Vol. 1.] Woodivorth . — Wing Veins of Insects. 67 



nevertheless produced from cells that have developed under 

 similar conditions. These conditions have progressed from the 

 base of the wing toward its tip, so that these veins may fairly 

 be regarded as outgrowing veins. They maintain their con- 

 nection with the body cavity in the adult condition in the 

 groups which, in this respect at least, we may consider to have 

 retained the primitive condition; that is, the Odonata and 

 Ephemerida. In all other existing groups of insects the base 

 of the wing has been profoundly modified by the development 

 of the structures that make the flexion of the wing possible. 



The vein least affected by this specialization is the primary. 

 Its relative strength is probably the reason that all the other 

 veins of the wing gave way while the primary is merely made 

 flexible, becoming the axis of flexion. The changes in the 

 articulation of the wing thus brought about have already been 

 discussed, and it has been shown that the only close attach- 

 ment of the wing to the wing-roots is by the primary, though 

 it may be assisted by an adjacent vein. This fact has given 

 the primary vein still more importance in venation, but has 

 somewhat complicated its study, because of the tendency of 

 other veins to attach themselves to this one. The extent to 

 which this occurs will be evident from a study of the modifi- 

 cations in the different groups. 



Anterior to the primary vein the effect of flexion has been 

 most marked in the specialization of the base of the marginal 

 vein. This vein has been broken into a number of movable, hol- 

 low segments, separated from each other by areas of soft skin; 

 a tendon terminating in a muscle passes through these segments 

 to be attached to the distal elements of the series. Thus the 

 insect is enabled to pull the distal element toward the base of 

 the wing, the front edge of which is thereby drawn forward till 

 the organ is expanded ready for action. Insects usually pos- 

 sess but a single anterior vein, which loses its basal connection, 

 attaching itself to tlie primary and becoming weak except 

 when it takes part with the primary in the articulation. This 

 specialization of the marginal vein makes it necessary, if there 

 is to be any considerable expansion of the wing anteriorly, 

 that the membrane extend beyond the vein; that is, that the 

 vein no longer lie along the margin. When this occurs and 

 the vein extends any considerable distance, the membrane so 

 produced is served by branches on the outside of the anterior 



