82 THE HABITS AND BIONOMICS OF THE HOUSE-FLY 



coloured faeces produced by feeding flies on coloured syrup are not 

 deposited until several hours after the meal. Graham-Smith in 

 his experiments found they were not depijsited within two hours 

 after feeding. He made a number of interesting observations on 

 the rate of defaecation and the effect of different kinds of food on 

 the same. The yellowish to dark broAvn faecal spots or fly "specks" 

 are Avell known. A series of ten flies which had been given one 

 feed of milk produced in a period of 22 hours an average of 12-5 

 vomit spots and 3"9 faecal spots. A series which were allowed to 

 feed on milk whenever they wished produced an average of 17 '9 

 vomit spots and 4-5 faecal spots in 22 hours. The average number 

 of " spots " both faecal and vomit, produced by the two series in 

 22 hours was 16'4 and 20"4 respectively. In a further experiment 

 the average number of spots, fixecal and vomit, which were de- 

 posited per fly in 24 hours was 307. Three series of flies were fed 

 on syrup, milk and sputum for several days. Those fed on syi"up 

 produced an average of 4'7 deposits (faecal and vomit) per fly 

 per day, those fed on milk 8'3 and those fed on sputum 27'0. In 

 the latter case Graham-Smith points out that the faeces were 

 much more voluminous and liquid than usual and in fact the flies 

 seemed to sufler from diarrhoea. 



The rate of deposition and number of the faecal vomit spots 

 deposited is highly significant in connection with the question 

 of the bacterial contamination of food and especial attention is 

 drawn to the bearing of the abundance and character of the spots 

 deposited by flies feeding on sputum on account of its bearing on 

 the dissemination of the tubercle bacillus. 



Mention will be made later of the influence of food on the 

 development of the larvae ; the experiments which were carried 

 out showed that the larvae develop more rapidly in certain kinds 

 of food, such as horse manure, than in others. It has yet to be 

 discovered what are the chemical constituents which favour the 

 more rapid development. It was found that insufficient food in 

 the larval state retarded development and produced flies which 

 were subnormal in size. Bogdanow (1908), in an interesting- 

 experiment, fed M. domestica through ten generations on un- 

 accustomed food such as meat and tenacetum in different propor- 

 tions, and he found that the resulting flies did not show any change. 



