14-1: University of California Publications. [Entomologt 



is that which is mechanically the strongest for the amount of 

 material used, a hollow cylinder, though the veins may become 

 rod-like, or flat, or even disappear in situations where they 

 have become useless. There is no essential difference in either 

 the structure or origin of the different sorts of veins; this 

 indicates that their functional utility is the important reason 

 for their existence. Insects have veins in their wings, pri- 

 marily because they are serviceable. 



The arrangement of veins is in like manner the expression 

 of the mechanical needs of the wing. The venation is con- 

 ceived of as receiving nothing from the precursor of the wing 

 except veins that were developed in the same way and to meet 

 identical needs with those of the organ after it became adapted 

 to flight. A system of trachea? that were developed for the 

 purpose of respiration can not, according to this view, have 

 any relation with the veins subsequently developing in order 

 to strengthen the organ, since tlie functions are incompatible, 

 and since the production of a vein in a tracheal gill would, by 

 just that much, reduce the breathing surface. 



The detailed study of the venation of the various groups of 

 insects has only strengthened the idea of the predominating 

 influence of the functional requirements, and confirmed the 

 writer in the view that in this we may find a basis for a true 

 theory of venation. The theory, stated very briefly, is, (1) that 

 each vein is produced for mechanical reasons: the marginals 

 to stiffen the edges of the wing, the primary to serve as the 

 dominant vein, the anterior and posteriors to supplement 

 the primary at the points of greatest need on either side, the 

 systems of independents and cross veins to occupy the areas 

 of the wing remaining poorly provided with longitudinal 

 veins, and, finally, plication veins in the Elytroptera to meet 

 the special requirements at the points of folding; (2) that the 

 production of different types of venation has proceeded along 

 comparatively narrow lines bounded by inflexible physiological 

 requirements, by w^iich the distinctness of the groups has been 

 maintained. 



There will be found, therefore, in all venations certain fac- 

 tors in common, and in each group certain features that can 

 not be so strictly compared, because they were produced under 

 conditions Avhere the mechanical requirements were different. 

 To tlie extent to which these requirements are uniform, and 



