Vol. 1.] Wood worth. — Wi)ig Veins of Insects. 65 



very regular intervals and lie parallel with each other, and 

 sometimes, when long, come to take a position almost parallel 

 with the primary vein. 



For rather small wings, branches afford what is probably the 

 most satisfactory mechanical solution of the problem of sup- 

 porting the wing membrane. When the wing becomes larger, 

 however, the torsion on the principal vein would begin to be a 

 serious matter, so that other means of strengthening the wing 

 become necessary. 



The ontogeny of the vein branches is explainable on the 

 supposition that the tendency toward multiplication instead 

 of specialization of the cells of the wingpad when carried too 

 far results in the production of transverse wrinkles, which 

 open up courses for the food material in the vein to find its 

 way outward. If this occurs before the specialization of the 

 membrane cells has gone too far, it may result in the extension 

 of the vein area in that direction. The course of a branch 

 produced in this manner would be governed by the same forces 

 that determined the course of the primary vein; that is, the 

 vein branch would follow the lines of least resistance. This 

 would be at first approximately at right angles to the primary 

 vein, and as soon as it gets beyond the influence of this vein 

 it would turn forward to reach the margin at approximately 

 right angles. Sometimes branches occur that leave the pri- 

 mary in the way indicated, then turn forward and run parallel 

 to it for a distance, and then turn again to meet the margin 

 squarely. Such a vein follows both of the lines of mechanical 

 weakness; that is, parallel to the vein and at right angles to 

 it. Of course, veins do not bend abruptly, and the tendency 

 is always to go straight, so there is always a compromise 

 effected, which reduces the angle of branching considerably 

 below ninety degrees. 



An increase in the size of a wing usually results in an in- 

 crease in the number of veins. There are many notable 

 exceptions to the rule that the size and number of veins are 

 correlated, but these only . emphasize the general rule. In 

 many of the lower insects the tendency of a wing to develop 

 a vein wherever there is room is very evident. To this tend- 

 ency we may look for the explanation of many of the facts 

 relative to the perfection of the venation system of insect 

 wings. 

 5— V 



