334 
Warhle Flies 
found an encysted warble larva in the gullet. One reason for this may 
be that the larvae are constantly moving, and this may also account 
for their ability to pass through other tissues on their final journey to 
the back. It is noteworthy that nearly all the Helminths select either 
the abdominal cavity and respiratory tract or the skin for their habitat; 
for instance, Gongylonema scutatum is found in the walls of the gullet, 
Strongyles in the lungs, bots in the stomach, etc. Possibly the presence 
of a freer supply of oxygen than could be obtained in the deep tissues, 
may lead them to select these situations, though the food supply may 
also be an important factor in their choice. 
The final position taken by the larvae is in fibrous tissue, of somewhat 
the same nature as that found in the gidlet. The reaction of the body 
against them is manifested by oedematous swellings. Very probably 
this reaction would end in their destruction, since they have now taken 
up a fixed position. But shortly after reaching the back they bore 
through the skin, and bacteria gain entrance to the cavity in which 
they lie. Hence it woiild seem that the presence of these bacteria helps 
the larvae in maintaining their position, as the tissues have now two 
foes to cope with instead of one. In summing up, I,think the reader 
wdll agree that the above experiment supports the view that the 
larvae gain entrance to the body by penetrating the unbroken skin. 
This view awaits confirmation and I am recording these observations 
merely for the purpose of adding my quota to the evidence already 
gathered. 
The next point of interest concerns the position taken up by the larvae 
under the hide. While studying the structure of warbles I have noticed 
that the opening through the hide is oblique, and that the larva lies 
with its ventral surface upward. The caudal end is bent and protrudes 
through the opening, and this gives one the impression, from an outside 
view, that it is placed perpendicularly beneath the hide instead of 
horizontally, as is actually the case (Plate XXV, Figs. 3 and 4). 
Perhaps this has already been noticed by other observers, but I have 
seen no record of it. The point I wish to make is that, in the horizontal 
position, the effect of the skin movements is reduced to a minimum. 
Probably for a somewhat similar reason the second-stage larvae lie 
longitudinally in the oesophagus. The contraction of the longitudinal 
muscles would simply cause the larva to shorten its body as it does 
naturally. If it lay transversely, the circular muscles would bend the 
body sideways. 
