LIFE HISTORY 23 



reduced to the minimum and do not produce soluble 

 ferments in appreciable quantity; third, that the larvae 

 accelerate putrefaction of bodies by assisting in the 

 increase of microbes; fourth, that the larvae nourish 

 themselves at the expense of the products of germ 

 chemistry — the germs can develop rapidly and spread 

 in all directions only by the assistance of the larvae; 

 there exists between these two agents of putrefaction 

 a true symbiosis. These conclusions, although reached 

 by a study of two species of the genus Lucilia, are un- 

 doubtedly applicable to the larvae of other flies feeding 

 in animal material. 



The Pupa and Pupariitm 



Before beginning its transformation to the pupa, 

 the full-grown larva empties its alimentary canal, con- 

 tracts from its own skin, the skin itself forming a 

 nearly cylindrical pupal case, the posterior portion be- 

 ing slightly larger in diameter than the anterior and 

 both ends being equally rounded. It is then about six 

 mm. in length and of the shape shown at figure ii. 

 At first this pupal skin remains pale yellowish, but 

 rapidly changes to red and finally to a dark chestnut 

 color. The insect inside loses its tracheal system, which 

 is withdrawn by the surrounding skin and eventually 

 remains inside of the skin or pupa shell but outside of 

 the insect itself. The insect rapidly assumes a true pupal 

 shape, and at the end of thirty hours, according to 

 Doctor Hewitt, most of the parts of the future fly can 

 be distinguished, although they are sheathed in a pro- 



