50 Uninersity of California PuUications. [Entomology 



Now, in the area in front of the second posterior vein, although 

 there is no vein traversing it, the band is interrupted, in many 

 cases at least, in just such a way as it would be if a vein were 

 present. In this particular case students are pretty well 

 agreed that this really represents a lacking vein. 



Folds undoubtedly have a close relation to veins. If com- 

 parisons be made between two closely allied insects in the 

 wing of one of which there is one less vein than in the other, 

 this lacking vein, except wdien near the margin, is always found 

 to be represented by a fold that occupies the same position as 

 the vein. On the other hand, there are many folds in the 

 membrane of the wings of insects that are never represented 

 anywhere by veins. 



Each of these three modifications of the membrane is thus 

 seen to indicate relationship with veins. The question arises, 

 Do they represent veins that have disappeared, or are they 

 places where veins are in the course of development? The 

 fact that the lower insects commonly have more veins than 

 the higher has led many observers to the conclusion that 

 these modifications represent lost veins. In some instances 

 this is very evidently the case. A broader view of the matter 

 would be the conception that there is between every two veins 

 a possible vein-path, or more than one, and that these may 

 be indicated by either of the peculiarities we have just been 

 considering, and that there is no essential difference between 

 the indications of a forming vein and of a suppressed one. 

 This path partakes of many of the characteristics of veins, so 

 that the calling forth of a new vein is not a process that must 

 result in an entirely new structure, but there is at hand a 

 region almost ready to take on that kind of development. 

 Likewise, the suppression of a vein involves merely the lower- 

 ing a little of the conditions that increase the tendency toward 

 vein development. These paths of potential veins exist in all 

 possible conditions from nothing to a complete vein. The 

 suppression of a vein may be supposed to involve the reduc- 

 tion of the adjacent vein-paths to the minimum, and their 

 replacement by a single path that represents the destroyed 

 vein; the formation of a new vein to involve the development 

 of two new vein-paths. 



Resume. — The many diverse conceptions of the structure of 

 veins, which have been based on the assumption that other 



