﻿12 BULLETIN 1369, U. S. DEPARTMENT OF AGRICULTURE 



age in this respect. Losses caused by this annoyance inchide 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 larvae through the skin; (2) the irritation pro- 

 duced in the gullet and in other internal organs due to the migrating 

 larvae; (3) inflammation produced along the spinal cord and on the 

 main branches of the nervous system by the burrowing of the larvae 

 along the spinal canal and the ingress and egress of that canal; 

 and (4) the irritation produced by the later larval stages in the 

 subclermal 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 larvae 

 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. hovis, owing to the fact that with the former many 

 larvae 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 larvae after they have passed through the skin. 

 It is possible, however, that this is of minor importance. As the 

 larvae increase in size, and especially when they are numerous, there 

 is no doubt that their migration through the body influences the 



