27 



strengthened above by an enlarged knob on the costa. This cross- 

 vein will be termed the nodal cross-vein (nc). The sudden bend of 

 E is a mechanical device correlated with the shifting of the nodal 

 cross-vein from an originally perpendicular to an oblique position. 

 The costal knob rests upon the curved fused part of Sc with C, and 

 the result appears to be a rather loose joint. 



Fig. 4. 



M,. 



1-2 



The Nodus in Mesophlebia. 



The nodus of recent dragon-flies (text-fig. 5 G ) differs from that of 

 Mesophlebia in two important respects. Firstly, the joint is a much 

 more complete one, the proximal and distal parts of the nodus being 

 more closely fused. Secondly, the lower half of the nodal cross-vein 

 has become bent at an. obtuse angle to the rest, and is continued 

 downwards between R and M 1-2 by a thickened oblique vein called 

 the subnodus (Sn), which in its turn is usually continued directly 

 by M2. In Mesophlebia, M2 leaves Ml-2 a considerable distance 

 distad from the level of the nodus, and the subnodus is quite absent. 



Fig. 5. 



R 



M.. 



N 



R 



M.-* 



C 



R. 



C 



Sc 



M.- 



R 

 Mi- 



N 



X 



3 Mi " 6 



Six stages in the Phylogeny of the Nodus. 



'C 

 Mi-i 



