﻿THE CATTLE GEUBS OR OX WARBLES 



55 



the hole in the skin, usually keeping them just a short distance be- 

 loTv the surface. During the course of development of the fourth 

 stage the hole may be completely closed hj scabs, but an effort 

 is always made to keep it open. This frequently results in the 

 formation of what the writers have called " a perforated plug.'' 

 This apparently consists of secretions which harden within the hole, 

 becoming somewhat free from its edges, and the larva maintains a 

 minute hole through the center of the mass. 



During the development of the fourth stage the body is nearly 

 horizontal, the dorsal side outward, and the posterior end bent 

 upward so that the spiracles are close to the aperture in the skin. 



The duration of the fourth stage of H. lineatum at Dallas, Tex., 

 has been observed in one instance to be from 5 to slightly more than 

 13 days, and in another from 10 to 13 days. It is thought, how- 

 ever, that the minimum periods mentioned are possibly erroneous, 

 owing to the chance of failure to record the molt to the fourth 

 stage with accuracy, and that 15 days is nearer a true minimum for 

 this stage. The maximum period recorded in one instance was be- 

 tween 44 and 53 days. The average duration of this stage based 

 upon 21 individuals as recorded at Dallas was 24.5 days. The 

 duration of this stage in the backs of cattle in New York is prac- 

 tically the same as in Texas. Some of the records for this stage are 

 given in Table 7. 



In H. hovis the length of the fourth stage was observed with 

 fair accuracy in 18 specimens in New York. The minimum period 

 among these was 21 -f- days, the maximum period 35 to 38 days, 

 and the average 27.1 days (see Table 5). 



Tablk 5. — Duration of fourth stage of Hypoderma bovis in New York 



Locality 



Larvae molted to 

 fourth stage 



Larvse molted to 

 fifth stage 



Duration of 

 fourth stage 



Herklmer- 

 Do... 

 Do... 

 Do... 

 Do... 

 Do... 

 Do... 

 Do... 

 Do... 

 Do... 

 Do... 

 Do... 

 Do... 



Middletown. 



Do 



Do 



Do 



Do 



1921 



May 10 (before) . 



do 



do 



May 15 (before). 



May 17 -.- 



May 19 (before). 

 May 24 (before). 

 May 25 (before). 



do. 



May 29 



June 2 (before).. 



do 



June 6 (before).. 



June l._ 

 June T.- 

 June 9-_ 



do.. 



do.. 



do_. 



June 17- 

 June 18- 

 June 20- 

 June27- 

 June 25- 

 July3.. 

 July 11., 



1922 



Apr. 1 to 3... 

 Apr. 8 to 9... 

 Apr. 15 to 17. 

 Apr. 19 to 20. 

 Apr. 22 to 24. 



1922 



May 8to9... 

 May 5 to 6... 

 May 15 to 16. 

 May 14tol6. 

 May 20 to 24. 



Days 



22+ 



28+ 



30+ 



25+ 



23 



21 + 



24+ 



24+ 



26+ 



29 



23+ 



31+ 



35+ 



35 to 38 

 26 to 28 

 28 to 31 

 24 to 26 

 26 to 32 



The molting of the fourth instar is apparently preceded by a 

 short period of compjirative ina(;tivity. The fifth-stage sjiiracles 

 appear as pale yellow objects deep l)eneath (lie integiiincnt two or 

 three days before the moU takes place. As the time for molting 

 approaches the spiracles become more distinct in outline and just 



