. 
54 BULLETIN 1369, U. S. DEPARTMENT OF AGRICULTURE 
immediately after the larve reach it, and never more than three 
days later. Usually the first indication of a break in the skin is 
in the nature of a minute, irregular hole which is usually accom-- 
panied by sight bleeding. Within a day or two after the first break 
is made the edges of the hole become more smooth and round. In 
several instances the writers have observed a circular plug of skin 
about 2 millimeters in diameter and bearing hair, seated in a freshly 
cut hole. In two cases these plugs were still attached at one side as 
though the cutting process was not complete. 
The only evidence to indicate which end of the larva does the 
cutting is that in every case where larve have been extracted the 
posterior end is always outward. This has been found even before 
the cutting of the hole was completed. It is conceivable that the 
larve may do the major part of the cutting with the mouth parts 
and then turn around and complete the enlarging of the hole with 
the posterior end. When these early stages are removed by pres- 
sure they appear to be in a position perpendicular to the skin rather 
than lying horizontally under it as is the normal position in sub- 
sequent stages. 
DEVELOPMENT OF THIRD-STAGE LARVZ 
The first molt under the skin, from the third to the fourth stage, 
takes place soon after the hole is completed. The determina- 
tion of this period with exactness is very difficult, but the records 
of the writers show that the molt in H.-lineatum may take place in 
about 24 hours after the hole is completed, or the stage may last 
from 6 to 8 days. The average period, based on 17 records made 
at Dallas, Tex., with considerable exactness, is 4.5 days, and the © 
average period based on 13 records made in New York is 4.26 © 
days. The period from the cutting of the hole in the skin to the 
molt in H. bovis, as observed in New York, ranged from 1 to 6 
days with an average of 3.35 days. 
For several hours prior to the molt the spiracles of the fourth 
stage can be seen beneath the skin of the larve. The exuvia are 
very delicate, but seem to be shed almost in their entirety at one 
time. The break in the larval skin is very irregular. The posterior 
ends of the tracheal tube where they join the spiracles are rather 
highly chitinized and they are shed along with the skin. _ 
The writers’ observations show that not infrequently the third- 
stage larva may move to a new location under the skin, usually 
within a few inches of the first, and cut a second hole or even a 
third hole before it molts. 
ENCYSTMENT AND DEVELOPMENT OF FOURTH-STAGE LARVZ 
It appears that about simultaneously with the molt to the fourth 
stage the tissues of the host change their form of resistance from 
an active leucocytosis to a segregation of the insects by the forma- 
tion of encystment sacs (fig. 26). With the development of these 
pouches the general inflammation of the connective tissue subsides. 
The skin of the third stage remains in the sac, which is formed 
very rapidly, and gradually the walls of the sac become thicker as 
the growth of the larva continues. During this and all subse- 
quent stages the larva remains with its posterior spiracles toward 
