No. 384.] THE WINGS OF INSECTS. 909 
aries unite strongly the three posterior longitudinal veins, only 
its inner side is bounded by a principal vein, its anterior and 
outer sides being formed from two cross-veins approximated 
upon vein M4. Primitively it differed little from an ordinary 
rectangular cell. The accompanying diagram (Fig. 65) shows 
the successive positions assumed by its anterior and inner sides 
kal 
Fic. 66. — Di illustrati procession of the triangle, and the deflection of the second 
Cu-A cross-vein in the fore wings of Libellulidæ. a, the first, and 4, the second Cu-A 
cross-veins ; 1, 2, 3, and 4, successive positions. 
Fic. 67. — Diagram deme, the recession of the triangle in the hind wings of the 
Libellulide. 1, 2, 3, and 4, successive stages. 
and by the two branches of the cubitus at their departure from 
it. This epitome of its history presents only steps in its de- 
velopment that are still preserved in the wings of living genera. 
In the Libellulidæ differentiation between fore and hind 
wing has changed the relation between arculus, triangle, and 
anal vein. Doubtless these were once similarly placed in the 
two wings, the triangle being a little beyond the arculus, and 
the anal vein meeting its hind angle in both wings (as, for 
