370 THE WINGS OF HYMENOPTERA 



between the tracheation and venation of the wings in the Hymenoptera. 

 An examination of the wings of young pupae of this insect revealed the fact 

 that in this insect the laying out of the wing-venation precedes the trachea- 

 tion of the wing. After the wing- veins reach that stage of development in 

 which they appear as pale bands, the tracheae grow out from the base of the 

 wing into them. Figure 390 represents the wings of a pupa taken at a stage 

 that illustrates this pushing out of the tracheae into the previously formed 

 wing-vein cavities. 



It is obvious that tracheae developed after the vein-cavities are formed 

 will follow the paths offering the least resistance to their progress ; and it is 

 not to be expected that they will preserve their primitive arrangement 

 under these conditions. 



This brings us to the conclusion already announced, that in deteiTnining 

 the homologies of the wing-veins in the Hymenoptera we are forced to base 

 our conclusions on a study of the veins themselves; and that a method of 

 study which is of the highest importance in determining the homologies 

 of the wing-veins in many other insects, is of little use here for this s]3ecial 

 purpose. 



id) METHODS OF MODIFICATION OF THE PRIMITIVE HYMENOPTEROUS TYPE 

 OF WING-VENATION 



The working out of the details of the modification of the wing-venation 

 in the several families of the Hymenoptera can only be done by those who 

 make extended studies of these families, a work that does not fall within the 

 scope of this essay. I wish merely to indicate, in this chapter, the essential 

 features of the hymenopterous type of wing- venation, the more prominent 

 of the methods by which this type is modified, and to illustrate by a few 

 examples the nature of the results that have been attained in the modifica- 

 tions of the primitive hymenopterous type. If this is done, data will be 

 available that will facilitate more extended studies in this field. 



Our knowledge of the various modifications of the primitive hymenop- 

 terous type of wing-venation that occur in the more generalized of the two 

 suborders of this order, the Chalastogastra, is very complete. This is due 

 to the labors of Dr. A. D. MacGillivray, who has published the results of a 

 very extended and detailed study, illustrated by nearly one hundred figures 

 of the wings of members of this suborder (MacGillivray '06). This most 

 excellent work, with its great wealth of details, furnishes all the data needed 

 for an understanding of the venation of the wings of the Chalastogastra, and 

 gives an invaluable basis of comparison for the study of the wings of the 

 more specialized suborder, the Clistogastra. 



I shall not attempt to give an abstract of Dr. MacGillivray's paper; 

 because a mere abstract could not take the place of the complete memoir, 



