1929 ] 
Tracheal System of Pyrausta nubilalis 
333 
center of the trachea at approximately a right angle. The 
projecting arm thus formed is somewhat larger than the 
other arm and swells slightly toward its free end. The 
lever is chitinous. Between the apex of the lever and the 
end of the closing bow opposite to that to which the lever 
is attached is a third chitinous piece. This is the closing 
hand. 
The contraction of a muscle extending between the free 
arm of the lever and the closing band causes the lever to 
move. The attachment of the lever to the closing bow be- 
comes the fulcrum, and the action of the muscle pulls the 
apex of the angle of the lever inward, causing the space 
between the lever and the closing bow to become constricted. 
The closing band is so attached to the apex of the angle 
that the end of the band is depressed inward, causing the 
space between it and the closing bow to be constricted. 
The trachea is thus pinched together. 
This structure is cast with the exuvia when molting takes 
place. 
Kranchner (1880), who first described the tracheal clos- 
ing apparatus of insects, expresses the opinion that it 
originates as a local thickening of the tsenidium. 
The description of the spiracles and closing apparatus is 
taken from a former paper, (Crowell, 1926). 
The Longitudinal Tracheae 
In the embryo the tracheal system arises as ectodermal 
invaginations which grow deeper, and from which branches 
are sent out. One running forward fuses with one run- 
ning backward from the next anterior spiracle, and one 
running backward fuses with one running forward from 
the next posterior spiracle. Thus, longitudinal connections 
are formed between the spiracles, and these connections 
form the main longitudinal trachese. In the thorax there 
is a longitudinal system on each side dorsal to the main 
system. It is formed by the joining of dorsally running 
branches. 
The longitudinal trachese lie on each side in an area be- 
tween the small dorso-recti muscles and the great ventro- 
