THE CATTLE GRUBS OR OX WARBLES 83 
_ below freezing. A mature larva of H. lineatwm placed in a freezing 
room for 24 hours at a temperature of about 7° IF. failed to pupate, 
whereas a fresh pupa exposed in the same way produced a normal 
adult. Nine mature larve of H. bovis subjected for from 24 to 
26 hours to temperatures in a freezing room of 7 to 9° F. failed 
to pupate. Five larvee of this species placed in the room at 25 
| to 29° F. for 26 hours gave an adult emergence of 80 per cent, and 
five larve kept at from 32 to 33° IF. for 14 days pupated promptly 
upon removal and 60 per cent emerged. 
SOIL CONDITIONS AND DRAINAGE 
Naturally soil character and drainage are linked with rainfall 
in their relationship to Hypoderma. The writers’ observations in- 
_ dicate that porous, well-drained soils are more favorable for Hy- 
‘poderma than heavy, flat-lying lands. 
In the valley of the Red River of the North, where Hypoderma 
seldom occurs, there is probably a soil condition which prevents 
fully developed larvee from producing adults. It is known that 
animals infested with Hypoderma have been shipped into that sec- 
tion, but the pest has never become well established. 
HOST RESISTANCE AND OTHER CONTROL FACTORS 
The great disparity in the number of grubs found in different 
cattle in the same herd is well known. The writers’ investigations 
indicate that this difference in degree of infestation is brought 
about by a combination of several causes. Probably the main rea- 
son why one animal will show a heavy infestation of grubs while 
another will be free from grubs or comparatively so, is that there was 
a marked difference in the number of eggs which were attached to 
them. The reason why one animal may receive a much larger 
number of eggs than another, however, is not altogether apparent. 
Certainly the element of chance enters here strongly. The indi- 
vidual idiosyncracies of the animals are also factors. Some ani- 
mals seem more able to detect the presence of flies than others and 
secure protection from them. 
A large series of observations made on individual animals year 
after year shows that the extent of infestation may vary greatly. 
The writers have found, however, that certain animals are uni- 
formly resistant to the grubs from the time the eggs hatch to the 
time the larvee complete their development or die within the host. 
This individual resistance is probably due in a large measure, as 
pointed out by Hadwen (35, 40), to eosinophilia. That there is 
a very definite reaction against the larve is apparent when they 
penetrate the skin of such resistant hosts. The exudate at the point 
of entrance is usually more profuse than ordinarily, and the infil- 
tration of the connective tissue beneath is pronounced. Apparently 
this strong reaction against the invading parasite is present 
throughout its course in the body of the animal. It is after the 
larvee have reached the subcutaneous tissues of the back, however, 
that the destruction of the larve is most apparent. The reaction 
against the larve appears to differ considerably in different hosts. 
In some, large swellings occur; in others, the reaction seems to be 
