120 The Common House Fly. [Sess. 



pearly-looking, banana-like structures. Each is about 1 mm. 

 or -^ inch in length. Under a high magnification the surface 

 is seen to be covered with minute hexagonal markings like 

 pavement epithelium. In from eight hours to three or four 

 days the larvae or maggots hatch out from the eggs ; the 

 warmer the medium the shorter the period. If closely- 

 examined, it is seen that at one end of the egg a slight split 

 or crack appears, which gradually enlarges until the larva 

 crawls out and leaves the collapsed egg-shell. 



The larva is about 2 mm. long, glistening white in colour, 

 with a pointed slender head, and gradually increasing in size 

 backwards. It rapidly disappears from sight, as it burrows into 

 the substance on which the egg was laid. It casts its skin 

 twice during this period of its life, and therefore has three 

 stages of growth. When fully grown it is nearly 12 mm. 

 long, devoid of legs, and with twelve distinct segments. At 

 the head end there is a pair of oral lobes, the mouth 

 opening between these, and this can be withdrawn entirely 

 within the second segment. Above the mouth orifice a 

 black hook-shaped process projects, which is used for loco- 

 motion and for tearing up the food so as to allow it to 

 absorb the liquid material on which alone the larva 

 subsists. The anterior spiracles are found in the second 

 segment, and at the hind end are two eyelike spiracles. Each 

 of the latter consists of three sinuous slits, leading to longi- 

 tudinal respiratory tubes. At each segment on its ventral 

 aspect is a pad with recurved spines, which assist the larva in 

 its onward progress. As it gets older, it becomes fat and 

 creamy-like, and subsists entirely on vegetable food, though, 

 in the absence of this, horse-manure forms the favourite food, 

 as does also human excreta. 



In from five to eight days the larva is mature, but under 

 unsuitable conditions the period may be prolonged for from 

 six to eight weeks. It then crawls to a dry and sheltered 

 spot, and, after a short rest, the pupa stage is entered upon. 

 The anterior segments are indrawn, and it assumes a cylin- 

 drical shape. The larva retracts itself from its outer skin, 

 which remains as the puparium, and its organs, after under- 

 going disintegration, are formed anew as those of the future 

 fly ; and this all occurs within twenty-four hours. The 



