62 University of California PuUications. [Entomology 



has not been entirely able to divest himself of his former idea 

 of an essential connection l)etvveen the trachea' and veins, even 

 after becoming a convert to the Miillerian theory of wing origin. 



The mechanical theory of the origin of venation really 

 affords, I believe, an explanation of wing structure much more 

 reasonable and adequate than either of the other two suggested 

 solutions of the problem. In an organ adapted for such a 

 special function as flight, where the accurate adjustment of the 

 mechanical detail is of such supreme importance, it is very 

 natural to think that the structure upon which the strength 

 of the wing depends should be more profoundly affected by its 

 own functional requirements than by the accidents of the 

 structure of a temporary adaptive organ occurring in an 

 ancestor that lived in or before the Silurian epoch, particularly 

 since the detail of structure of that organ could have been of 

 no importance in the beginning, and can not have any onto- 

 genetic necessity at the present day. 



The more rational conception is that there existed at the 

 beginning, and has existed through all time to the present day, 

 a mechanical necessity, in accordance with which the primitive 

 venation was produced and all its essential features have been 

 maintained through all the vicissitudes of the ages. This idea 

 accords entirely with what has been conjectured to be the 

 course of the development of the precursor of the wing. 



In this ancient organ we may suppose that the general out- 

 line of the venation of insect wings had already been blocked 

 out, because that organ was called upon to meet similar needs. 

 Exactly how far the venation was specialized before the organ 

 became an organ of flight may never be known, and does not 

 particularly concern the present discussion. The terms wing 

 and wingpad will be used, in what follows, in the sense that 

 they include the precursor of the wing. 



Another matter that can not be followed with any assurance 

 of chronological accuracy is the order of vein specialization, 

 because there are a number of elements in a venation that 

 are to a large extent independent of each other. In other 

 respects the course of the development of venation can be 

 traced very satisfactorily. 



One of the earliest veins to appear is the marginal. Its 

 ontogeny can be imagined by supposing the wingpad to have 

 become somewhat more bag-like than usual after one of the 



