036 



MR. G. S. COTTERELL ON THE LIFE- HISTORY 



The Egcj. 



The egg is creamy- white when first laid, becoming darker as 

 the incubation period increases. It measures from 2 to 3 mm. 

 in length and is slightly curved. 



It is especially characteristic in having two wing-like exten- 

 sions of the chorion at its anterior end (text-figs. 4 & 5, a). 

 These are covered with extremely short setje on their under 

 surfaces, and serve as a support in the dung. The egg is laid 

 obliquely at an angle of from 15° to 25° with the surface of the 

 dung, so that the two extensions lie flat on the surface. The 

 micropyle (text-figs. 4 & 5, c) is situated between the ex- 

 tensions, and is drawn out into a short crest (text-figs. 4 & 5, ^) 

 at the anterior end between the two extensions. Its surface is 

 finely sculptured. The two extensions and the micropyle are all 



Text-figure 4. Text-figure 5. 



Text-fig. 4. — Egg of S. stercoraria, showing supporting wings. 

 Text-fig, 5. — Hatched egg of S. stercoraria. 



exposed on the surface. The wall at the base of the egg (text- 

 fig. 5, e) is thickened, giving greater strength, as this is the part 

 of the egg which first comes into contact with the dung when 

 oviposition is taking place. The egg is broken by means of a 

 transverse split behind the micropyle (text-fig. 5). 



In summer the whole surface of a deposit of cow-dung may 

 be seen covered with the wing-like extensions of the eggs. The 

 incubation period varies from one to two days according to tem- 

 perature. This period is much more constant than other periods 

 in the life-history, as temperature is the only governing factor. 



The newly-hatched larva breaks the egg behind the micropyle, 

 crawls out on to the surface of the dung, and seeks a convenient 

 crack by which to gain access to the softer parts of the dung. 

 This may take some considerable time if the dung has been de- 

 posited for some time, and consequently become dry on the surface. 



