76 THE GENERAL FEATURES OF WINGS 



nated as IMia, IMib, and IMic, respectively, beginning with the oldest, 

 i. e. the longest one. It will rarely be necessary, however, to resort to this 

 somewhat cumbersome terminology; as it will be siifficient in most cases 

 to indicate the number of distinct intercalary veins in a given area. 



The Adventitious veins. — -In certain insects there occur secondary veins 

 that are neither accessory veins nor intercalary veins as defined above; 

 these may be tenned adventitious veins. The more important of them are 

 described in later chapters in the course of the descriptions of the wings in 

 which they occur. Examples of them are the supplements of the wings of 

 certain Odonata and the spurious vein of the Syrphidse. 



The anastomosis of veins. — The typical arrangement of the wing-veins 

 is often modified by an anastomosis of adjacent veins; that is, two veins 

 will come together at some point more or less remote from their extremities 

 and merge into one for a greater or less distance, while their extremities 

 remain separate. This is illustrated by Nemoura (Fig. 12) where tracheae 



; 2345 e 7 



Fig. 63. — Diagram illustrating the formation of regular cross-veins 

 (After Needham). 



Sc2 and Ri extend for a short distance in the same vein cavity. In the fore 

 wing of Prionoxystus (Fig. 62) there is an anastomosis of veins R3 and 



R4+.^- 



The cross-veins. — It has been shown that so far as the principal 

 longitudinal veins are concerned the wings of all orders of insects are 

 modifications of a single primitive type, and a diagram representing our 

 conclusions regarding this type has been given on an earlier page (Fig. 6). 



In the discussion of our hypothetical type no reference is made to cross- 

 veins ; for it is evident that a homology, similar to that which exists between 

 the longitudinal veins of the several orders of insects does not exist between 

 the cross-veins of these orders. 



It is shown in Chapter IV that in the wings of the more generalized 

 members of the Pateodictyoptera, the order of fossil insects from which the 

 orders of recent insects were evolved, there were no definite cross-veins, 

 but instead an irregular networic of veins in the spaces between the longi- 

 tudinal veins (see Fig. 8). A similar network exists to-day in the fore 

 wings of the Acrididac, for example. 



The transformation of an irregular network of veins into regular 

 transverse cross-veins is discussed by Needham ('03) and is illustrated by 

 the accompanying diagram (Fig. 63). His conclusions were based on his 



