1876.] The House Fly. 479 
| body contracts into a barrel-shaped form, as seen in Figure 28, 
| D, turns brown and hard, forming a case (puparium) within 
which the body of the larva transforms into that of the pupa. 
| Weismann has made the discovery that in the larval flesh fly when 
about to transform into the pupa state, the head and thoracic 
‘segments die, and that the head and thorax of the pupa arise 
from minute disks attached to the smaller nerves or traches in 
the body of the worm. This is paralleled by the metamorphosis 
: of the “ pluteus ” into the adult starfish, and is a much more com- 
| plete metamorphosis than even that of the caterpillar into the 
chrysalis of the butterfly. 
Our house fly having as a maggot lived a life of squalor, im- 
mersed in its revolting food, with its new change of form, involv- 
ing the death of one half its body and the origin of a new head 
and thorax, with legs and wings, eyes, feelers, and mouth-parts; 
after a short pupal sleep of from five to seven days pushes off 
one end of its pupa case, and appears winged, with legs where 
before there were no traces of feet, and is animated by new in- 
stincts and mental traits. It is difficult to realize how striking 
are the changes, physical and psychological, which the house fly 
undergoes in the transition from the maggot to the volant, curso- 
rial being which puts a girdle, like Puck, around its little world, 
— the dining-room or parlor, — and like its mischievous proto- 
type plays all sorts of antics, tasting the sugar, lapping the 
molasses, now tickling the nose of the sleeping housewife resting 
from her pre-prandial toils, or adjourning to the library and scrap- 
mg with its spiny tongue the rich binding of the bookworm’s 
treasures, 
If in its Winged condition it is one of the most disagreeable 
features of dog-days, and people wonder why flies were ever 
made at all, it should be remembered that flies have an infancy 
a maggots, and the loathsome life they then lead as scavengers 
cleanses and purifies the August air, and lowers the death-rate 
of our cities and towns. Thus, while stables and piggeries and 
filth are tolerated by city and town authorities, the young of the 
house fly and the flesh and blow fly, with their thousand allies, are 
doing Something towards purifying the pestilential air, and avert- 
mg the summer brood of cholera, dysentery, diphtheria, typhus 
and typhoid fever, which descend like harpies upon the devoted 
wns and cities, It may be regarded as an axiom that where 
flies most abound there filth, death-dealing and baneful, is most 
abundant, and filth-diseases such as we have named most do con- 
Sregate, 
