334 The Warble-flies of Cattle 
thus proving clearly the identity of the larvae. It appears that only the largest 
oesophageal larvae are ready to proceed at once to form warbles, and that 
those less advanced continue their wanderings—sometimes returning to the 
oesophagus when experimentally placed beneath the skin. 
Ordinarily the course of a migrating larva is betrayed by no track, but on 
occasion sepsis has arisen and a greenish trace is visible, as has been noted 
by Ruser and by Hadwen. The oesophageal larvae experimentally inserted 
without disinfection always mark their course by a distinct track, and are 
therefore particularly instructive. The tracks, whether occurring naturally 
or experimentally, are always in the connective tissue. 
It might be supposed that the larvae in the oesophageal wall would all 
be found to be travelling in the same direction, but this is far from being the 
case. Their orientation is quite irregular and they clearly spend a considerable 
time in this organ, moving in any direction. It is, however, observed that they 
are at first chiefly congregated towards the pharyngeal end of the oesophagus 
and that they progressively descend, the last remaining larvae being always 
near the entrance to the stomach. 
The choice of the oesophagus for a prolonged stay on the way to the back 
is of great interest. As Hadwen remarks, there are two points of supreme 
importance to the parasite: it must avoid organs where the reaction to its 
presence would be so great as to endanger its life or that of its host, and it 
must provide against the possibility of encystment. Encystment is avoided 
by constant motion, and the oesophageal wall is admirably suited for its 
purpose, as the reaction is slight unless the larvae are very numerous. 
There appears to be no invariable route from the oesophagus to the back. 
Occasional larvae found outside the wall of the oesophagus (Carpenter, etc.) 
seem to indicate a direct passage to the anterior region of the warble area, 
but the general course is to proceed along the gullet to the diaphragm. One 
route thence has been definitely traced by Hadwen (1915), the greenish track 
of the larva being visible the whole way. The larva had entered the diaphragm, 
passing under the pleura enfolding the termination of the oesophagus, and 
had then struck downwards and outwards, following the radial arrangement 
of the muscle fibres. This brought it to the spot where the diaphragm meets 
the ninth rib, and it proceeded up the posterior border of this rib, following 
the course of the blood vessels, and arrived at the spinal canal, which it 
entered by a posterior foramen. 
This is the only course absolutely demonstrated, but it is clear that any 
of the ribs upon which the diaphragm impinges might be utilised, and it is 
quite possible, and Hadwen considers it probable, that many larvae do not 
use the ribs at all, but take a shorter course directly up the diaphragm. 
The remarkable observations here briefly described have removed many of 
the obscurities which so long enveloped the larval life of Hyjpoderma, but they | 
throw no light on the period between oviposition and the arrival of the larva 
in the gullet. 
