190 ZOOLOGY. 



leaves, before the sheep go to pasture in the morning. 

 For sheep, which in late summer rub their noses up 

 and down tree-stems, walls, hedges, or their own legs, 

 substances that cause sneezing may be employed, 

 e.g. cheap snuff, which is best introduced into the 

 nose by means of a quill-feather. Later on, when 

 the maggots have passed from the nasal cavities into 

 the frontal sinuses, etc., sneezing does no good. Opera- 

 tions seldom succeed in removing all the maggots. 



The Bot Flies (Oastrus, or GastrophilvJ) live in 

 various parts of the stomach (in left side of horse's 



Fig. 121.— Horse Bot Fly (Gastrus equi): a, egg on a hair, strongly magnified ; c, 

 younger (magnified), and 6, older larva \ d, opened pupa-case ; e, fiy. 



stomach) and intestine. I give, first of all, a compressed 

 tabular view of the external characters and mode of 

 life of the four British species. (See next page.) 



Diseases caused by Bot Flies. — The maggots bore 

 into the walls of the stomach and intestines until they 

 reach the layer in which the blood-vessels, lacteals, 

 and lymphatics ramify; they then suck the juices 

 found in these vessels, and also serous fluids. In 

 small numbers they are often almost harmless, but 

 when a great many are present they hinder the secre- 

 tion of the digestive juices. They may also set up 

 inflammation of the intestinal coats, or may cause 

 death by internal bleeding if they perforate the wall 

 of an artery. In foals they often bore right through 

 the wall of the intestine, and enter the abdominal 

 cavity, where they may set up inflammation of the 

 peritoneum or of the mesentery. There may be as 



