32 
Jo urnal of Agricultural Research 
Vol. XXVIII, No. 1 
11. The original maggots perhaps took to living in soft mud or in water, and, 
in adaptation to their new environment, they developed a pair of dorsal spiracles 
at one or both ends of the body in connection with the dorsal longitudinal tracheal 
trunks, while the primitive spiracles of the lower or lateral tracheal trunks were 
closed. They thus became propneustic, metapneustic, or amphipneustic by a 
new breathing system, not by a modification of the primitive peripneustic system. 
The dorsal spiracles are independently renewed with each molt. 
12. The puparium of the higher Diptera is the hardened skin of the last active 
larval instar. In Rhagoletis it is loosened from the underlying hypoderm, and a 
fourth larval instar is formed within the puparial shell. This larva has a minutely 
papillated cuticle of its own, but it does not shed the linings of the trachese and 
alimentary canal of the third instar. This prepupal larva soon molts in its turn, 
whereupon the true pupa is formed, and its unbroken cuticle remains as a mem¬ 
branous envelope about the pupa within the puparial capsule. How far this 
condition is general in the Diptera has not been determined. 
13. In the pupal stage all the sunken imaginal parts are everted, including the 
frontal sacs and the buds of the mouth parts, legs, and wings. Structures that 
have been acquired by the larva for its own special purposes are lost by histoly¬ 
sis; those that have been modified are remodeled into imaginal organs; and 
imaginal organs or tissues that have been suppressed grow rapidly into the adult 
form. In the higher Diptera the pupa goes through two external phases in its 
development, a cryptocephalic substage and a phanerocephalic substage. 
14. The pupa supplants the anterior dorsal larval spiracles with a correspond¬ 
ing pair of its own, but it loses, without replacing, the posterior larval spiracles. 
In Rhagoletis the pupa obtains air from the anterior larval spiracles of the pupa¬ 
rium conducted into the.pupal chamber through the cast larval tracheal tubes 
which remain connected with the anterior larval spiracles and which are rup¬ 
tured within the prepupal envelope of the pupa. 
15. The pupal metamorphosis is for the purpose of restoring the ancestral 
form of the insect, but it involves a restoration complicated by the addition of 
all the special characters added by the adult during its own evolution. In gen¬ 
eral, the more the larva has departed from the ancestral path, the more the adult 
has modified its inheritance, and consequently the more arduous becomes the 
work of the pupa in making the one over into the other. Nowhere in insects 
has this independence between young and adult been carried to such a degree as 
in the higher Diptera, and consequently a more intense reconstruction goes on 
in the fly pupa than in any other. 
16. Physiologically the life of the insect is divided into but two periods—the 
larval period and the imaginal period. The pupa is a constructive proimaginal 
stage separated from the mature imaginal stage by a secondarily acquired molt. 
It is not a preimaginal phylogenetic stage. In most cases it has developed special 
characters of its own, and often retains one or more special characters of the 
larva. 
17. The adult represents the primitive form of the insect on which has been 
built up the special adult characters acquired by each species in its evolution. 
In insects with metamorphosis the adult restores characters of its ancestors that 
were discarded by the larva. In the higher Diptera, for example, the fly abol¬ 
ishes the breathing system established by the larva, reduces the enlarged dorsal 
tracheal trunks to normal size, and reopens the primitive lateral spiracles along 
the lower lateral tracheal trunks. These spiracles remain through the larval 
stage as mere groups of special cells in the hypoderm, which appear first, in 
Rhagoletis, as minute stigmatic spots on the puparium. The imago of insects 
with complete metamorphosis is not a new creature built up independently from 
embryonic cells; its first stage, the pupa, follows the last larval stage by a process 
of replacement which is only a modification of that which takes place between 
any two larval stages, and which again occurs in normal form at the molt between 
the pupa and the adult. 
