16 THE STABLE FLY. 
of ensilage, particularly when mixed with straw, as is often the case 
when the bottom of a silo is cleaned out. Experimentally, a few 
specimens have been reared from pure cow manure, but this substance 
seems to be unattractive to the adult and not favorable for the breed- 
ing of the larve on account of its very compact texture. A Russian 
investigator, Prof. Portchinski, has made the statement that the larvee 
have been found in the leaves of growing plants. This, however, must 
be avery rare occurrence. ‘This species has never been found breeding 
in human excrement and does not frequent malodorous places, which 
are so attractive to the house fly. Hence it is much less likely to 
carry typhoid and other germs which may be found in such places. 
The development of this insect is somewhat slower than that of the 
house fly, and it is therefore more essential, in order that it may breed 
out successfully, that the eggs be deposited in rather large accumu- 
lations of material. The larve are very sensitive to drought and 
soon succumb if the material in which they are breeding is not kept 
rather moist. 
HABITS OF THE ADULT. 
Both the male and female of this species feed on the blood of ani- 
mals. They appear to discover their host mainly by sight and 
usually, especially on cattle, pass quickly to the lower portion of the 
legs (fig. 6), particularly on the outside, where the hair is somewhat 
shorter than on other parts of the animal and where they are less 
likely to be struck by the tail of the host. When the flies are very 
abundant their attack is by no means confined to the legs, as both 
cattle and horses have been seen practically covered with flies on all 
parts of the body. They seldom remain on the host long without 
inserting the beak. Before blood is extracted they are easily dis- 
turbed and often move about several times before settling down for 
final engorgement. After the beak is well inserted and the blood 
begins to flow they usually become engorged in from two to five min- 
utes. During feeding the abdomen becomes greatly distended (see 
fig. 5) and often of a distinctly reddish color. When the appetite of 
the fly has been satisfied it withdraws its beak and flies rather slug- 
gishly to some near-by object, where it rests while digesting its meal. 
During this process numerous drops of clear liquid excrement are 
voided. This also takes place while the fly is feeding. The inser- 
tion of the beak is accompanied by a rather severe, sharp pain. 
This accounts for much of the worriment caused to the host by this 
species. After blood extraction has begun little or no pain is felt. 
When the proboscis is withdrawn a drop of blood usually exudes from 
the wound. Numerous small flies have been seen to frequent the 
blood which exudes in this way, and it is not improbable that the 
screw-worm fly may deposit its eggs on these spots and thus cause 
infestation of the host with these maggots. 
540 
