62 John He7iry Com stock 



addition. The extreme result of this method of specialization 

 is presented by the Orthoptera and especially by the hind wings 

 of the Acrididse. Here we find a widely expanded anal area, 

 with regularly alternating concave and convex veins. Such a 

 wing is not fitted for striking vigorous and rapid blows upon the 

 air as is required for rapid flight ; but is adapted to a sliding 

 flight, a sliding up like a kite or down like a parachute. Such 

 a method of flight would naturally reach its highest develop- 

 ment in jumping insects, like the Acrididae. 



A study of the illustrations just given shows that where the 

 tendency of natural selection is towards the development of a 

 rapid flight there is usually a cephalization of the function of 

 flight, i. e., the hind wings are greatly reduced, and the fore 

 wings become the chief organs of flight. This is well shown 

 by the more specialized Zygaenids (Fig. ii) ; and the extreme 

 of such a cephalization is presented by the Diptera. That 

 such a cephalization is not absolutely necessary to rapid flight 

 is shown by the dragon-flies (Odonata) ; but here the abdo- 

 men is greatly elongated, which gives a similar result. 



On the other hand where an expansion of an anal area has 

 taken place in order to provide for a sliding flight, it is the 

 hind wings that are specialized by addition, z. e., the opposite 

 of cephalization takes place. The Acrididse have already 

 been cited as an illustration of this. 



The region lying between the subcostal and anal areas is 

 traversed by three principal veins and their branches. The.se 

 veins as already indicated, are the radius, viedia and cubitus, 

 the radius lying next to the subcosta, the cubitus next to the 

 anal area, and the media, between the radius and cubitus. 

 Very remarkable modifications take place in the structure of 

 these veins and in their relation to each other. Some of the 

 modifications will be discussed in detail later ; in this place I 

 wish only to make some very general statements. 



If a large series of wings be examined it will be found that 

 the area of each of these veins may be specialized either by 

 addition or by reduction, i. e., it may be either widened or 

 narrowed. When the tendency of natural selection is to 

 widen one of these areas, the points of origin of the branches 



