I 



96 University of California PuUications. [Entomology 



relation of the veins they denominate cubital and. anal, which 

 has evideiitly influenced their conception of the method of the 

 development of the triangle. If the ontogenetic evidence 

 afforded by the tracheae at this point is reliable, the trunk of the 

 first posterior vein (their Cu) should be marked Cu plus A, 

 since both tracheae occupy the cavity of the vein Cu, and the 

 main trunk of the second posterior designated as the recurrent 

 fourth of the anal, as the trachea indicates in their figure 61 

 of the hind wing of Cordulegaster. In an insect like this, 

 where the apical portion of the second posterior (Cu of Com- 

 stook and Needham) has come to lie so as to be almost a direct 

 continuation of the first posterior (their Cu), it is not strange 

 that the relatively strong trachea of the latter vein should 

 send a brancli to occupy the cavity of the second posterior, 

 even to the exclusion of the one that it originally contained. 

 Indeed, a similar shifting of trachea? occurs at the nodus, as 

 was first tigured by Brongniart. The interpretation of this sug- 

 gested by Comstock and Needham (p. 904), that we here have 

 a case of actual crossing of veins during nyniphal stages, is 

 impossible. 



But to return to the posterior veins; it must be l)orne in 

 mind that we are dealing with structures in a region that is 

 constantly liable to suppression, because a little narrowing of 

 the wing would involve the disappearance of parts of these 

 veins. Examples of this reduction are to be seen in many 

 Anisoptern, and an extreme case in Lais, where both posteriors 

 are involved. 



■ The development of the triangle probably occurred in insects 

 in which this region was of rather more than usual importance 

 in the process of flight. It will be recalled that the character 

 of the stroke as to speed and path determines the relative im- 

 portance of the different parts of the wing. We can suppose 

 that the triangle first appeared in the wing of a primitive 

 Odonatid, in which there was an increasing functional signifi- 

 cance in the anal area; a wing with a venation resembling that 

 of a Calopterygid, which is not far removed from the Prot- 

 odonata (Fig. 27). The first step in the process was a more 

 abrupt bending backward of the posteriors, which occurs below 

 the arculus, to meet the increasing demands of the anal area. 

 In Archilestes (Fig. 30) we see, in spite of the narrowing of 

 the wing, a similar position of the posteriors. Now if the wing 



