26 
Journal of Agricultural Research 
Vol. XXVIII, No. 1 
tissues and their replacement by new ones generated from the imaginal buds. 
Finally, some organs undergo histolysis alone and are not replaced, while others 
are newly formed in the pupa by simple histogenesis. These various processes, 
however, as we shall see, are probably not so diverse as they have seemed. 
The best examples of parts that follow the path of normal development from * 
embryo to imago are the organs of reproduction. But even here growth is 
almost arrested during the larval period. In the pupa the organs resume 
activity, but in the case* of the apple maggot, according to Illingworth {15), 
the ovaries of the female fly do not produce mature eggs tfll from 14 to 24 days 
after emergence. 
The heart, the nervous system, and the tracheal system undergo more or less 
change in the pupa in order to arrive at the forms they are to have in the fly. 
Yet, in most cases, the metamorphosis of these organs is a matter of direct 
transformation by the growth of their own tissues. Alterations of this sort 
take place also in insects with incomplete metamorphosis, and constitute a 
bond of unity between the young of such insects and the maggot, grub, or 
caterpillar of insects with metamorphosis erroneously called “complete.” 
The larval hypoderm, salivary glands, and alimentary canal have their tissues 
destroyed but replaced in situ by new ones proliferated from the imaginal buds, 
and such regenerated parts often take on forms strikingly different from those 
of the larva. Many of the muscles undergo a similar metamorphosis, though 
the new tissues have never been traced to imaginal muscle buds, being formed 
by a reorganization of the elements of the old muscles. This process, however, is 
apparently not confined to the pupal stage, for Mobusz {35) has described the 
regeneration of the stomach epithelium in the larval instars of a beetle (Anthrenus) 
as well as in the pupa. It may be suspected, therefore, that reconstruction may 
occur also in insects with incomplete metamorphosis. 
Many of the larval muscles, especially in the higher Diptera, are completely 
destroyed by histolysis and are not replaced by corresponding new ones; On 
the other hand, some muscles that have no larval prototypes are newly generated 
in the pupa. Those of this type most probably arise from the mesoderm layer 
of the imaginal discs of the body wall. The destruction and regeneration of 
muscles must take place also in some insects with incomplete metamorphosis, 
but the histology of the process has not been studied in this group. 
The metamorphosis of the hypoderm and of the alimentary canal has been de¬ 
scribed by so many writers, who agree on most of the details, that only a general 
statement need be made here. The writer has not studied the subject in 
Rhagoletis. At the beginning of metamorphosis the ectodermal cells of the 
imaginal buds of the hypoderm begin to divide and multiply. Wherever there 
are appendages the latter grow rapidly, but the peripheral parts of the buds 
spread out and constitute the new hypoderm of the body wall. The old cells 
and the basement membrane are absorbed into the blood or are devoured by 
phagocytes as the new tissues crowd upon them and take their places. This is 
subsequent to the loosening of the larval cuticle, though the latter may not be 
cast off till some time later. 
Likewise, the epithelium of the foreintestine and of the hind intestine is re¬ 
placed in the same way from imaginal buds in the intestinal walls. In the mid¬ 
intestine, or ventriculus, however, the larval epithelial cells are shed into the 
lumen, and the imaginal epithelium is regenerated from groups of deep-seated 
regenerative cells. According to Rengel {42) these regenerative cells in a beetle 
{Ttnebrio molitor L.) are the same that also regenerate the ventricular cells lost 
during the process of digestion in both the larval and the adult periods. The new 
epithelium proceeds at once to digest and absorb most of the discarded larval 
epithelium. 
