6 
Journal of Agricultural Research 
Vol. XXVIII, No. 1 
ventral lateral trunk which is always connected with the primitive spiracles. In 
the maggot the lateral spiracles are undeveloped and the lateral tracheal trunks 
are reduced in size till, by comparison with the highly developed dorsal trunks, 
they become inconspicuous and appear to be mere commissures between the 
descending branches from the dorsal trunks. But the lateral spiracles appear 
on the puparium (PI. 5, A) and are opened in the pupal stage by the shedding 
of the tracheal linings. Finally they become functional in the adult, where 
the lower lateral tracheal trunk regains its importance and the dorsal one is 
reduced again to usual size. 
Perhaps the lateral spiracles are present in some form in the hypoderm of 
the larva, but it is impossible to see them by surface examination until the skin 
hardens to form the puparium. Van Rees {47) noted in Calliphora that the 
lateral stigmatic tracheae do not penetrate the hypoderm in the larva or in the 
puparium, but that each ends in a small island of cells which forms a part of 
the outer surface of the body. These islands regenerate the epithelium of the 
stigmatic tracheae and that of neighboring parts of the longitudinal trunks. 
Wahl {50) regards them as true hypodermal imaginal buds and says that they 
are present on the metathorax and on the first seven abdominal segments in 
Eristalis, as noted also by Giacomini {12). Van Rees {47) finds them on all 
the abdominal segments but the eighth in Calliphora. Pratt {40) says that 
there is a series of eight microscopic, rudimentary stigmata on young larvae of 
Melophagus ovinus L., two on the thorax and six on the abdomen. In the pupa¬ 
rium of Rhagoletis (PI. 5, A) eight pairs of spiracles appear on the abdominal 
segments, but the last is very small. 
It is thus evident that the two sets of spiracles in the Diptera have no relation 
to each other. The temporary, dorsal larval spiracles are developed in con¬ 
nection with the dorsal longitudinal tracheal trunks; the primitive spiracles 
belong to the lower lateral trunks. The fundamental tracheal system is the 
;same in both larva and adult, but the longitudinal trunks are proportionately 
^developed in each stage according to the spiracular system in use—the larva 
and pupa using the dorsal system, the adult using the ventral system. 
The larvae of the lower flies are peripneustic like the adults, and the presence 
of the stigmatic islands of Van Rees in the hypoderm of the larvae of higher forms 
shows that even in them the lateral spiracles have not been entirely lost. Their 
final opening is but a restoration to primitive conditions. The tracheal system 
of a cecidomyiid fly larva {Cecidomyia resinicoloides Williams) as described and 
figured by Williams {55) consists of two large dorsal trunks extending between 
the anterior and the posterior spiracles, and of two slender lateral trunks con¬ 
nected with lateral spiracles. The anterior spiracles are on the pro thorax; the 
posterior spiracles are on the last abdominal segment, which is the ninth. The 
lateral spiracles occur only on the first seven abdominal segments. The condi¬ 
tion here strongly suggests that both respiratory systems are present and func¬ 
tional at the same time in this larva, and that the lateral spiracles are in an 
early stage of repression. The spiracle trachese are still well developed in the 
metathorax, though the spiracles of this segment are lacking. 
The larva of the higher flies, then, temporarily discards the primitive lateral 
spiracles and develops breathing organs of its own in connection with the dorsal 
tracheal trunks. It acquires first a pair of spiracles at the rear end of its body, 
and is metapneustic in its earliest instar. Next it acquires a second pair at the 
anterior end of the body, and is amphipneustic in the second and third instars. 
The pupa retains only the anterior spiracles of the larval system, and is pro- 
pneustic. The adult discards all the larval spiracles, restores the primitive 
breathing system of its ancestors, and is again peripneustic. 
