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when the more primitive or generalized members of different 

 orders are compared with each other. In most of the orders 

 of insects the greater number of species have become so 

 modified or specialized as regards the structure of their wings 

 that it is difficult at first to trace out the primitive type. 



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 type, 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 progenitor of that group. 



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

 form may be regarded as generalized when compared with forms that are 

 still more highly specialized. 



The specimens indicated for the student to study in the 

 following part of this course have been selected with care to 



Fig. 2.— Wing of Rhyphus. 



illustrate gradually increasing degrees of divergence from 

 the primitive type. In the case of each order studied, the 

 work begins with a comparatively generalized form, and 

 passes step by step to those that are more specialized. 



The flies of the genus Rhyphus afford good examples of 

 comparatively generalized wings. By studying a wing of 

 one of these flies and the accompanying figure (Fig. 2) the 



