BREEDING TECHNIQUE 343 



and to settle which one he is going to use for his experiments. In order 

 to do this a preliminary experiment should be carried out. 



The eggs should be washed out of the tubes with some saline solution, 

 which is then poured, together with the eggs on to some fresh horse 

 dung placed at the bottom of a glass jar * such as that used for pre- 

 serving pathological specimens. The jar should be covered with a piece 

 of muslin securely fastened with some tape to prevent stray flies from 

 laying their eggs in the horse dung. It should also be remembered that 

 if the dung is not quite fresh it may already contain the eggs of some 

 other muscid, and if this happens much confusion will arise. It some- 

 times happens that fresh dung cannot be obtained, and in this case it is 

 best to keep the dung in the jar well covered for twenty-four hours ; 

 if it contained any eggs at the time it was collected the larvae will have 

 hatched out and can be found by disturbing the superficial layers. 



As soon as the larvae hatch out of the eggs, the jar should be placed 

 for a short time in the sun, just long enough to make the glass warm ; it 

 should then be kept in the shade and at night put inside the mud 

 enclosure alread described (see page 314). By the fourth day the larvae 

 will be mature, and as they will be preparing to migrate preparatory to 

 pupating the cloth should be removed from the jar on leaving the 

 laboratory in the evening. Several specimens of the larvae should be 

 removed and dropped into almost boiling water, and then preserved in 

 seventy per cent alcohol. These specimens are necessary in order to 

 study the structure of the larva ; the particular points to note, and those 

 which prove of the greatest use in separating the species, are the number 

 of the papillae on the anterior spiracles, and the structure of the 

 posterior spiracles. 



If the jar is placed in the enclosure and the cloth removed, the larvae 

 will crawl out during the night and will be found next morning in the 

 sand around and under the jar ; a number will have already pupated. 

 Several of the puparia should be collected and dropped into hot water, 

 and then preserved either by placing them in seventy per cent alcohol in 

 a tube or by pinning them to pieces of cork, care being taken to label 

 the specimens. In addition some of the puparia should be placed in test 

 tubes, one in each tube, and when the flies hatch out the pupal cases 

 should be kept along with the imagines in the collecting box. If this 

 preliminary experiment is carried out the worker w r ill soon satisfy 



* The jar used by the writers is shown in Plate XLIV, fig. 5 ; it measures 9 inches in 

 height and 5J inches in diameter, and can be purchased from Messrs. Baird and Tatlock, 

 Hatton Gardens, London. 



