THE CATTLE GRUBS OR OX WARBLES Ad5 
occurs it is probably due to the fact that the lower eggs, being 
closer to the skin, are kept at a slightly higher temperature. 
Several hours before hatching, the segmentation and rows of 
spines are distinctly discernible through the shell. The larva be- 
comes active shortly before hatching and can be observed through 
the eggshell pressmg the mouth parts forward against the end of 
the egg. Finally the suture across the end of the egg is ruptured 
and the larva crawls out rather rapidly. 
LARVAL ACTION AFTER HATCHING 
The larva of H. lineatum after hatching usually crawls directly 
down the hair which bears the egg and after feeling about with the 
mouth parts begins to burrow directly into the skin at the base of 
the hair. The body is usually more or less extended along the hair, 
and during the initial efforts there is considerable twisting, expan- 
sion, and contraction. After the first few segments have been 
worked into the skin the larva becomes more quiet. The burrow- 
ing is slow but usually rather steady in case of vigorous specimens. 
Progress is retarded as the middle body segments reach the surface 
of the skin, and usually it is slightly accelerated when the larva 
has become almost completely imbedded. The time occupied for 
a larva to disappear after it has begun burrowing has been ob- 
served to be about one and one-half hours and sometimes consider- 
ably longer. | 3 
In nature it appears that several larvee frequently enter the host 
through the same hole, and this no doubt greatly facilitates the pene- 
tration after the first larva has gained entrance, 
Considerable difficulty was experienced in watching the penetra- 
tion of the larve into the skin, mainly owing to the difficulty of 
holding a host quiet for considerable periods of time but also to the 
fact that the larve, on account of their small size and delicate struc- 
ture, are very easily lost sight of. Most of the writers’ observations 
on penetration were made with freshly hatched larve taken from an 
incubator. Even with the most careful handling there was a high 
mortality, apparently much greater than when the eggs are attached 
to the host in the natural way. It was observed repeatedly that 
larvee removed from an incubator and placed on hair would wander 
about for a time before attempting to burrow and in so doing it 
seemed that they became more sensitive to drying, probably through 
the rubbing off of the gelatinous material with which their bodies 
are covered when they first emerge from the egg. Many larve put 
on hosts in this manner began burrowing, but perished during the 
process. In making observations a calf was usually placed on a 
table and firmly held or even strapped down so that the actions of 
the larve could be followed under hand lenses or binoculars. Where 
the hair had been clipped closely, early tests with the penetration 
of larve through the skin were unsuccessful, and all subsequent 
| observations on penetration were made on hair one-fourth inch long 
| orlonger. In order to reduce the chances of escape without detection, 
| it was the usual practice to clip closely or shave a narrow area around 
a tuft of hair and to place the larve upon this tuft. 
Several attempts were made to observe the penetration of larvee 
on the hands or arms of man. Usually the larve were watched 
