120 STEPS IN SPECIALIZATION 



(d) THE LONGITUDINAL VEINS 



(i.) The number and branching of the longitudinal veins as repre- 

 sented in the hypothetical primitive type of Comstock and Needham, with 

 the addition, near the margin of the wing, of accessory veins, which were 

 inconstant in niimber and position. 



From this primitive type of wing-venation three quite distinct lines of 

 specialization arose, as indicated under 2, 3, and 4, below. 



(2.) The number of the wing-veins increased by the addition of 

 accessory veins. 



(3.) The niunVjer of the wing-veins increased by the addition of inter- 

 calary veins. 



(4.) The number of the wing- veins reduced by the coalescence of veins, 

 and also, in many cases, by the atrophy of veins. 



Note. — In some cases the method of specialization of the anal area of 

 the wings is quite different from that of the preanal area. Thus, in the 

 series of orders in which the characteristic method of specialization of the 

 preanal area is by a reduction in the number of veins there are forms in 

 which the anal area is expanded and the number of veins is increased. 



TABLE OF THE METHODS OF SPECIALIZATION OF THE WINGS CHARACTERISTIC 

 OF THE ORDERS OF INSECTS 



This table is merely the result of an effort to indicate the more striking 

 of the methods of specialization of the wings characteristic of each of the 

 orders of insects. It is not a key for determining the orders of insects. It 

 is not available for this purpose; because, in many cases, the wings of an 

 insect do not show the type of specialization characteristic of the order to 

 which the insect belongs. Thus, for example, while the most characteristic 

 modification of the courses of the wing-veins in the Diptera and Hymenop- 

 tera is due to the coalescence of veins proceeding from the margin of the 

 wing towards the base of the wing, there is no indication of this type of 

 coalescence of veins in some of the nematocerous Diptera. 



A. Wings specialized by the development of superntmierary veins in the 

 preanal area. 

 B. Supernumerary veins of the accessory type. 

 C. Wings developed externally. 



D. Wings retained throughout life. Wings without a striking 

 contrast in the thickness of the veins of the anterior part of the 



wing and those of the middle portion Orthoptera 



DD. Wings deciduous, there being near the base of each wing a 

 transverse suture along which the wing is broken off after the 

 swarming flight. Wings with the veins of the anterior part of the 



