252 THE HOUSE FLY—DISEASE CARRIER 
apples and the refuse of cider mills and fermenting 
vats of grape pomace. It is a rather rapid breeder, 
and a generation may develop in twenty days, more or 
less. It is attracted especially to preserves and canned 
goods, and frequently damages raspberry vinegar. It 
is often very difficult to prevent the fly from entering 
fruit jars. 
There are about thirty species of Drosophila in 
North America, and the majority of them breed in 
the juices of decaying and fermenting fruit. Aside 
from the one just mentioned, D. amoena, D. funebris, 
D. graminum, and D. transversa are occasionally found 
in houses. Another species, D. cellaris, occurs in cel¬ 
lars in fermenting liquids, such as wine, cider, vinegar, 
and beer; also in decaying potatoes. Another species 
damages flour paste; and still another mustard pickles. 
One species, D. Haveola, does not need decaying vege¬ 
tation for its larval food, since its larvae mine the leaves 
of cabbages and radishes. 
The fruit flies may be dangerous inhabitants of 
houses, since they are nearly all attracted to excreta, 
and some of them breed in human excrement. The 
larva and puparium, as well as the adult fly of D. 
ampelopliila, are shown in Fig. 27. 
The Bluebottle or Greenbottle Flies 
The Blow Flies ( Calliphora erythrocephala Meig., 
Lucilia cccsar L., Phormia terramoves Desv.) 
Several species of bluebottle or greenbottle flies oc¬ 
casionally gain entrance to houses, and all are danger- 
