16 BULLETIN 597, U. S. DEPARTMENT OF AGRICULTURE. 
The other two species appear at Aberdeen, S. Dak., about the same 
time as G. haemorrhoidalis, but are most abundant just before a 
killing frost. The @. haemorrhoidalis are least abundant at this 
time, and are present in greater numbers during the early half of the 
season. After a killing frost one seldom finds a Gastrophilus except 
when warm temperatures prevail during a few days. 
In the “ nose-fly ” district one must bear in mind that the period 
during which flies oviposit is that when farmers are most busy, and 
the most favorable time for fly ovipositions is when the weather is 
most favorable for working horses. The adults appear during the 
plowing of corn and sorghum, and the annoyance continues during 
the mowing of hay, the harvesting and thrashing of grain, and the 
marketing of farm products. 
GASTROPHILUS HAEMORRHOIDALIS (Linnaeus). 
SYNONYMY. 
Oestrus haemorrhoidalis Linnaeus, 1761. 
Gastrophilus haemorrhoidalis Leach, 1817. 
Gastrus haemorrhoidalis Meigen, 1824. 
ATTACHMENT IN RECTUM AND DROPPING OF LARVZ. 
During the early spring and summer the fourth-stage larve are 
normally concealed within the rectum, where lesions have been noted 
in post-mortem examinations. Later they effect a temporary attach- 
ment to the margin of the anus, where they become accustomed to the 
air temperatures, assume a greenish color, and apparently increase 
their motility. Larvee, normally exposed to the air at the rectum, 
after dropping seem to conceal themselves at grass roots so as to be 
protected from the heat of the sun much more easily than those 
unexposed at the margin of the anus. When a larva is attached 
so that only the posterior end is exposed at the rectum one will find 
the anterior end a pinkish color, while the posterior will be greenish. 
Larvee exposed at the rectum have been observed for the length of 
time they remain attached, and the shortest period was slightly more 
than 40 hours, while the longest was 71 hours. The heat of the sun 
for a few minutes was sufficient to cause larve to drop when an 
attempt was made to photograph a larval attachment at the anus. 
At various times during the day larvee appear at the anus, as many 
as 13 sometimes being visible at one time. The larve are likely to 
drop under most any condition, but do not drop with manure, as is 
supposed. When manure is dropped during their attachment they 
seem to use more effort in clinging and are only pushed aside during 
its passage. (See Plate I, figure at upper right.) 
PREPUPATION AND PUPAL PERIODS. 
In the normal pupation of a larva which has been exposed at the 
margin of the anus there is a gradual change from greenish to yel- 
