Muscidae 431 



more flies daily. The walls should be lined with suitable shelves on 

 which to place the cages and jars. Electric heaters thermostatically 

 controlled or temperature regulators for radiators may be used for 

 holding the temperature fairly constant. A basement room is quite 

 suitable as temperature variations are usually less, and if well insulated, 

 cooling units will not be needed in hot summer weather. Relative 

 humidity should be kept above 50 per cent. 



In starting a culture it is important to obtain large, healthy, un- 

 parasitized flies. These are placed in a screen breeding cage where 

 mating and oviposition take place. A cage of approximately 12" x 12" 

 X12" with one sliding glass side and a wooden bottom is quite suitable. 

 Egg laying usually starts when the flies are 3 days old. A dish contain- 

 ing a small amount of the bran mixture just described should be placed 

 in the cage for egg deposition and should be replaced each day. Food 

 consisting of milk diluted with equal parts of water should also be 

 placed in the cage in a small dish and renewed daily. Glass battery 

 jars approximately 6 inches in diameter by 8-12 inches high may be 

 conveniently used as rearing jars or where larger numbers of flies are 

 desired galvanized iron pails (12 qt. capacity) are suitable and are not 

 as easily broken. The rearing jars are filled about % full of the alfalfa 

 meal-wheat bran mixture and the eggs dropped on top. For a battery 

 jar of the size mentioned about 600 eggs are sufficient. For larger or 

 smaller jars a proportional number should be used. It is well to make 

 an approximate count of the number of eggs used so that overcrowding 

 will not occur else a population of undersized flies will emerge. The 

 tops of the jars are covered with cheesecloth held on with a rubber band 

 and the jars then placed on a shelf and incubated for 9-1 1 days at which 

 time the adult flies should be emerging. The house fly is positively 

 phototropic and advantage is taken of this for transferring the flies to 

 stock cages by means of a cone shaped top made to fit the jar or con- 

 tainer. Stock cages of the same size as breeding cages may be used to 

 keep the emerging flies in until needed for experimental work. As 

 many as 1000 or more flies may be kept in a cage of this size but it is 

 probably advisable not to overcrowd. Diluted milk should of course 

 be supplied daily as food. The flies vary in resistance to insecticides 

 at different ages being of low resistance in the writer's experiments, just 

 after emerging and of greatest resistance at about 48 hours of age after 

 which resistance decreases as they grow older. The resistance of the 

 flies varies from day to day and season to season but is fairly uniform 

 on any one day. For this reason in a series of comparative tests it is 

 well to use flies all of the same age and to finish the series as soon as 

 possible, preferably in 2 to 4 hours. 



By shrouding the cage in black cloth except for a hole at the top 



