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 Puparium 
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 n. 
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is withdrawn by the surrounding skin and eventually 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- 
