12 BULLETIN 1369, U. S. DEPARTMENT OF AGRICULTURE 
age in this respect. Losses caused by this annoyance include marked 
reduction in milk flow, failure to put on flesh normally, mechanical 
injury due to the wild efforts of the animals to escape attacks, and 
not infrequently loss from the miring down of cattle when they 
rush into ponds or mud holes, or their injury or destruction when 
running over embankments or cliffs. These attacks usually occur 
at a time when the stock are in a weakened condition in early spring, 
hence the danger of miring and abortion is increased. The exas- 
peration and often death loss resulting from stampedes caused by heel 
flies is well known to stockmen. ‘The second group may be divided 
into four sections: (1) The soreness and pain produced by the pene- 
tration of the young larve through the skin; (2) the irritation pro- 
duced in the gullet and in other internal organs due to the migrating 
larve; (3) inflammation produced along the spinal cord and on the 
main branches of the nervous system by the burrowing of the larve 
along the spinal canal and the ingress and egress of that canal; 
and (4) the irritation produced by the later larval stages in the 
subdermal tissues of the back, with accompanying pus formation. 
The exact extent of loss produced by these various stages and 
activities of the insect can not be determined, but it may be well to 
point out some facts along this line. Considering the loss produced 
by the flies at the time they are depositing eggs, many dairymen 
affirm that they know immediately by reduced production of milk 
when the heel flies become active. Of course the amount of loss at 
this time depends to a large extent upon the number of flies, the 
quantity of feed supplied the cattle, and the protection afforded 
them during the day. In regions where the grubs are abundant some 
dairymen estimate the milk loss at from 10 to 25 per cent during 
the period of fly activity. The season when the adult flies are active 
varies, lasting from one to four months. It can be well understood 
how this marked reduction in milk flow is brought about when the 
frantic efforts of the cattle to escape attack are observed. They are 
frequently seen standing closely bunched in the shade or in the 
middle of a pond or stream during the major part of the day when 
they should be grazing. : 
The annoyance produced by the penetration of the young larvee 
through the skin is by no means small, although the period when it 
occurs is comparatively short. This irritation is indicated by the 
violent licking of the heels and other parts where the penetration 
is taking place, accompanied by intermittent kicking and stamping 
of the feet. There are also extensive external lesions, their loca- 
tion indicated by hair matted and rough from the exuding serum 
and sometimes by rather extensive areas denuded of hair or even by 
sloughed areas. This injury is more severe in the case of H. lineatum 
than -with H. bowis, owing to the fact that with the former many 
larvee penetrate the skin at nearly the same spot, the eggs from 
which they emerged having been laid together, whereas the eggs 
of the latter are placed singly. | | 
Little is known as to the annoyance produced by the migration 
of the first-stage larve aftér* they have passed through the skin. 
It is: possible, however, that this is of minor importance. As the 
larvte increase in size, and especially when they are numerous, there _ 
is-Ho doubt that-their migration through the body influences the 
