42 BULLETIN 597, U. S. DEPARTMENT OF AGRICULTURE. 



The G. iniestinalis are the most resistant of the Gastrophilus to 

 treatments, but these, being found in the stomach, are in the most fa- 

 vorable place for treatment. No immediate effects of the gas upon last- 

 stage Gastrophilus larvae are observed, as they remain motionless for 

 some time. Finally they contract so as to conceal the hooks of attach- 

 ment and then drop from the stomach section. This sometimes occurs 

 within 30 minutes after the fumigation begins, but most often it is 

 after 1 or 2 hours. Occasionally the contraction is not so great and 

 larvae remain attached but drop at the slightest touch. A number of 

 observations were made upon animals treated by local veterinarians. 

 During the spring and early summer records were kept on 23 of these 

 horses, which were treated when they contained only last-stage larvae. 

 The treatment was given as recommended by the Bureau of Animal 

 Industry. Within 36 to 48 hours the first bots appear in the faeces, 

 though if the physic acts well they may be found after 24 hours. The 

 writer observed bots to pass for a period of 5 days, beginning about 

 36 hours after the treatment. With the first droppings a few living 

 larvae may be found which detached from the stomach before the 

 treatment, and in one case G. nasalis was reared from such larvae. 

 G. haemorrhoidalis may appear at the anus as usual if this treat- 

 ment is given in the spring, as by this time numbers have previously 

 migrated to the rectum. 



While Table IX shows that carbon clisulphicl gas is capable of 

 killing last-stage larvae within 3J hours, the ideal time for treatment 

 of horses would be in the autumn when all larvae are young and the 

 G. haemorrhoidalis are still within the stomach and duodenum. The 

 last-mentioned experiments in Table IX show some fumigation re- 

 sults with small Gastrophilus. It will be observed that 1 hour was 

 an amply sufficient time, though a few last-stage larvae were present 

 as late as September 18. If not caused to detach these would possibly 

 drop during the winter and succumb to low temperatures. In Dallas, 

 Tex., 45 minutes was sufficient to kill young larvae on December 9, 

 1916. 



The fact that G. nasalis attaches in the throat, where the larvae are 

 not in a position to be affected by the carbon disulphid treatment, em- 

 phasizes the fact that " an ounce of prevention is worth a pound of 

 cure." 



REPELLENTS. 



The rubbing of horses upon posts, bowlders, and other convenient 

 objects suggested a device for use in pasture whereby horses could 

 rub their lips upon a repellent. The short period of effectiveness of 

 repellent substances and the inability to obtain one that will remain 

 on the lips during grazing are difficulties which would seem to be 

 overcome by such a device. A keg reservoir was devised whereby a 



