Vol. 1.] Woodtvortli. — Wing Veins of Insects. 79 



the membrane. If one side of the wing in the vein region 

 does not take part in the formation of a longitudinal vein, it 

 is not difficult for a. cross vein to extend its influence across to 

 the other side. Not uncommonly, where the longitudinal vein 

 is very much to one side, the cross veins actually establish 

 a connection across from side to side. The vein thus formed 

 across the longitudinal vein sometimes becomes quite strong 

 and important. An extreme case of this sort has been 

 described in this paper in the triangular cross vein at the base 

 of the wing of Eschna. (See page 31.) 



The advantages a series of cross veins would have, in case 

 they came to lie parallel to the margin, is evidently the explana- 

 tion of the veins of this nature that are so common in the wings 

 of certain neuropterous insects. The combined cross veins in 

 this case come to bear the same relation to the outer margin 

 that the primary vein originally had to the front margin. It 

 is doubtless only another expression of the power that tends 

 to hold all the longitudinal veins parallel with one another. 

 Sometimes there will be more than o-ne line of cross veins 

 parallel with the margin. The cross veins on the disk of the 

 wing that sometimes extend as a series of continuous veins 

 at right angles to the longitudinal veins, are certainly related 

 structures, though they have no relationship with the margin. 



What gives the cross veins that follow the margin their high 

 development and permanency is the fact that they connect 

 veins which reach the margin more or less obliquely. The 

 influence of these cross veins causes the elements of the longi- 

 tudinal ones to alternate with each other while the cross veins 

 become continuous and prominent. The cross veins become 

 coordinated in spite of the tendency impressed on them by 

 the longitudinal veins — they become stronger, and then the 

 logitudinals conform to them in the same way they would to a 

 marginal vein. 



The space between the primary and the first posterior vein 

 is larger than any other interspace in the wing — usually 

 nearly or quite a third of the whole wing area. The conse- 

 quent weakness of this region is not wholly relieved by the 

 insertion of the independents. Here, then, is a place where a 

 series of coordinated cross veins serving as a connective 

 between these two veins would be of particular utility. Some- 

 times the veins are not coordinated, but are equally important 



