THE CATTLE GRUBS OR OX WARBLES | 55 
the hole in the skin, usually keeping them just a short distance be- 
low the surface. During the course of development of the fourth 
stage the hole may be completely closed by 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. bovis the length of the fourth stage was observed with 
fair accuracy in 18 specimens in New York. The minimum period 
among these was 21+ days, the maximum period 35 to 88 days, 
and the average 27.1 days (see Table 5). 
TABLE 5.—Duration of fourth stage of Hypoderma bovis in New York 
Tonalit Larvee molted to Larve molted to Duration of 
Caan, fourth stage fifth stage fourth stage 
1921 1921 
Days 
Leia ahaa Ysy pee gOS cot a ee I ea ee ee ee May 10 (before) ______- JUIN Q eles a See 22+ 
TD XO) is Sep a SR Oa a AGN gel (0 Ogee Sa a eae aa AP OUONES 7c Se 28+ 
EU) Cp eS ms one avn cr aNR ae EE  Wa 50) ge AIR ea JUNC OMe we esc. 30+ 
TD Yo) ee Satna ee OL bial Eels orate She pen ee May 15 (before) _._____|_____ GOMES eed 25+ 
BUD) (Peace settee Det TL Rk Se Belay Lzdge stele Seas We ay ea Omer eee owe 23 
MD) Oras pes Se ae ae ES ANS ESE May 19 (before) _______|____- COTE ek, 21+ 
BI) Re ieee Oe ee ea Tene ate MGT May 24 (before) ______- Afb aVe\ Types eae Gee 24-- 
DD) OR Be eh aaa Nats OU eee AE alte 2 May 25 (before) ______- UDO LS excess 24+ 
IS) Cee eu pee om ena sErGsc 22 ae Oa = Saya June 20 shel 26+ 
EID) (0 ae ata ce th al ae I ike a BV E29 ee aloe Mises MUM Dee eee 29 
UD 0) Aa eM Nee OR a Neco ta Pc June 2 (before) _______- JUDIE OD yh teen ere east 23-- 
[DY teeta 8 eo A ol rar ce PE 2 Wa ls ARI ak pn PS OR ORS ak ne UU iy SNA eee ae 31+ 
1 Yop ks lp Min a ae pata ne tebe a abe Seema June 6 (before) ______..- delay TU aos ee 35+ 
1922 1922 
VET ale to wien eo et ce SN cA Telit Oe ak May 8 to9_______- 35 to 38 
ID) ORAL ak HE ERAN TN a eae kaa! Ve ING Oe te HO) Ge MMLERY 1 HO Sees S 26 to 28 
LD (opie Ges oes ar OE a 2 ee Rr rs De ADE. 15 COM 222s e== 22s) IME US OR oS 28 to 31 
5 YY NT ge aa EC es ea Apr lO o20 ee May 14t015_____-_ 24 to 26 
VO 0 Sa eeeenee ts sees eae re aerate yeaa pa pe aaa ISS Ob Fy PPA HO) PH a oe yee May 20 to 24_-___-_ 26 to 32 
The molting of the fourth instar is apparently preceded by a 
short period of comparative inactivity. The fifth-stage spiracles 
appear as pale yellow objects deep beneath the integument two or 
three days before the molt takes place. As the time for molting 
approaches the spiracles become more distinct in outline and just 
