440 
Journal of Agricultural Research 
Vol. XXI, No. 7 
The anterior spiracles located above and on either side of the mouth 
are described in detail by Carpenter and Pollard (j). These organs, in 
so far as they have been studied, do not lend sufficient definite char¬ 
acters to be of any material value in differentiating the two species. 
In order to arrive at an interpretation of the structure of the posterior 
stigmal plates, so far as is necessary in the present paper, it will be best 
to consider the last or fifth larval instar. 
If a transverse section of the stigmal plate is viewed with a medium- 
power microscope it will be seen that the plate is composed of three 
layers which may be designated as the external, the middle, and the 
internal. The internal part, often visible in newly molted specimens 
viewed from the external surface with a deep focus, is composed essen¬ 
tially of a series of irregular, chitinous, plate-like structures which vary 
greatly in outline but are fairly constant in number for each species. 
These radiate from the tracheal chamber. The middle layer is composed 
of a series of tubes or stems that arise from the internal plate-like struc¬ 
tures and terminate at the surface in a disk, the margin of which is most 
heavily chitinized, giving a ring-like appearance. This is surrounded 
by supporting tissues. Surface views of the plate often show the separa¬ 
tion and even the form of the internal series of structures by light sutures 
dividing these disks or ring-like structures into sections similar to those 
in the internal layer, but not so clearly defined. 
From surface views of the plates they naturally vary according to 
the size and number of the disks or ring-like structures, and it is this 
variation which is so useful in differentiating the larvae of the fourth 
instar. 
Brauer (/, p. 125) mentions only three larval stages of H. bovis. The 
first stage he regards as unknown and calls the two large spiny stages 
appearing in the backs of cattle the second and third. In 1888, 
Hinrichsen (6), a veterinarian in Husum in South Jutland, published an 
article in which he related his findings of another stage of Hypoderma 
larvae in the spinal cavity of cattle. This is the first mention made of 
what had so far been considered as the first-stage larva or the stage 
which Brauer regarded as unknown. 
In 1890, Cooper Curtice (4), of the Bureau of Animal Industry, 
United States Department of Agriculture, first noted the appearance of 
a similar stage of a Hypoderma larva which he terms H. bovis, appear¬ 
ing in the esophageal walls, under the pleura near the eleventh rib and 
in the subcutaneous tissue of the backs of cattle. He speaks of this 
stage as the first-stage larva. This stage was unarmed with spines and 
unlike the two larger armed stages described by Brauer in the backs of 
cattle. This was really not the first-stage larva, as C. V. Riley (9), 
of the Bureau of Entomology, in 1891, first described the real first-stage 
larva of H . lineatum which he obtained from the egg just before hatch- 
