THE WINGS OF INSECTS 389 



Figure 410 represents the wing of an adult fly of the genus Rhyphus. 

 This wing is comparatively generalized, but in several respects it depa-ts 

 from the primitive t>'pe; the radius has been reduced to a three-branched 

 condition; the media is also reduced to a three-branched condition; only 

 vestiges of the first and third anal veins remain ; these are represented by 

 dotted lines; and a part of the stem of media is atrophied. 



By studying a wing of Rhyphus and the accompanying figure (Fig. 410) 

 the student can gain a good idea of the type of the wings of insects bslonging 

 to the order Diptera, and have a standard with which to compare wings of 

 insects of other orders. 



Longitudinal veins and cross-veins. — The veins can be grouped under 

 two heads: first, loigiiudmal veins, those that normally extend lengthwise 

 the wing ; and second, cross-veins, those that noTTnally extend more or less 

 nearly transversely. In Figure 410, three of the cross- veins are indicated 

 by arrows, near the middle of the wing; two other cross-veins are repre- 

 sented near the base of the wing. All other veins represented in this figure 

 are longitudinal veins. 



The insertion of the word normally in the above definitions is impartaat; 

 for it is only in comparatively generalized wings that the direction of a vein 

 can be depended upon for determining to which of these two classes it 

 belongs. A little later the student will study wings in which the direction 

 of some of the longitudinal veins has been so modified in the course of 

 specialization that they extend transversely {i. e., cephalo-caudad), and 

 some cross- veins extend in a longitudinal direction {i. e., proximo-distad) . 



Simple veins and branched veins. — Veins are either simple or branched. 

 The veins lettered Sc and 2d A in Figure 410 are simple veins; between these 

 there are three branched veins. 



In the case of branched veins the entire vein, including all of its branches 

 is often referred to as a single vein. Thus the third vein in the wing 

 Rhyphus, counting the thickened, cephalic margin of the wing as the first 

 vein, is termed the radius or vein R; and by this expression we include both 

 the main stem of the vein and its three divisions. On the other hand, each 

 division of a branched vein is oiten termed a vein. Tiius tlu first division 

 of the radius, counting from the cephalic margin of the wing, is termsd 

 radius-one or Vein Ri, and the second division, radius-twj or vsin Ri, and 

 so on till all are numbered. 



Note. — In the most generalized flies known to us, the radius is five-branched. 

 But in most flies some of the branches of this vein coalesce so that the number of apparent 

 branches is less than live. In Rhypus veins R^ and Ry coalesce so as to appear as a 

 single vein. In order to indicate that this apparently simple vein is composed of two 

 veins, and in order that homologous veins in different insects shall bear the same designation, 

 this compound vein is termed radius-two- plus-three or vein R0+3. In the same way, 

 what appears to be the third branch of the radius in Rhyphus is really the fourth and 

 fifth coalesced, and is, therefore, designated as radius-four- plus-five or vein R[+a. The 

 tracing out of the homologies of the branches of veins is often ver^" difficult; but it is of 

 the greatest importance in determining the relationships of different genera or of families. 



