THE GENERAL FEATURES OF WINGS 67 



reduction has taken place; it may be due either to the atrophy of one or 

 more veins, or it may be due to the coalescence of veins. In either case if 

 the manner of reduction be determined the homology of the remaining vein 

 or veins can be indicated. 



But in some cases, as for example, in the Odonata, there has been a 

 specialization b}' reduction, which has resulted in the preservation of a 

 single anal vein, and there is available no data showing the course of this 

 evolution. The three anal veins may have coalesced into one: or one anal 

 vein may ha\'e atrophied and the remaining two coalesced; or two of the 

 veins may have been lost, leaving a single vein. Obviously, in this state of 

 uncertainty, it is best to designate the single vein as the anal vein (A) without 

 any predicate. 



When a single anal vein is present the identity of which can not be 

 determined and this vein is branched, we are not warranted in designating 

 these branches to make use of the names istA, 2d A, and 3d A, as this would 

 indicate homologies that we do not know to exist. Therefore in such cases 

 the branches of vein A are designated as .4i, A2, A3, etc. 



The reduction of the number of wing-veins. — In many wings the num- 

 ber of the veins is less than it is in the hypothetical type. In some cases 

 this is due to the fact that one or more veins have faded out in the course of 

 the evolution of the insects showing this deficienc}^; frequently in such 

 insects vestiges of the lacking veins remain, either as faint lines in the 

 positions formerly occupied by the veins or as short fragments of the veins. 

 A much more common way in which the niimber of veins has been reduced 

 is by the coalescence of adjacent veins. In many wings the basal parts of 

 two or more prinicpal veins are united so as to appear as a single vein; 

 and the number of the branches of a vein has been reduced in very many 

 cases by two or more branches becoming united throughout their entire 

 length . 



The evolutionary process by which the number of the veins has been 

 reduced, whether by the fading out of veins or by the coalescence of adja- 

 cent veins, is termed specialization by reduction. 



The coalescence of principal veins is usually made evident by a study of 

 their branches, the branches of two or more veins arising from a common 

 stem. The coalescence of the branches of a vein is often shown, by a study 

 of a series of allied forms in which different degrees of it can be observed, 

 the point of separation of two branches being nearer and nearer to the 

 margin of the wing in successive forms until the margin is reached. 



As the venation of the wings affords some of the most available charac- 

 teristics for use in the classification of insects, it is of the utmost importance 

 .that the same name should be applied to homologous veins in the wings of 

 different insects. For this reason when a vein consists of two or more of 

 the primitive veins united, the name applied to the compound veins should 



