Vol. 1.] WoodwortJt . — Wing Veins of Insects. 43 



In this connection it should be recalled that a vein (Fig. 

 14) is produced in halves, one in each membrane, and that 

 the lumen is the result of their not fitting together closely, 

 they being arched in cross-section. Sometimes the curvature 

 exists in only one membrane, the other contrilmting merely a 

 flattened plate, which is sometimes only slightly or not at all 

 thickened. There is no essential difference between these one- 

 sided veins and symmetrical ones, as all transitions occur 

 from a case where a vein is shared equally between the two 

 lamina to one where, so far as any appreciable structure is 

 concerned, it belongs wholly to one membrane. This transi- 

 tion may sometimes occur in the course of a single vein. 



The apical end of a vein, especially in Hymenoptera, is often 

 solid; that is, the lumen ends long before the tip of the vein 

 is reached, and there is no change in any of the other charac- 

 teristics of the vein. These peculiarities may be seen in Fig. 15. 

 The structure in the latter case is unquestionably identical 

 before and after reaching the point 

 where the lumen ceases, and for 

 some distance both lamina con- 

 tribute to the formation of the solid 



vein; finally, when the wing appears pj,, ^^ ^^^^.^^^ ^^ ^,^^ ,^.,^,^ 

 to involve but a single layer, there ^^'i"? ^^ ^,?i«' showing veins k- 



^ •' ' coming solid. 



is still no change in structure, only a 



gradual diminution in size, which begins long before the lumen 

 disappears, and is uniform and regular all along its course. 

 Furthermore, there is a great deal of variation within a species 

 regarding the point where the lumen ceases. In some species 

 the lumen may extend to the extreme end of the vein; in 

 others, it may not exist beyond the point where the vein leaves 

 the cross vein. 



The evidence is thus sufficient to support the proposition 

 that the presence or absence. of the lumen does not indicate 

 any essential difference in the nature of a vein, but signifies, 

 rather, different conditions of the same organ. 



The presence or absence of a trachea in the vein has ap- 

 peared a matter of more significance, especially to German and 

 American entomologists, on account of the association of 

 the wings with tracheal gills. Not a few writers have sup- 

 posed the vein to be merely a specialized trachea. The only 

 basis for this idea is the tracheoid character of many veins. 



