July i, 1921 
Larval Stages of H. bovis and II. lineatum 
445 
While the presence or absence of spines on the posterior border of the 
tenth ventral segment is a good distinguishing character of the fiftil-stage 
larva, it is much simpler to distinguish the two species by the shape 
and structure of the posterior stigmal plates. The flat or level surface 
of the plate of H. lineatum (fig. 5) compared with the distinctly cup¬ 
shaped or funnel-form surface of the plate of II. bovis (fig. 6) is so well 
marked that the two species can be identified with the naked eye at 
sight. If the hair surrounding a warble hole through the skin of an 
animal is parted and a slight pressure is applied to the skin so as to 
bring the anal segment of the warble near the surface of the skin it can 
be viewed through the hole, and a glance is sufficient for determination of 
the species without extracting it from the host. The size of the stigmal 
plate of H . lineatum is from 0.837 to 1.07 mm. and that of H . bovis 
from 0.971 to 1.27 mm. at its greatest diameter and does not change 
in size with either species throughout this instar. The structure of the 
stigmal plates of H. lineatum is so formed that the light-colored space 
between the inward-curving borders around the tracheal opening of the 
Fig. 5 .— Hypoderma lineatum: Posterior stigmal Fig. 6—Hypoderma bovis: Posterior stigmal plates of 
plates of fifth-stage larva. Greatly enlarged. fifth-stage larva. Greatly enlarged. 
respiratory area is usually more than twice as great as the corresponding 
area in H. bovis. This difference is especially noticeable in the younger 
or lighter-colored specimens. As the larva of the fifth stage grows to 
maturity and darkens from a light or cream color to black the color 
of the stigmal plate also changes uniformly, and the age of the specimen 
can thereby be fairly accurately determined while the grub is still in 
the host. When the larva has just molted to the fifth instar the color 
of the plate is light orange throughout. As the larva advances in 
age the border of the plate turns to a dark brown. A little later the 
entire plate turns dark brown, and finally just prior to emergence the 
plate darkens to black. In a series of experiments with various lar- 
vicides, conducted at Dallas, Tex., the results of which are to be pub¬ 
lished later, the color changes of the stigmal plate were very useful in 
determining the age of the larvae in the host and the effects of the larvi- 
cide on specimens of certain ages. With the dark brown or black larva 
the finer structure of the stigmal plate is no longer discernible with the 
naked eye, but the flat surface plate of H. lineatum and the cup-shaped 
