308 THE WINGS OF HYMENOPTERA 



branches of the radial sector arise from a common stem ; and the tips of all 

 of them have moved away from the apex of the wing, veins R2 and R3 

 following the costal margin of the wing, and veins R4 and R5 following 

 vein Ml. 



Reference has been made above to the curving of vein Ri away from the 

 margin of the wing to make room for a stigma (Fig. 383, si). The stigma is 

 bounded in front and at its base by the tip of vein Sc2, which anastomoses 

 with the radius for a considerable distance but separates from that vein at 

 the base of the stigma. 



In the above account reference has been made only to the venation of 

 the fore wings. In the hind wings of most Hymenoptera the venation has 

 been so greatly reduced that it is exceedingly difficult to determine the 

 homologies of the wing-veins. 



The suborder Chalastogastra includes those Hymenoptera in which the 

 reduction of the venation of the hind wings is least. The extended study 

 of the wings of this suborder made by MacGillivray ('06) enabled him to 

 prepare a diagram of a typical hymenopterous hind wing, which is of great 

 value as an aid to the determination of the venation of the hind wings of 

 Hymenoptera, this diagram was followed in the preparation of Plate X. 

 The lacking veins are indicated by dotted lines. 



From the foregoing account it will be seen that even in the most general- 

 ized of living Hymenoptera there exists a highly modified wing-venation. 

 But notwithstanding this it is possible to determine the homologies of all 

 of the wing-veins. 



Some of the modifications of this primitive hymenopterous type will be 

 discussed in a later section of this chapter. 



[c) THE TRACHEATION OF THE WINGS OF THE HYMENOPTERA 



The tracheation of the wings of the Hymenoptera was studied by Com- 

 stock and Needham in order to ascertain if the courses of the tracheas fur- 

 nish any data regarding the homologies of the wing-veins in this order. We 

 had found, as shown in preceding pages, that in the more generalized insects 

 there is a close correlation between the venation and the tracheation of the 

 wings; and that it can be accepted as a firmly estabhshed fact that the 

 courses of the wing-veins of primitive insects were determined by the courses 

 of preexisting tracheae. 



But we also found that in the Trichoptera there is little correlation 

 between the venation and the tracheation of the wings, a remarkable reduc- 

 tion of the wing-trachese having taken place. A similar reduction of the 

 tracheae of the wings exists in most families of the Di]^tera; and even when 

 a large portion of the tracheae are retained, as in certain Asilids, they 

 afford little aid in determining the homologies of the wing veins. 



