HOUSE FLIES AND BLUEBOTTLES 201 



instead, the last larval skin simply hardens and is not 

 shed at all, the true pupa being formed inside it. In 

 most cases, the larva burrows into the ground before 

 accomplishing this change. The hardening process ap- 

 pears to be completed very suddenly ; the skin contracts, 

 especially in length, and becomes hard, brittle, and dark 

 coloured, until what was a few minutes before a soft, 

 wriggling, conical maggot, becomes a short cylindrical 

 body with rounded ends, enveloped in a hard reddish- 

 brown skin, faintly marked with a succession of grooved 

 rings transversely to its length, and utterly incapable 

 of the slightest movement. Evidently, therefore, this 

 so-called pupa is not strictly comparable to the chrysalis 

 of a butterfly or moth, notwithstanding some superficial 

 resemblance. Each, it is true, is covered by a hard 

 reddish-brown skin; but that of the moth is the true 

 pupa skin, whereas that of the fly is not a pupal skin 

 at all, but the last larval skin, and corresponds to the 

 thin, crumpled, and collapsed skin that is left at the 

 tail end of the moth's chrysalis. The true pupa skin 

 of the fly is to be found inside, in the shape of a thin 

 membrane in which the contained pupa is enveloped. 

 Such a pupa is described as coarctate, and the brown 

 skin is called the puparium. 



When the fly is ready to issue from the pupa-case, it 

 is found to be furnished with a large membranous pro- 

 tuberance on the head between the eyes. This is the 

 outer surface of a hollow sac, which is capable of great 

 distension, and is used in pushing off the. top of the 

 barrel-shaped pupa case, which separates along the line 

 of one of the circular grooves. By muscular contractions 

 the fluids of the body are forced into this sac, distending 

 it, and causing it in turn to press against and ultimately 

 force off the lid, thus liberating the fly. In Fig. 65 the 



