1940] 
Wing Pads of Pteronarcys 
117 
lacuna is divided into three smaller lacunse by cell processes 
forming* partions. As the wing flange broadens posteriorly, 
the three lacunse become separated in space by added cell 
processes which form a typical middle membrane. 48^ pos- 
terior to the first appearance of the lacuna, the trachea be- 
comes associated with the most mesal lacuna. Posterior to 
this point the innermost lacuna bifurcates. 120p. from the 
opening of costa and 120pi from the posterior margin of the 
wing pad, the trachea goes deeply into the ventral epithelium 
and tapers to an end. 
If one numbers the lacunse inward from the margin, num- 
ber three appears to be the new structure, and is therefore 
probably radius. The subcosta in the early growth phase 
third instar runs midway between costa and the hsemoccele, 
and sends a short branch to the anterior margin of the wing 
near its apex. The second lacuna in the late growth phase 
third instar has the same position and structure as it did in 
the early growth phase, whereas the new (third) lacuna runs 
next to the hsemoccele and forks at mid-length of the wing. 
The left metathoracic wing has the same anatomy with minor 
differences. The two remaining wings were compressed lo- 
cally when they were sectioned so that one cannot follow the 
trachea. The three lacunse, however, are readily identified. 
Four additional lacunse, each about 50p. in length, make 
their initial appearance at the apex of the wing pad. Further 
study is necessary to determine their identity. 
Fourth Instar 
Head width, .88 mm. Four specimens were sectioned seri- 
ally and examined. The anatomy of the wing pad is the 
same as that of the late growth phase third instar. 
General Consideration and Conclusions: Before one is 
able to characterize vein lacunse, one has to study blood la- 
cunse in the prothorax. When the costa is formed in the 
second instar, a blood lacuna develops in the prothoracic 
flange and like the costa runs in an arc to communicate an- 
teriorly and posteriorly with the hsemocoele. Similarly, at 
the time of differentiation of subcosta, the prothoracic flange 
adds another lacuna which is entad of and parallels the first. 
(Later instars show that several more lacunse are added.) 
Prothoracic flange epithelia do not oppose their basement 
