I 



Butlerjiy Migrations in British Guiana. 159 



one side to the other with ease. The river was at least 

 a hundred and fifty, and possibly two hundred yards wide. 

 Three butterflies were timed and took twenty, twenty-four 

 and twenty-six seconds to cross. Taking twenty-four 

 seconds as an average and a hundred and fifty yards as the 

 distance, this gives twelve miles per hour (if the river were 

 two hundred yards wide it would be sixteen miles per hour). 

 This is, I think, a not unreasonable estimate. 



In flying the insects kept close to the surface over which 

 they were passing; over the forest they seemed never to 

 be more than a few feet above the tops of the trees, and in 

 any but the smallest clearing they descended rapidly to the 

 ground and flew between small bushes and trees about four 

 to eight feet up. On crossing the river they kept still 

 lower, being seldom more than three feet above the water 

 and occasionally almost touching it. 



Any attempt to estimate the number of butterflies in a 

 flight of this kind must of necessity be only an approxima- 

 tion. Taking ten butterflies per minute per two hundred 

 yards as an average, this gives 5,400 per hour per mile, 

 or 135,000 for a day of five hours on a front of five miles. 

 At the rate noted at the junction of the Aruka and Barima 

 on the 1st August (150 per minute) 84,000 would pass in a 

 single hour on a front of one mile. 



On the 10th August I left the district, and was later 

 informed by Mr. A. A. Abraham, Manager of the Govern- 

 ment Experimental station at Issororo, that the butterflies 

 disappeared shortly after my departure. 



The chief points of the above migration may be summar- 

 ised as follows : (1) The migration lasted at least ten days ; 

 (2) both males and females were represented, the former 

 predominating (my total captures were seven males and 

 three females). (3) They flew constantly in a south- 

 easterly direction at a speed of about twelve miles per 

 hour across the prevailing north-east trade wind. 



2. In the middle of October 1916 Mr. Abraham wrote 

 to me from Issororo to say, " Since my last note to you [end 

 of September] I have observed that the butterflies are 

 returning at the same rate and numbers to the bank of the 

 river from which they migrated." This is a particularly 

 interesting record, as it confirms several other records of 

 migration in opposite directions in the same locality about 

 which I had been doubtful. 



3. In September 1916 I observed a very difi'use niigra- 



