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504 0. GORDON HEWITT. 



would be reared and ready to assume their customary activity 

 under the benign influence of the sunny days of June. 



I have made many experiments with a view to finding out 

 the rapidity with which house-flies breed. Anyone who has 

 endeavoured to keep flies alive in an enclosed space will 

 appreciate the difficulty of the task, those who have not done 

 so would hardly realise it. Fewer insects seem less tenacious 

 of life when enclosed even in a comparatively large enclosure 

 of six or nine cubic feet. It is a remarkable fact, as one 

 would imagine a priori that these insects, flying about 

 everywhere as they do, could be easily kept in a roomy cage 

 if given the necessary food and water. This, however, has 

 not been the case in my experience ; the longest period which 

 I have been able to keep them in captivity in summer is 

 seven weeks. I am pleased to find that Griffith has succeeded 

 in keeping a male fly sixteen weeks, and has obtained four 

 batches of eggs from females in captivity. In one of my 

 experiments I was successful in obtaining flies of the second 

 generation bred in captivity. I found that the flies became 

 sexually mature in ten to fourteen days after their emergence 

 from the pupal state and, four days after copulation, they 

 began to deposit their eggs, that is, from the fourteenth day 

 onwards from the time of their emergence. 



From these results it may be seen that in very hot weather 

 the progeny of a fly may be laying eggs in about three weeks 

 after the eggs from which they were hatched had been de- 

 posited. As a single fly lays from 120 150 eggs at one time 

 and may deposit five or six batches of eggs during its life, it 

 is not difficult to account for the enormous swarms of flies 

 that occur in certain localities during the hot summer months, 

 and algebraical calculations are not required to more vividly 

 impress the fact. 



IV. DEVELOPMENT. 



As I have already stated, M. domestica may become 

 sexually mature in about ten to fourteen days after emer- 



