1 6 DURHAM DIPTERA. 



(C.II.) A small number above the line indicates a section of 

 a long vein, or, if connected with a cross-vein or cell, 

 it indicates the ist, 2nd, or 3rd of its kind, numbering 

 from the base of the wing. 



The longitudinal veins are numbered 1 to 7. 

 The cross veins bear the number of the vein on 

 which they rest, i.e., regarding the fore margin as the 

 top of the wing. 



V.I. is read "The first vein" or shorter "V one." 

 V.4.b. is read "The lower branch of the 4th vein." 

 0.2.b. is read "The lower 2nd cell." 

 X.5 2 . is read "The 2nd cross-vein on the 5th 

 vein"; and so on, but it is far better to say simply 

 " cross five two," as that is shorter, and the mental 

 picture is developed in the proper order, first the 

 kind of vein, then its general position, and then its 

 exact place. 



The cells bear the number of the long-vein upon 



which they rest, that is the one which bounds them 



on their lower side. If cross-veins be present, and 



therefore more than one cell along the upper side of 



a vein, they are distinguished by the addition of 



" inner" " middle" or " outer" or of a small figure 



\ 2 , or 3 , thus 0.5 1 . or 0.5. In. is the first or inner cell 



resting on the fifth vein. They are said to be open 



when they are bounded at their outer end by the 



costa or the margin of the wing. On Plate V, 0.2. is 



open in 19, closed in 20. 



(2) How to Read a Fly's Wing. — First notice that there 



are two principal vein-systems. Each arises from 



the base of the wing in a single main stem, which 



throws out one or more branches. These systems 



are quite unconnected with one another, except 



at one place, where a small cross-vein unites 



them. (Sometimes they are also united near the 



tip of the wing by the bending up of the fourth 



vein). This cross-vein is the key of the position. 



