﻿THE CATTLE GRUBS OR OX WARBLES 75 



fore February 1 or after April 15. The maximum abimdance usually 

 occurs between February 20 and March 15. In the plateau region 

 of southwestern Texas in certain instances larvse have been found to 

 mature early enough in the fall to permit of the issuance of a con- 

 siderable number of heel flies in the fall. It has not been possible 

 to determine if this really takes place, but the finding of a young 

 larva in the gullet of a cow on January 4 is fairly conclusive proof 

 that some eggs are deposited in October or November. It is certain 

 that some heel flies emerge and attack cattle during December, and 

 heel-flj^ activity has been observed on numerous occasions in Jan- 

 uary. The season of adult activity in New Mexico, Arizona, and 

 California appears to be about the same as observed at Dallas. 

 Throughout the central belt of States^ — that is, between the latitudes 

 35 and 45° N. — the period of activity is approximately between 

 March 15 and May 1, and in the northern tier of States between 

 April 1 and June 30. These dates are only approximate, and vary 

 much during different j-ears, as well as with altitude and local 

 conditions. 



In New York adults of H. Uneatum may begin oviposition as early 

 as the middle of April, are most abundant during the first three 

 weeks of May, and cease activity about the end of June. In the 

 territory where H. hovis occurs it is difficult to determine the date of 

 cessation of activity of the adults of H. lineatuni^ since their seasons 

 of activity overlap. 



In New York considerable numbers of adults of H. hovis begin to 

 o^dposit during the first week in June and apparently some emerge 

 during the latter part of May. The flies are very annoying to cattle 

 during June and the first half of July, are less so during the re- 

 mainder of July and the first half of August, and during the re- 

 mainder of August and up to the middle of September only strag- 

 glers are abroad. Since normal larvse have been found in the backs 

 of cattle in New York after September 1, some fly activity is possible 

 throughout that month. No flies have been seen, however, nor have 

 the effects of their attack been noted, after September 14. 



Hadwen {36) states that at Agassiz, British Columbia, adults of 

 H. hovis appear in the early part of June and continue up to the be- 

 ginning of August, and that adults of H. Uneatum< were out from 

 April 15 to April 24. Carpenter (19) records the emergence of 

 adults of H. hovis in June and July, and deposition of eggs of H. 

 lineaturro on May 8 and June 16, at Athenry, Ireland. 



The season of oviposition of both species is about coincident with 

 that of adult emergence. 



SEASONAL DEVELOPMENT OF LARV^ 



Rather abundant data regarding the seasonal occurrence of larvse 

 of //. lineatum^ in the gullets and viscera of cattle are presented on 

 page 47 and Tables 3 and 4. In general, approxinuitely two and one- 

 half months elapse between the deposition of eggs and the earliest 

 appearance of larvae in tlie gullets. 



In the vicinity of Dallas, 'i'cx., the larva? first appear in the sub- 

 mucosa of the gullets of cattle at about the end of March and con- 

 tinue to enter that organ until the maximum is reached during Sep- 



