﻿60 BULLETIN 1369, U. S. DEPARTMENT OF AGRICXJLTUEE 



surface, where they could be seen distinctly. Not only the time of 

 molting was observed in this way but the changes in color of the 

 last stage could be observed and the approach of complete develop- 

 ment noted and emergence watched for. 



In order to eliminate any possible adverse effect ^n the develop- 

 ment of the larvae by these repeated observations, the appearance in 

 the subdermal tissues of several hundred third-stage larvse was deter- 

 mined by the method indicated, following which they were not dis- 

 turbed except to make occasional notes on their presence until about 

 the date of emergence from the host. The average time for the 

 development in the case of those larvse examined for molts, etc., 

 was practically the same, however, as in the case of those which 

 were not disturbed. 



As a method of checking up on the observations on the various 

 larvae it was found best to make a rough diagram of the backs of the 

 hosts, indicating the position of each larva and assigning it a num- 

 ber. In order to facilitate the location of each larva the position 

 on the animal's back was indicated by clipping the hair. In the 

 early observations the hair was clipped close to the skin immediately 

 over the warble hole. Later, as it was thought that the development 

 might be interfered with by exposure, clips were made in the hair 

 of the host just above or below the position of the warble. 



The minimum period of development of larvae of H. lineatwrrb in 

 the backs of cattle at Dallas, Tex., based on about 200 records, was 

 between 35 and 47 days. Another period noted was 38 to 40 days, 

 and still another 39 to 46 days. In several other cases the period 

 ranged between 39 and 50 days. The maximum period was between 

 78 and 89 days. The average total developmental period, based on 

 104 warbles examined at frequent intervals, was 56.3 days. A few 

 representative records of the development in the backs of cattle are 

 given in Table 7. 



The average period of development of 222 other larvae of H. 

 lineatum in which the date of appearance under the hide and the 

 date of emergence from the host were determined without disturb- 

 ing the larvae by making observations on molts was 57.74 days. 



At Uvalde, Tex., D. C. Parman recorded the duration of the 

 larval period in the backs of cattle for larvae of H. lineatum. The 

 minimum period noted was 43 to 45 days, the maximum 54 days, 

 and the average 49.4 days. 



Six accurate records of the period of development of H. hovis 

 in the backs of cows were made in New York. The maximum was 

 77 days, the minimum 65 days, and the average 72.8 days. 



Apparently the duration of development in the backs of cattle 

 is not influenced by individual, breed, or age of the host. The 

 writers' records indicate that the development may be slightly 

 shorter at Uvalde, Tex., than at Dallas, and that the time spent in 

 the subdermal tissues of cattle averages slightly longer in the case 

 of larvae coming up to the backs in the early part of the season. 

 For example, the average period of those which appeared in the 

 subdermal tissue in October and early November was about 58 

 days, whereas in those which appeared during the latter part of 

 December and in January it was about 51 days. 



