66 



University of California Puhlicaiions. [Entomology 



Two sets of veins early to be developed constitute the series 

 which we may denominate the anteriors and the posteriors. 

 These are independent series, being developed on separate 

 sides of the primary vein, but they arose in exactly the same 

 manner. These are the veins that radiate from the base and 



occupy the membrane not 

 served by the branches of 

 the primary. They would 

 be produced in a series one 

 after the other as the size 

 of the wing increased, and 

 may be numbered in the 

 order of their development, 

 which is from the primary 

 outward. The stages of 

 development are illus- 

 trated in the accompany- 

 ing diagrams. (Fig. 23.) 

 These anterior and pos- 

 terior veins, as they are 

 produced,will extend them- 

 selves across the membrane 

 at its weakest points and 

 thus divide the area more 

 or less equally into two 

 parts. Probably the first 

 of these veins to be devel- 

 oped was the first posterior. 

 The time of the appearance 

 of the • next posterior in 

 reference to the first ante- 

 rior can not be decided, as 

 the two series have no 

 necessary relation to each other. The anteriors and posteriors 

 may become branched, but as they become successively closer 

 together the probability of branching rapidly diminishes. The 

 first posterior is often branched, but seldom as much so as the 

 primary. 



All the veins so far considered have arisen directly or indi- 

 rectly from the base of the wing. While they are hardly to be 

 designated as outgrowths of the cells of the body wall, they are 



FIG. 23. Diagram illustrating proposed theory 

 of the development of venations. 

 AM, anterior marginal vein ; PM, posterior 

 marginal vein; P, primary vein ; B, branches; 

 lA, anterior vein; IP, 2P, :^P, 4P, first, second, 

 third and fourth posterior veins; 1, inde- 

 pendent veins; 12, IS, and fractions i, J, J, etc., 

 possible ways of designating independents. 



