THE WINGS OF PLECOPTERA 



255 



A more striking example of specialization by reduction is afforded by 

 certain forms in which the hind wings are so reduced in size that they are 

 smaller than the fore wings and have fewer veins. An excellent illustration 

 of this is the common Chloroperla cydippe (Fig. 253). In this species the 

 radial sector of the fore wing is nearly reduced to an unbranched condition 

 and in the hind wing this reduction is complete. The most remarkable 

 feature, however, is the extreme reduction of the anal area of the hind wing, 

 which results in the hind wing being much smaller than the fore wing. 



Sci 



Fig. 253. — Wings of Chloroperla cydippe. 



It is unnecessary to cite other examples of specialization by reduction, 

 as they will be easily recognized when met. 



Specialization by addition. — A few examples of specialization by addi- 

 tion are illustrated by the accompanying figures. The most convenient 

 method of discussing these examples is to treat each of the principal veins 

 of the wing separately. 



The subcosta. — In many genera there are in addition to the very con- 

 stant humeral cross-vein other veins that extend from the subcosta to the 

 costa. These are commonly termed cross- veins; but as they are preceded 

 by tracheae and the true cross-veins are not preceded by tracheae in this 

 order, they are morphologically accessory veins. These accessory sub- 

 costal veins are present in the wings of Isogenus (Fig. 245); and the 

 presence of a trachea in each was clearly evident in a pupal wing of Acro- 

 neuria, which I studied. 



The radius-one. — Accessory veins, closely resembling those described in 

 the preceding paragraph, extend from radius-one to the costal margin of 

 the wing in some genera. These are also shown in Figure 245. 



