232 



THE WINGS OF ODONATA 



On the other hand it is suggested by Tillyard that "forms Hke Diphlebia, 

 which show a pecuhar obhque \-ein under vein Mi far distad from the 

 subnodus, need also a thorough investigation ; since in such cases the vein, 



x4 at present taken to be M2 



V I — I , I y^ -Mi may eventually prove to be 

 V , 1 . 1 -I — U — XX/?, none other than R3 itself." 



Returning to the Anisop- 

 tera, one more interesting 

 modification of the radial 



Fig. 233. — A detail from the region of the nodus of 



Petalura gigantea, showing the bridge and two 



obhque veins, o' and o (From Needham '03). 



sector will be noted. In the wings of some dragon-flies, there exist two 

 oblique veins; this is well-shown in the wings of Petalura gigantea (Fig. 



233)- 



The existence of two oblique veins is doubtless due to a division of the 



radial sector trachea into two branches one of which follows the course of 



trachea M2 for a greater distance than the other This condition was 



observed by Needham in the wing of a nymph of Cordulegaster diastatops 



(Fig. 234) ; but in the wing of the adult of this species only one oblique vein 



is recognizable ; evidently the second has become transverse as is the case 



with the oblique vein in Lestes rectangularis (Fig. 232). 



In connection with this division of the radial sector trachea, there arises 

 a problem, the solution of which may be of considerable taxonomic impor- 

 tance. Is this condition a primitive one or has it been acquired? If it be 

 a retention of a primitive condition, the second oblique vein is vein R2+3 

 and the first is R4+5; but even in this case the two branches coalesce so 

 that in its distal part vein Rg is unbranched. 



The intercalary veins. — A result of the development of the fan-lilce type 

 of wing in this order is the development of a greater or less ntmiber of 

 secondary longitudinal veins. Many of these veins appear to be of the 

 accessory vein type ; but there is no doubt that they arose as intercalary 

 veins, and became attached directly to the principal veins secondarily. 



All of the stages in the devel- 

 opment of an intercalary vein 

 can be seen by studying the dis- 

 tal end of a few wings of mem- 

 bers of this order. As a result 

 of the formation of folds in the 

 wing there is a marked tendency 

 for the zigzag lines of cross- 

 veins between adjacent rows of 

 cells to become straightened 

 along the crest of a fold or along the bottom of a furrow, and thus an 

 intercalary vein is formed. The proximal end of such a vein is usually 

 connected bv cross-veins to both of the two veins between which it is 



Fig. 234. — Tracheation of the nodal region of 



a wing of a nymph of Cordulegaster 



diastatops (From Needham '03). 



