ROLE OF STOMOXYS CALCITRANS. 



491 



to thirty seconds. The flies used in this series were laboratory 

 bred, applied on a single occasion only, with the exception of 

 those in experiments 10 and 11, wherein the flies had been used 

 previously in experiment 9. (Table VI.) 



Two attempts at mechanical transmission by interrupted feed- 

 ing were made with horses as the second hosts. The method 

 pursued was the same as before, but the flies employed were not 

 laboratory bred. It was aimed in this series to exaggerate the 

 normal conditions as much as possible by using as virulent a 

 strain of surra as could be obtained and by transferring the 

 infected flies to the weakest animals available. Unfortunately 

 the number of flies employed was not adequate, due mainly to 

 the great length of time required to feed them and the desire to 

 save needless suffering of the horses strapped to the operating 

 table. Four to five hours were required to complete each 

 experiment. 



The experiments were performed in the screened operating 

 room of the laboratory where the horse was strapped to the 

 operating table and bitten as rapidly as the flies could be trans- 

 ferred from the infected guinea pig on the adjoining table. 

 As many as 4 flies could be fed simultaneously on the horse 

 in this manner. Individual tubes were used to hold each 

 parasite. On July 1, 1912, horse 275 was bitten by 25 flies 

 which were applied at intervals of from ten seconds to two 

 minutes after contaminating their labiums with blood of guinea 

 pig C. The time required to complete the meal varied with the 

 individual flies from forty seconds to flve and one-half minutes. 



On the following day, July 2, horse 279 was similarly treated 

 with 38 flies. These were fed for from twenty to thirty seconds 

 on surra guinea pig A, which had a maximum infection at this 

 time. The flies required from thirty seconds to six minutes to 

 become completely engorged on the second host. 



Details of the work appear in the following table: 



Table VII. — Experiments and interrupted feeding of Stomoxys on horses. 



Infected 



aruinea 



pig 



used. 



Average 

 number 

 of trypa- 

 nosomes 

 present 

 in blood 

 per field. 



No. of 

 horse 

 em- 

 ployed. 



Number 

 of flies 

 applied. 



Length of 

 time fed on 



surra host 



(average per 



fly). 



Interval be- 

 tween feed- 

 ings (average 

 per fly). 



Time required 

 to complete 

 meal on the 



horse (average 

 per fly). 



Duration of 

 experiment. 



C 



A 



50-60 

 65 



275 

 279 



26 

 88 



Seconds. 

 20 

 26 



Seconds. 

 80 

 26 



Minutes. 



3.5 

 4.0 



Hrs. mins. 



4 20 



5 00 



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