— 86 — 



(lettered 2d I). In this wing also cell III is divided into 1st 

 III and 2d III ; and cell V2, into 1st V2, and 2d V2. 



Margins of wings. — An insect's wing is more or less 

 triangular in outline ; it, therefore, presents three margins; 

 the costal margin (Fig. 7, a-b) ; the outer margin (Fig. 7, 

 b-c), and the inner -margin (Fig. y, c-d). 



Angles of wings. — The angle at the base of the costal 

 margin (Fig. 7, a) is the humeral a?igle ; that between the 

 costal margin and the outer margin (Fig. 7, b) is the apex 

 of the wing ; and the angle between the outer margin and 

 the inner margin (Fig. 7, c) is the anal angle. 



THE WING- VEINS OF DIPTERA. 



As the chief object of the following work is to give the 

 student training in tracing the homologies of wing- veins, 

 comparatively little information will be given directly. The 

 student will be furnished in each case with the wing 

 to be studied, and his studies should take the following 

 course : — 



Directions for the study of wings. — Make a drawing of 

 the wing, based upon a careful study of it with a compound 

 microscope, using a low power. The drawing should be 

 first made with a pencil ; after it has been criticised by the 

 teacher, the lines should be inked. Make the drawing 011 a 

 sufficiently large scale so that each vein can be represented 

 distinctly ; in most cases the drawings should be somewhat 

 larger than Figures 7 and 8. 



Number each vein and cell of the wing. 



Write a description of the wing, noting the more important 

 features of its venation, and especially the more important 

 departures from the primitive type of the order as indicated 

 by the generalized form first studied. In the Diptera the 

 wing of Rhyphus (Fig. 2) may be used as a generalized 

 type, although in certain respects other wings will be found 

 to be more generalized. 



