1913] Wing Veins of MembracidcE 93 



from the point of its coalescence outward; R2 turns upward to 

 meet Sc plus Rr, R3 turns outward and downward and coalesces 

 with the tip of R4 ; R4 turns upward to unite with R3 ; R5 contin- 

 ues outward to touch the end of Mi; Mi bends upward to R5; 

 M2 turns backward to unite with the tip of M3; M3 continues 

 forward to meet M2; M4 also turns backward to meet the 

 tracheae of Cui+2 and 1st A which have continued outward, 

 and the tips of the other trachese have proceeded distad in their 

 natural position, extending to points which enable them to 

 coalesce with the tracheae ahead. 



In this way a strong marginal vein has been formed along 

 the lines laid down by these trachcce which is as strong and 

 sometimes stronger than the longitudinal veins themselves, 

 since it contains at various places in its course, the tips of two, 

 three, and sometimes even four tracheae. 



Variation 



This study would be incomplete if some mention were not 

 made, in the consideration of the fore wing, of the variations 

 which often occur. The venation which has been outlined has 

 been in the main that of the normal structure. Considerable 

 variation occurs, however, often within a species, and this 

 deserves some mention. 



The wing of Thelia bimacidata has been chosen as an illus- 

 tration because this species shows perhaps the greatest range 

 of variation found in any one species. In the diagram shown 

 (Fig. 24) the dotted lines represent the maximum variation, and 

 all stages between the normal and this maximum may be found. 

 It will be noted that Ri sometimes leaves radius two-plus-three 

 at a point very close to the fork of radial sector. This would 

 represent a less specialized condition than the normal. R4+5 

 and Mi+2 sometimes approach each other with a wide curve 

 and barely touch, instead of coalescing in the usual manner, 

 and this does away with the sharp bend of M1+2. M3 and M4 

 are occasionally separate, forming an additional cell M3. As 

 might be supposed from the discussion of the cross-veins, that 

 one between media and cubitus shows the greatest irregulari- 

 ties. It ranges from the most proximal position shown by the 

 dotted lines at the left, to one very close to the point at which 

 media branches, and in some cases even disappears altogether, 

 media and cubitus bending toward and touching each other. 



