22 ,. BOAED OF AGRICULTURE. 



male, and the color is lighter. The thorax is ash-colored, 

 with four black lines and small black spots. The abdomen is 

 light ash, more or less spotted with blackish. The female has 

 a tapering abdomen, with a long ovipositor, which is curved 

 forward when about to deposit the eggs. 



The flies appear during the whole summer, and according 

 to Mr. Riley, deposit the young larvae previously hatched in 

 the oviduct, on the margins of the nostrils of 

 sheep. " The moment the fly touches this part of '^^*' 

 the sheep, they shake their heads and strike the 

 ground violently with their fore-feet ; at the same 

 time holding their noses close to the earth, they 

 run away, looking about them on every side to 

 see if the fly pursues. They also " smell to the 

 grass " as they go, lest one should be lying in 

 wait for them. If they observe one, they gallop back, or take 

 some other direction. As they cannot, like the horses, take 

 refuge in the water, thev have recourse to a rut or dry, 

 dusty road, or gravel pits, where they crowd close together 

 during the heat of the day, with their noses close to the 

 ground, which renders it difficult for the fly conveniently to 

 get at the nostril." (Clark, 1796). This description, al- 

 though written long ago, applies equally well to the habits 

 both of the fly and sheep, as observed at the present day. 

 The young larvae at once proceed to ascend into the nostrils 

 by means of their hooks and writhing motions, thus causing 

 great irritation and an increased secretion of mucus, upon 

 which they feed. They finally reach the frontal and maxil- 

 lary sinuses and other cavities, connected with the nose, and 

 attach themselves securely to the mucus membranes by means 

 of the two sharp hooks upon their heads. They thus cause 

 great inflammation, if in considerable numbers, and severe 

 illness, or even at times convulsions and death result. Tiiis 

 disease is often known as " grub in the head " among farm- 

 ers. The disease is indicated by loss of appetite, frequent 

 coughing or sneezing, unusual discharge of matter from the 

 nose, slow and weak gait, dullness, inability to stand, tossing 



Figure 24. — Larva of the Sheep-bot [CEstrus ovis), natural size. From Clark. 



