1940] Wing Pads of Pteronarcys 119 
II. Mesothoracic wing pad, x294, second instar, left side. Preeostal 
sinusoid and a middle membrane present. No lacuna or 
trachea developed. 
III. Prothoracic flange, x294, second instar, left side. False lacuna 
present but a middle membrane not developed. 
IV. Mesothoracic wing pad, x294, late growth phase second instar, 
left side. Costal lacuna is present. 
V. Prothoracic flange, x294, same specimen as no. IV, left side. 
Blood sinusoid present. 
VI. Prothoracic flange, x294, early growth phase third instar, right 
side. Two blood sinusoids are present. Middle membrane is 
not developed. 
VII. Mesothoracic wing pad, x206, same specimen as no. VI, right side. 
Subcosta has formed. 
VIII. Mesothoracic wing pad, x290, fourth instar, left side. Three 
lacunse are visible. 
Key to Abbreviations 
BC Blood cells 
C — Costa 
FL False lacuna 
12, 13, 14 Integument second instar, etc. 
M _ Middle membrane 
P Preeostal sinusoid 
SC Subcosta 
R Radius 
T Trachea 
WD Wing disc 
