350 VETERINARY HYGIENE 



undipped lambs for the greater protection the fleece of the latter 

 affords. They subsist mainly on blood sucked from their host, 

 but may derive some nourishment from the grease of the wool. 



Destruction of keds is assured by suitable dipping of the sheep 

 with dips that contain arsenic. Melophagus ovinus cannot live for 

 any length of time away from a host. Georgina Sweet and 

 Seddon kept the parasites under a variety of conditions to test 

 their viability. They found that a moderately cool and uniform 

 temperature is the most favourable condition for its existence away 

 from a host and without food, especially if it be dry. Thus keds 

 were found to live on bare soil on a lawn for two and three-quarter 

 days ; on moist grass in a cellar for six and three-quarter days ; on 

 moist grass on a lawn for five and three-quarter days ; on bare soil 

 in a cellar eleven days; and on dead leaves on soil in a cellar for 

 eleven and three-quarter days. If extremes of temperature be 

 present, then moisture is necessary for the life of the ked, dryness 

 soon proving fatal.* 



HORSE EOT FLIES. 



The horse bot flies, Gastrophilus equi (G. intestinalis) and G. 

 Hamoidalis, of the family Oestrida, are on the wing during the 

 month of August. 



The female of the former lays her eggs chiefly on the hairs over 

 the metatarsus and metacarpus of the horse, and that of the latter 

 chiefly on the lips. It is generally considered that the larvae on 

 hatching out set up an irritation that causes the horse to lick the 

 region affected and thus the larvae reach the stomach where they 

 attach themselves almost exclusively to the cardiac region. 



Roubaudf concludes from his researches that the eggs do not 

 hatch spontaneously, but that the ripe eggs wait, perhaps for weeks, 

 to liberate their larvae only on mechanical contact with the lips, 

 tongue or teeth of the horse. The primary larvae, according to 

 Roubaud, freed by contact with the mucous membrane of the lips 

 and gums, immediately bury themselves under the epithelium. 

 They migrate and grow under the epithelium and, when they have 

 attained a certain size, become free and are swallowed. It is known 

 that Gastrophilus larvae, unlike those of Hypoderma, cannot pene- 

 trate the skin. The bot larvae remain in the stomach of their host 

 for about ten months and then pass out with the faeces to pupate 

 in the soil, from which they emerge as adult flies in about six weeks. 



* Vet. Journ., 1917, LXXIII. 

 t C. R. Acad. Sci., CLXIV., p. 453. 



