27 



ready to undergo its chauge into a fly (Fig. 7a, a, a, and b) — the charac- 

 ters outlined in its first stages have been j)erfected. It is this stage 

 that is commonly seen by those who split open the head of an affected 

 sheep. The mature grub averages over 20°^™ in length and 7'"'" in 

 width, or about three-fourths of an inch long by one-tliird of an inch 

 wide. Its width and length when measured depend much upon its 

 state of contraction. Its back is very convex, its abdomen is slightly 

 curved but generally flat, its outline is a very elongated oval, with an 

 acute head and obtuse posterior end. Half-grown specimens are more 

 pointed at the ends and decidedly flatter on the abdomen. From the 

 young to the mature larval state there is a decided change of color. 

 At first they are white and semi-transparent. They quickly grow 

 whiter and soon after take on a tinge of yellow, which, as they mature, 

 grows darker and darker. Then, too, on the back of every segment, 

 except the first and last, a dark narrow band appears which eventually 

 changes from a brown black to a dead black. These bauds are rather 

 narrower in front, increasing in width backward. On the side of each 

 segment below these bands there is, in mature specimens, a row of dark 

 dots. The spines show the same changes, at first white, then tipped with 

 brown, and finally changed to black. The spines occur only on the 

 abdomen and the closely adjacent edge. They all point backward and 

 assist the grub in moving around. 



When the larva is mature it escapes from the nostrils, falls to the 

 ground, bores into it for an inch or two, and, according to Riley (o. 

 c, p. 102), contracts during the next forty-eight hours to half its for- 

 mer size, becomes smooth and hard and of a black color, tapering as 

 in the larva towards the head (Fig. 14). It remains in this state 

 three or four weeks, or according to some authorities from fifty to sixty 

 days, depending on the weather. When the fly has matured within 

 the case it pushes ofi'a little round cap, ascends from the ground through 

 the hole left by the larva in its descent, and emerges into day to com- 

 plete the cycle of its existence. 



The office of the fly seems to be merely to reproduce its kind. On ac- 

 count of its rudimentary mouth it is unable even to eat. After emerging 

 the fly crawls upon some neighboring grass or twig and rests there until 

 its wings and body have hardened. During the first part of its life, ac- 

 cording to Brauer (o. c, p. 149), it is very sluggish, sitting around in the 

 cracks and crevices of the walls of sheep stalls, and is so dull that it 

 m«8t be dragged out. When placed on the hand it seldom flies off. 

 This dullness vanishes as soon as the fly has reached perfection and the 

 right temperature of the air comes. It is then off with a whirr, first 

 vertically in the air, and then in the direction of the flocks. 



The effects it produces on sheep and how and where the injury is in- 

 flicted may now be stated. 



Bracy Clark, an English veterinarian, describes the effect of the attack 



