16 BULLETIN 597, TJ. S. DEPARTMENT OP 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 G. 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-ffy " 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 LARVAE. 



During the early spring and summer the fourth-stage larvae 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. Larvae, 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. 



Larvae 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 larvae to drop when an 

 attempt was made to photograph a larval attachment at the anus. 

 At various times during the day larvae appear at the anus, as many 

 as 13 sometimes being visible at one time. The larvae 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- 



