— 7 6- 



Note. — The student should have a clear idea of the significance of 

 the terms generalized and specialized, which are now much used in 

 biology. Generalized indicates a primitive condition, a nearness to 

 ancestral forms. Thus the most generalized member of a group (as 

 a family or an order) is that member which most clearly resembles 

 the ancient progenitor of that group. Specialized, on the other hand, 

 indicates remoteness from the primitive t} pe, an adaptation to more 

 special conditions of existence. Thus the most specialized member 

 'of a group is the one that departs most widely from the ancient pro- 

 genitor of that group. 



These terms are used in a comparative sense ; thus, a highly special- 

 ized form may be regarded as generalized when compared with forms 

 that are still more highly specialized. 



A TYPICAL WING. 



The flies of the genus Rhyphus afford good examples of 

 comparatively generalized wings. By studying the accom- 

 panying figure (Fig. 2) of one of these, the student can 



Fig. 2. — Wing of Rhyphus. 



gain a good idea of the type of the wings of insects belong- 

 ing to the order Dipt era, 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, longitudinal veins, those 

 that normally extend proximo-distad ; and second, cross- 

 veins, those that normally extend more or less nearly 

 cephalo-caudad. In Figure 2, three of the cross- veins are 



