THE WINGS OF INSECTS 



395 



IDENTIFICATION OF THE \VIXG-VEINS AND OF THE CELLS OF THE WINGS IN 



DIFTERA 



The Diptera constitute a hit^hly specialized order of insec ts in which only 

 one pair of wings has been presen'ed as organs of flight, and in most cases 

 the venation of the remaining pair of wings exhibits striking variations from 

 the primitive type. But in some members of the order the venation of the 

 wings is nearly typical; and, as varying degrees of departure from the 

 typical form exist, by studying a carefully selected series of wings one can 

 obtain a knowledge of the manner in which the more striking variations 

 from the ]:)rimiti\'e t}'pe have been evolved. 



In the wings of Protoplasa fitchii, one of the most generalized members 

 of the order Diptera, the variations from the typical form are comparatively 

 slight (Fig. 415). 



Note that while the costal trachea in the wings of nymphs and pupas 

 extends parallel with the costal margin of the wing, but at some distance 

 from it, as shown in Figure 409, in the wings of adults the costal vein coin- 

 cides with the costal margin of the wing (Fig. 415, C). 



In Protoplasa fitchii the subcosta is forked and the radius is five- 

 branched; these are two generalized features rarely found in the Diptera, 

 usually the subcosta is not forked and the number of the branches of the 

 radius is reduced. 



In all Diptera known to me except Protoplasa fitchii, media is only three- 

 branched. In lettering his drawings, the student may omit any reference 

 to vein M4, as is done in the figure of the wing of Rhyphus above. 



2d A Cu, 



Fig. 415. — A wing of Protoplasa fitchii. 



In most Diptera the first anal vein has atrophied ; but there is usually a 

 vestige of it, which is a suture-like line parallel with \-ein Cu; this is kno^^^l 

 as the anal furrow; it is represented in the figure of the wing of Rhypluis 

 by a dotted line. In Rhyphus there is also a vestige of the third anal vein, 

 represented in the figure by a dotted line. 



Directions for the study of wings. — Make a drawing of the wing, based 

 upon a carefvil stud)- 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; ink the lines representing media 



