DURATION OF LIFE 43 



A chart made at Jinja in 1910 (see Chart II) 

 shows very clearly the direct relation between relative 

 humidity of the atmosphere and the number of flies that 

 could be caught. 



In order to estimate the length of life of the wild fly, 

 the method adopted was one that had been used by 

 Dr. Bagshawe to ascertain how far the fly would follow 

 canoes or men. 



Large numbers of flies were captured, a portion of one 

 leg was amputated through the middle of either femur or 

 tibia (limbs may be lost at a joint naturally, but never 

 in this way), and then the fly was liberated. By con- 

 tinually catching flies at the same point day after day 

 it could be ascertained for how long a marked fly had 

 lived. 



Since a fly has six legs and each can be marked in two 

 ways, twelve markings are possible : one method of marking 

 was kept for the flies of each week. At Jinja, between 

 July 27th and October 15, 1910, some eight thousand 

 flies were so marked, and eight markings were used, an 

 average of a thousand to each marking. 



Between August 3rd and February 11th, careful exami- 

 nation was made of each fly caught by the boys, who 

 worked every day for four or five hours. On January 

 31st, 1911, a female fly was taken that had been marked 

 on one of the days July 27 to August 2, 1910, and on 

 December 29th a male was taken that had been marked 

 on one of the days July 27 to August 2. This gives 

 a period of life of at least 182 days for the female and 

 149 days for -the male, but it is of course impossible to 

 say for how long before the marking the flies had been 

 alive : probably they were hatched fairly recently. 



On Bugalla Island in 1912 this experiment was repeated, 

 and between March 18th and April 26th nine thousand 

 flies were marked in six different ways. The last marked 

 flies to be caught were noted on January 17, 1913 : two 



