﻿26 BULLETTIir 1369, U. S. DEPARTMENT OF AGRICULTUEE 



than is H. lineatum. Frequently cattle will run for a knoll if shade 

 is not at hand, and when the breeze is strong the grouping of the 

 stock on high ground seems to give some immunity from attack. 

 When these natural protections are not accessible the flies, especially 

 H. hovis, will keep up their attack intermittently, causing the cattle 

 to run from one part of the pasture to another, in small pastures 

 causing them to run round and round until they froth at the mouth, 

 breathe heavily, and even drop from exhaustion. Under range con- 

 ditions the stock usually have better opportunity of escaping, but the 

 tendency to a general stampede is increased, especially if cattle are 

 being worked in large herds. 



When grazing, cattle usually detect the presence of a fly very 

 quickly, largely by sight and hearing, but also by touch. Cattle 

 readily difi'erentiate between the presence of a heel fly and other in- 

 sects. The listening attitude assumed when a fly is in the vicinity is 

 characteristic, as is also the action of shaking the foot (fig. 5, a) to 

 dislodge a fly, the rolling action of the tail, and the look of fright 

 when a fly attacks in earnest. 



DESCRIPTION OF STAGES 

 THE EGG 



The egg of Hypoderma lineatum (fig. 6, a; fig. 7) is dull yellowish white 

 and the surface is smooth and shining. It is narrowly ovoid, slightly larger 

 at the base than at the tip, and its greatest diameter is at the middle. The 

 average length of the egg proper is 0.76 millimeter and the average diameter at 

 its greatest thickness is 0.21 millimeter. The unattached end of the egg has a 

 slight ridge across it from side to side along which the egg splits when hatching 

 takes place. ( Fig. 6, h. ) This ridge crosses the end slightly on the side toward 

 the hair and the micropyle is located centrally at the apex. The clasp with 

 which the egg is attached to the hair is oval in outline. The average length of 

 the base attached to the hair is 0.31 millimeter. The petiole averages 0.1 

 millimeter in length. It is flat and curved and very narrow when viewed from 

 the side. The position of the eggs v>'hen attached in a row is at an angle of 

 about 45° to the axis of the hair, except the last one toward the tip of the 

 hair, which is u.sually more nearly parallel with it. 



The eggs of H. bovis are sim lar to those of H. liueatiDu except in size, meas- 

 uring 0.81 millimeter in length and 0.29 millimeter in width. The clasping 

 base is slightly more truncate at the end toward the tip of the hair and the 

 petiole arises from the clasp more nearly at its middle than in H. lineatum. 

 The most striking difference between the eggs of these two species is the attach- 

 ment of the petiole to the base of the egg. With H. lineatum the attachment is 

 on the side of the base of the egg away from the hair whereas with H. hovis the 

 attachment is more nearly in the middle of the base and the petiole is more 

 elbowed. (Fig. 8.) Furthermore, the eggs of H. lineatum are normally at- 

 tached to the hair in rows, whereas with H. bovis the eggs are always laid 

 singly. 



THE LAKVA 



FIRST STAGE 



The larva of Hypoderma- lineatum when hatched from the egg measures 

 from 0.55 to 0.65 millimeter in length and from 0.15 to 0.18 millimeter in width 

 at its greatest diameter. The width is greatest at the posterior end and the 

 larva tapers to the head. It is creamy or dull white in color, and densely 

 covered with spines on all segments, the anterior borders bearing the 

 heaviest spines in transverse rows, followed usually by six rows of spines, 

 more or less regularly placed, and slightly decreasing in size toward the pos- 

 terior border of the segment. The anal segment differs from all those pre- 

 ceding in that it bears spines of three distinct types. The posterior spiracles, 

 which are represented by two dark circular spots, are protected by two or 

 three rather large, triangular spines located near their borders. The cephalo- 



