1940] 
Wing Pads of Pteronarcys 
115 
opening is very small. The costal lacunae of the right fore, 
right hind, left fore and left hind wing pads measure 168[x, 
162 |a, 160[jl and 14(V respectively. 
With the exception of the right hind wing, the single 
trachea to the wing disappears in the dorsal epithelium of 
the bud, and does not approach costa or form a lacuna. In 
the right metathoracic flange, however, the trachea is worthy 
of note for its exceptional behavior. The trachea travels 
into the costa of this wing, the only instance where it does so 
in twenty-eight wing pads of early instars studied by serial 
section. 60pi from the site of apposition of the trachea to 
the dorsal wing epithelium, and 18\l past the anterior end 
of costa, the trachea enters the lacuna. In the next 42 jjl, 
the trachea moves across the lacuna into the ventral epithe- 
lium where it may be followed for 22[x to its end. As the 
trachea enters costa 18^ away from the lacunal opening it is 
obvious that the lacuna is not caused by a boring in of the 
trachea, nor does the diameter of the lacuna taper to con- 
form to the tapering trachea. 
In each segment of the pterothorax a nerve leaves the in- 
terganglionic connective at a point immediately anterior to 
the ganglion. The nerve associates itself with the wing 
trachea and enters with it into the dorsal wing epithelium. 
The nerve could not be traced further. 
Because tracheae are often considered to be inductors of 
lacunae, it is well to digress upon the anatomy of the tracheal 
system. In the second instar a trachea extends to the wing 
for the first time. (It is the more anterior of the two 
branches which supply the wing of older nymphs. The pos- 
terior branch does not appear during the late third instar.) 
A large longitudinal trachea runs laterad of the alimentary 
tract, and in the wing bearing segments the longitudinal 
trachea receives perpendicularly an equally large branch 
from the anterior gill. From the junction of these large 
tracheae to the corresponding junction in the succeeding seg- 
ment runs a smaller trachea which arches out peripherally 
from the main longitudinal trachea. From the peripheral 
arch branches extend to the wing, leg, and posterior gill. 
In the late third instar a trachea extends for the first time 
to the posterior portion of the wing flange. This is the state 
of pterothoracic main tracheae during the second, third, and 
fourth instars. 
