84 BULLETIN 1369, U. 8. DEPARTMENT OF AGRICULTURE _ ' 
more localized. In the latter case there is a tendency for the skin 
to close the grub opening and cut off its air supply. In certain © 
animals there is also a strong tendency to form pus in and about © 
the cyst, and in such cases the grubs usually die. The writers’ obser- 
vations show that the mortality in the subcutaneous tissues of the 
back is very high. Their figures indicate that 60.5 per cent of those 
which have cut holes through the skin die before reaching maturity. 
Of this percentage approximately 16 per cent succumb in the third . 
stage, 46 per cent in the fourth stage, and 38 per cent in the fifth 
stage. It appears also that occasionally third-stage larve which 
reach the subdermal tissues die before they succeed in cutting 
through the skin. 
There is a marked difference in the degree of infestation of 
animals of different ages. Young stock, notably animals from 1 to 
3 years old, almost always show a higher percentage of infestation 
than mature animals. There is also some reason to believe that as 
cattle become very old the degree of infestation is again increased. 
Seymour-Jones (97) advances the theory that in young stock the 
tenderness of the muscle (panniculus carnosus) immediately under 
the skin enables the grubs to penetrate them, whereas in older cattle 
they do not all get through. | 
Hadwen (35, 40) is of the opinion that the difference in de- 
gree of infestation between young and matured animals may be 
explained by a gradual development of immunity following the 
repeated attacks of parasites. He believes that a first invasion of 
parasites in an unresisting host stimulates the production of both 
antisubstances and eosinophiles to neutralize their cast-off products, 
and in addition to this, that there must be a third substance which is 
antagonistic to the parasites themselves; he believes that this sub- 
stance is secreted by the eosinophiles, and that it paralyzes or kills 
the parasites which are surrounded by the eosinophiles in the tissues. 
The writers believe that the difference in the number of grubs 
found in young and old cattle may be explained in part by this _ 
reaction, but that other factors enter into the question. In general, 
young animals are more exposed to the attack of the flies, both be- 
cause they are not housed so long in the spring and because they are 
seldom placed in barns during the time when the flies are oviposit- 
ing. It is also true that the young stock are frequently not as well 
fed as the older animals. The statement is made frequently by 
dairymen and stockmen that poor animals are more heavily im- 
fested than fat ones. The writers have observed many instances in 
which very fat animals were fairly well infested with grubs, but 
their observations indicate that the poor cattle are usually more 
heavily infested. It appears logical to believe that well fed, healthy 
animals have more natural resistance to grubs than those in poor 
flesh and with lower vitality. It is also certain that cattle in an 
extremely impoverished condition will fall more ready prey to 
the flies at the time of oviposition. It seems evident, however, that 
the presence of a large number of grubs, especially in cattle not re- 
ceiving an optimum amount of feed, will reduce flesh condition. 
No marked difference in susceptibility seems to exist in different 
breeds. When water or shade is readily available it serves as a 
means of partially protecting from infestation the stock which have 
