334 
Psyche 
[December 
recti muscles. In this area there are no longitudinal 
muscles, but it is crossed by sterno-tergal muscles. 
Lying in front of all of the spiracles, except those in the 
prothorax, is a sterno-tergal muscle, and the longitudinal 
trachea except in the mesothorax and in front of the first 
abdominal spiracle, passes between these muscles and the 
body wall. In the prothorax the sterno-tergal muscle lies 
behind the spiracle. In the metathorax a large branch from 
the main trachea passes beneath this muscle. In the meso- 
thorax the muscle is present and a trachea arising in a 
manner similar to the branch in the metathorax passes 
between it and the body wall. Thus in each body segment 
through which the main trachea runs it is held to the body 
wall by a muscle passing over it. (Fig. 4. The trachea is 
much longer in proportion to its thickness than here in- 
dicated) . 
According to De Guyse (1926) the primitive position of 
the spiracle located on the prothorax was in the membrane 
between the prothorax and mesothorax, and it has moved 
ahead to its present position. That would account for the 
fact that the sterno-tergal muscle lies behind the spiracle 
rather than in front of it, as in the abdomen. 
DeGuyse (1926) explains the positions of the muscles in 
front of the first abdominal spiracle, and in the mesothorax, 
from a consideration of the larvae of Stkenopis (Hepialidae) , 
as follows: 
The thoracic spiracles are situated at a much lower level 
than those of the abdomen. In the abdomen the anterior 
lateral branch arising from the short spiracular trachea 
passes under the sterno-tergal muscle and anastomoses with 
the posterior lateral branch from the next preceding seg- 
ment. The anterior lateral branch from the first abdominal 
spiracle passes as usual under the muscle, but since the 
second thoracic spiracle is at a much lower level, its pos- 
terior lateral branch fails to meet the anterior lateral 
branch from the abdomnial spiracle. The latter loses it- 
self among the dorsal muscles. At the same time the lower 
visceral branch is found greatly enlarged, and anastomosed 
with a posterior branch of the first thoracic spiracle. The 
