89 



one or vem R^, and the second division, radius-two or vein 

 R^, and so on till all are numbered. 



Note. — In the most generalized flies known to us, the radius is five- 

 branched. But in most flies some of the branches of this vein coalesce so 

 that the number of apparent branches is less than five. In Rhyphus veins 

 R-x and Rz coalesce so as to appear as a single vein. In order to indicate 

 that this apparently simple vein is composed of two veins, and in order that 

 homologous veins in different insects shall bear the same designation, this 

 compound vein is termed radius-two-plus-three or vein R^ + 3. In the same 

 way, what appears to be the third branch of the radius in Rhyphus is really 

 the fourth and fifth coalesced, and is, therefore, designated as radius-four- 

 plus-five or vein Rj^^ 5, The tracing, out of the homologies of the branches 

 of veins is often very difficult ; but it is of the greatest importance in deter- 

 mining the relationships of different genera or of families. 



Names of the longitudinal wing-veins. — There have 

 been many different sets of names applied to the veins of 

 wings. Not only have the students of each order of insects 

 had a peculiar nomenclature, but in many cases different 

 writers on the same order have used different sets of terms. 

 This condition of affairs was incident to the beginning of the 

 science, the period before the correspondence of the veins in 

 the different orders had been worked out. But now the time 

 has come when it is practicable to apply a uniform nomencla- 

 ture to the longitudinal wing-veins of all orders ; and the 

 following set of terms has been proposed for that purpose : 



Costa. — The vein extending along the cephalic or costal 

 margin of the wing is the costa. 



Subcosta. — Immediately caudad of the costa and extending 

 parallel with it, is a vein, which is usually simple in flies ; this 

 is the subcosta (Fig. 2, Sc). 



Radius. — Immediately caudad of the subcosta there is a 

 vein which in generalized insects is always branched ; this is 

 the radius. In Rhyphus, the radius is three-branched (Fig. 2, 

 R,, ^.^3, and ^, + 3). 



