MUSCA : EARLY STAGES 337 



the bites of other flies, such as Philaematomyia insignis. It breeds 

 in cow dung. 



The eggs of Musca domestlca and its allies measure from '8 to 1 mm. 

 in length (Plate XLVI, fig. 1). They are of a creamy white colour, 

 cylindrical in shape, with one end broader than the 

 other; the dorsal surface has two curved rib-like thick- 

 enings. The larva (Plate XLVI, fig. 10) hatches out 

 in from twelve to twenty-four hours according to the temperature. It 

 measures when mature from 10 to 12 mm. in length and consists of 

 about twelve segments, but the exact number is a matter of dispute. 

 The second segment has at each postero-lateral border a fan-shaped 

 structure, the anterior spiracle, which consists of from five to eight papil- 

 lae ; the number of these papillae varies in the different species and is useful 

 in distinguishing the different larvae. The posterior spiracles are situated 

 on the posterior end of the last segment. They are dark brown D-shaped 

 structures, consisting of a chitinous ring enclosing three stigmatic slits. 

 The larva passes through three stages or instars, shedding its skin each 

 time. After each change the posterior stigmata become larger ; at first 

 they are small and have a single slit ; at the next change there are two 

 slits and in the last stage the slits become long and convoluted. Between 

 the third and fourth day or even longer, according to the time of the 

 year, the larva arrives at maturity, when it assumes a creamy white 

 colour, due to the large amount of fat body which has been accumulated. 

 It then slowly contracts, the outer skin at the same time hardening, until 

 it becomes about half its original size. The puparium is first of a creamy 

 yellow colour, but soon changes to a dark mahogany brown (Plate XLVI, 

 fig. 1 1) ; the posterior spiracles are now seen as two flat projecting tuber- 

 cles, at the broader and posterior end. The fly emerges about the sixth 

 or seventh day after the egg is laid. 



As the fly is about to leave the puparium, it splits off a circular piece 

 with the aid of its frontal or ptilinal sac, which is extruded just above the 

 bases of the antennae. When it has completely freed itself, the fly levers 

 its way through any obstruction by inflating and deflating the sac ; it 

 then crawls about, drawing in air into its respiratory system by the aid 

 of muscular action, and waits till its wings have become unfolded and 

 sufficiently hard to permit of flight. 



According to Howard, Newstead, Hewitt, Griffith and others, Musca 

 domestica prefers horse dung to any other nidus in which to lay its eggs. 

 It will, however, breed in human excrement, and that of 



( , _ breeding FlclDltS 



poultry, in bedding and other rubbish contaminated 

 43 



