404 



THE WINGS OF INSECTS 



accessory veins may be increased in an irregular manner by interpolation, 

 i. e. by the splitting of various members of a series of accessory veins. 



Illustrations of the adding of accessory veins distally are to be seen in 

 Corydalus and allied genera. The presence of fine twigs at the tip of 

 trachea R2 in the pupal wing of Corydalus (Fig. 419) indicates the method of 



increase, which is doubtless 

 as follows: the branches 

 have been added one after 

 another to the tip of tra- 

 chea R2, there being a 

 migration of the base of 

 each accessory trachea 

 towards the base of the 

 wing, thus making room 

 for the addition of new 

 branches. In this case 

 the first accessory vein is 

 the proximal one. It is 

 the oldest accessory branch 

 of the radial sector that is 

 lettered R2a in Figure 418. 

 The successive branches 

 are Rob, R2C, R2d. etc. 



In the wing of a n^Tnph 

 of a cockroach represented 

 by Figure 420 there are 

 many accessory tracheae 

 branching from the front 

 side of the radial trachea. 

 From the presence of the 

 fine twigs near the apex of 

 the wing, it is evident that 

 accessory tracheae are being 

 added distally. It is also 

 evident that the number 

 of accessory tracheae is 

 being increased by the splitting of some of these accessory tracheae, i. e. by 

 ainterpoltion. In cases of this kind it is impracticable to designate the 

 accessory veins individually. 



The pectinate type of radial sector. — The order Neuroptera differs from 

 all other orders of living insects in the fact that except in a few cases the 

 radial sector has been so modified that it is of the form known as pectinate 

 or comb-like; that is, it consists of a supporting stem upon which are 



Fig. 421. — Diagrams of several types of radius. 



