40 BULLETIN 1369, U. S. DEPARTMENT OF AGRICULTURE 
On January 4, 1924, 21 larve from 11 to 16 millimeters in length, 
from gullets, were introduced subcutaneously above the hock of 
another cow. On January 8 one larva cut through the skin 51, 
inches above the incision. On the following day 5 others punctured 
the skin, one of these being 30 inches above the point of introduc- 
tion. The following day 3 others appeared, 1 of these being near 
the spinal column on the right side about 7 inches behind the shoul- 
der, a distance of 40 inches in a line from the incision. A total of 
14 grubs perforated the skin, all being on the right side. 
It appears from these tests that when larve which are ready 
to leave the gullet are introduced subcutaneously they may not 
travel far from the point of introduction before puncturing the 
skin, and that they appear to pass upward under the subdermal con- 
nective tissue without penetrating deeply into the tissues as oc- | 
curs when young larve from the gullet are introduced. 
To recapitulate, in the tests carried out at the Dallas laboratory, 
28 animals were infested by allowing flies to oviposit on the legs 
and bodies and were prevented from reaching any part of their 
bodies with tongue or mouth. Of these 28 animals, 2 were acci- 
dentally killed and not examined. In one of those slaughtered 
for dissection no larve were found, but in this case no hatched 
eggs were observed on the animal and other evidence indicated 
that none of the eggs placed upon it were viable. One of the other 
animals was killed immediately after the eggs hatched and, al- 
though no larvee were found, there were strong indications of the 
penetration of larve through the skin. Two of the other animals 
killed showed an abundance of grubs along the gullet and on the 
viscera. Twenty of these animals were kept for observation during 
the fall and winter, and in every one of them grubs came to the | 
back during the normal grub season. In all of the 11 cattle which 
were infested by introducing larve from the gullets of slaughtered 
cattle under the skin on the legs, behind the ear, and on the back, 
larve appeared under the skin at the usual time during the fol- 
lowing fall and winter. Larve were also recovered in 2 animals 
which were dissected after larve had been introduced under the 
skin in this way. On the other hand, among the 8 animals to 
which larve and eggs were given by mouth, neither of the 2 which 
were carefully dissected showed the presence of larve, and in 
not a single instance among the 6 cattle held for observation did 
a grub appear along the back. 7 
In 1922 Carpenter, Phibbs, and Slattery (/9) report similar 
results in experiments carried out in Ireland. No grubs were 
recovered in calves to which numerous larve were fed but heavy 
infestations developed in carefully muzzled calves exposed to the 
oviposition of flies. 
The fact that the larve penetrate through the skin is established 
by the following evidence: x ae 
1—The eggs are usually laid on the hairs comparatively close 
to the skin, which would favor penetration. They are firmly at- 
tached and the eggshells, after the larve have escaped, remain at- 
tached to the hair. 
2—The eggs are not fitted with an operculum as in the case 
of the horse bots, the ends merely splitting as the larve emerge. 
