole the form of the larva, pupa, and imago are kept distinct in adaptation to their separate environments and habits.
Finally, as Pratt very truly remarks, the epigenetic period in insects, when new organs are forming, does not end with the birth of the larva from the egg, but extends through the larval, and even through the pupal period. “The principal significance of the pupal period and the metamorphosis is that it is the time when the larval characters which were adapted for use during a period of free life in the midst of the development, and which would be valueless to the imago, are corrected or abandoned.”